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ABSTRACT
LANIER, BRYAN KEITH. Study in the Improvement in Strength and Stiffness Capacity of Steel Multi-sided Monopole Towers Utilizing Carbon Fiber Reinforced Polymers as a Retrofitting Mechanism (Under the direction of Dr. Sami Rizkalla)
Wireless service is a fast developing market which places inherent demands on providers
to maintain constant, reliable networks through which the service is offered. In order to
facilitate this growing need, wireless providers must install equipment which creates and
strengthens these networks. Telecommunication towers are popular solutions for placing
antennas at elevations which develop the line of sight trajectory and signal coverage the
networks demand. However, as telecommunication towers have a finite limit to the
amount of equipment installation, they must be strengthened to support additional
equipment expansion.
Research completed at North Carolina State University proposes a strengthening solution
utilizing high-modulus carbon fiber polymers as a retrofitting mechanism for monopole
telecommunication towers. The experimental program, along with development of an
analytical model, investigates the behavior and validates the effectiveness of carbon fiber
in increasing the flexural capacity of existing monopole tower structures.
The experimental program consists of testing three large scale monopole towers using
high-modulus sheets, high-modulus strips and intermediate-modulus strips to determine
their respective effectiveness in increasing the flexural strength enhancement. The three
tests are designed using approximately the same reinforcement ratios, as well as
identically sized monopole towers, to compare the effectiveness of the three
strengthening systems regarding the increase in strength and stiffness. Design nominal
strength and stiffness increases were in the range of 20 to 50% which was found in the
measured values. The three tests were subjected to the same load setup and tested until
failure to capture the elastic and inelastic behavior and the strength increases, as well as
the failure mode of the strengthened tower.
The analytical models were designed to simulate the monopole’s behavior before and
after strengthening using conventional methods of analysis typically applied to tower
design. The analytical model is based on moment-area and transformed section theories
to predict the strain and deflection behavior in the elastic range of the steel and carbon
fiber. Parametric studies are conducted to study the effect of the numerous variables with
respect to strengthening these types of towers.
STUDY IN THE IMPROVEMENT IN STRENGTH AND STIFFNESS CAPACITY
OF STEEL MULTI-SIDED MONOPOLE TOWERS UTIZLING CARBON FIBER
REINFORCED POLYMERS AS A RETROFITTING MECHANISM
By
BRYAN KEITH LANIER
A thesis submitted to the Graduate Faculty of
North Carolina State University
In partial fulfillment of the
Requirements for the degree of
Master of Science
in
CIVIL ENGINEERING
Raleigh, North Carolina
Spring 2005
APPROVED BY:
_____________________________________ Dr. Sami Rizkalla, Chairman of Advisory Committee
_____________________________________ Dr. William Rasdorf, Member, Advisory Committee
_____________________________________ Dr. James Nau, Member, Advisory Committee
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BIOGRAPHY
Bryan Keith Lanier was born on December 2, 1977 in Lexington, North Carolina. He
graduated from West Davidson High School in May 1996 and enrolled at North Carolina
State University in the School of Engineering. He completed the Bachelor of Science in
Civil Engineering in May of 2001. After graduation, Mr. Lanier was employed by
SpectraSite Communications. His work there entailed the structural analysis and design
of steel and concrete telecommunication towers. During the spring of 2001, Mr. Lanier
was admitted into the Graduate School at North Carolina State University in pursuit of
the degree of Master of Science in Civil Engineering.
Mr. Lanier is the son of Dennis and Rita Lanier and has one younger brother, Jason Craig
Lanier, who also is a graduate of North Carolina State University from the Department of
Mechanical Engineering. Upon completion of his academic requirements, Mr. Lanier
will continue his career in the field of telecommunication/tower engineering.
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ACKNOWLEDGEMENTS
The author wishes to gratefully acknowledge the funds provided by the National Science
Foundation through the Industry/University Collaborative Research Center and the
Mitsubishi Chemical America, Inc for providing the materials needed for the
experimental program. The engineering department staff of SpectraSite
Communications, specifically Douglas Pineo, provided significant assistance in the test
design. J. John Harris, P.E., David Schnerch, and Dr. Amir Fam provided valuable
insight into the test design process. Jerry Atkinson, Lab Technician for the Constructed
Facilities Laboratory, and Bill Dunleavy, Electronics Technician, added valuable
practical knowledge and advice to the actual test setup. The members of the advisory
committee are gratefully thanked for their contributions and reviewing this thesis.
Finally, the author would like to acknowledge Kirk Stanford, Scott Wirgau, Randall
Wilson, Todd Garrison and Joel Howard for their support throughout entire graduate
school process.
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TABLE OF CONTENTS
LIST OF TABLES vi LIST OF FIGURES vii CHAPTER 1 - INTRODUCTION 1
1.1 General 1 1.2 Objectives 3 1.3 Scope 4
CHAPTER 2 - BACKGROUND & LITERATURE REVIEW 7
2.1 Introduction 7 2.2 Telecommunication Towers 8
2.2.1 Latticed/Self-Supporting Towers 8 2.2.2 Guyed Towers 10 2.2.3 Monopole Towers 11 2.2.3 Tower Design Loads 13 2.2.4 Industry Design Codes 16
2.3 Current Conventional Strengthening Methods for Monopoles 17 2.4 Carbon Fiber Reinforced Polymers 20
2.4.1 Previous Testing and Applications 20 CHAPTER 3 - EXPERIMENTAL PROGRAM 27
3.1 Introduction 27 3.2 Material Characteristics 28
3.2.1 Steel 28 3.2.2 Carbon Fiber 32 3.2.3 Epoxy 33
3.3 Design of the Specimens 33 3.3.1 Test I Design – High-Modulus Sheets 34 3.3.2 Test II Design – High-Modulus Strips 35 3.3.3 Test III Design – Intermediate-Modulus Strips 37
3.4 Fabrication of the Specimens 38 3.4.1 Monopole Surface Preparation and Cleaning 38 3.4.2 CFRP Preparation 39 3.4.3 Installation 40
3.5 Test Setup 42 3.5.1 Instrumentation 42 3.5.2 Load Application 44
CHAPTER 4 - TESTED RESULTS 55
4.1 Test I – Monopole Strengthened with High-Modulus Sheets 56 4.1.1 Stiffness and Strength 57 4.1.2 Discussion of Test Results 61
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4.2 Test II – Monopole Strengthened with High-Modulus Strips 63 4.2.1 Stiffness and Strength Results 63 4.2.2 Discussion of Test Results 68
4.3 Test III - Strengthening with Intermediate-Modulus Strips 70 4.3.1 Stiffness and Strength 71 4.3.2 Discussion of Test Results 76
CHAPTER 5 - ANALYTICAL MODEL 107 5.1 Elastic Flexural Stiffness Model 108 5.2 Test I Model 111
5.2.1 Deflection, Stiffness and Strain 112 5.2.2 Discussion of Tested vs. Modeled Results 113
5.3 Test II Model 115 5.3.1 Deflection, Stiffness and Strain 115 5.3.2 Discussion of Tested vs. Modeled Results 116
5.4 Test III Model 118 5.4.1 Deflection, Stiffness and Strain 118 5.4.2 Discussion of Tested vs. Modeled Results 119
5.5 Parametric Study Using the Proposed Analytical Model 121 5.5.1 Effect of Layers - Test I Model 122 5.5.2 Effect of Modulus - Test II Model 125
CHAPTER 6 - SUMMARY AND CONCLUSIONS 134
6.1 Summary 134 6.2 Conclusions 136 6.3 Recommendations for Further Testing 140
REFERENCES 141 APPENDIX
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LIST OF FIGURES
Page
CHAPTER 2 2.1 Lattice/Self-Supporting Tower 24 2.2 Guyed Tower 24 2.3 Tapered Monopole 24 2.4 Stepped Monopole 24 2.5 DualPole Installation 25 2.6 DualPole Cross-section 25 2.7 MUS Steel Band and Epoxy Application 25 2.8 MUS Installation 25 2.9 STSP Completed Installation 25 2.10 STSP Installation 25 2.11 WDMRS Base Installation 26 2.12 WDMRS Installation Looking Up 26 2.13 AMUS Completed Installation 26 2.14 AeroSolutions Adhesive Testing 26 2.15 HTSMTR Completed Installation 26 2.16 HTSMTR Installation 26 CHAPTER 3 3.1 Monopole Shaft Dimensions and Fabrication Method 47 3.2 Baseplate Dimensions and Anchor Bolt Orientations 47 3.3 Monopole Specimen 47 3.4 Stress/Strain Coupon Test Results 47 3.5 CFRP Sheets and Strips 48 3.6 Longitudinal and Transverse Sheet Layout, Test I 49 3.7 Clip Angles at Base, Test I 49 3.8 Longitudinal Strip Layout, Test II & III 50 3.9 Stiffener Placement, Dimension and View, Test II & III 50 3.10 Surface Preparation 51 3.11 Longitudinal and Transverse Sheet Installation 51 3.12 Adhesive Application 52 3.13 Strip Installation 52 3.14 Pi Gauge and Strain Gauge 52 3.15 Pi Gauge Layout 52 3.16 Base Potentiometer Layout 52 3.17 Typical Pi and Strain Gauge Locations 53 3.18 Potentiometer Locations Along Monopole Shaft and Baseplate 53 3.19 Monopole Loading Setup 54 3.20 Monopole Loading Layout 54
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CHAPTER 4
Page
4.1 Measured Deflection Locations for Tests I, II and III 79 4.2 Strain Measurement Locations for Tests I, II and III 79 4.3 Net Displacement at 0.25L, 0.5L, 0.75L and L, Test I – First and Second Load Cases 80 4.4 Longitudinal Strain at 150 – 200 mm from Base, Test I – First and Second Load Cases 80 4.5 Longitudinal Strain at 460 mm from Base, Test I – First and Second Load Cases 81 4.6 Longitudinal Strain at 1520 mm from Base, Test I – First and Second Load Cases 81 4.7 Longitudinal Strain at 2900, 3050 and 3250 mm from Base, Test I – First and Second Load Cases 82 4.8 Longitudinal Strain at 4570 mm from Base, Test I – First and Second Load Cases 82 4.9 Longitudinal Strain Profile at 32 kN, Test I – First and Second Load Cases 83 4.10 Net Displacement at 0.25L, 0.5L, 0.75L and L, Test I – Third Load Case with
Nylon Straps 83 4.11 Minor Localized Debonding of Sheets at 75 kN, Test I – Third Load Case with Nylon Straps 84 4.12 Monopole Load Application with Nylon Straps 84 4.13 Monopole Load Application with Steel Chains 84 4.14 Net Displacement at 0.25L, 0.5L, 0.75L and L, Test I – Third Load Case with Steel Chains 85 4.15 Buckling of Monopole Shaft and Rupture of Sheets, Test I – Third Load Case with Steel Chains 85 4.16 Longitudinal Strain at 200, 460 and 1520 mm from Base, Test I – Third Load Case with Nylon Straps 86 4.17 Longitudinal Strain at 2900, 3250 and 4570 mm from Base, Test I – Third Load Case with Nylon Straps 86 4.18 Longitudinal Strain at 200, 460 and 1520 mm from Base, Test I – Third Load Case with Steel Chains 87 4.19 Longitudinal Strain at 2900, 3250 and 4570 mm from Base, Test I – Third Load Case with Steel Chains 87 4.20 Longitudinal Strain Profile at 32 and 95 kN, Test I – First, Second and Third Load Cases 88 4.21 Vertical Strains at 610 and 1220 mm, Test I – Third Load Case with Steel Chains 88 4.22 Net Displacement at 0.25L, 0.5L, 0.75L and L, Test II – First and Second Load Cases 89 4.23 Longitudinal Strain at 80, 150, 200 and 230 mm from Base, Test II – First and Second Load Cases 89
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Page 4.24 Longitudinal Strain at 460 mm from Base, Test II – First and Second Load Cases 90 4.25 Longitudinal Strain at 1520 mm from Base, Test II – First and Second Load Cases 90 4.26 Longitudinal Strain at 2900, 3050 and 3250 mm from Base, Test II – First and Second Load Cases 91 4.27 Longitudinal Strain at 4570 mm from Base, Test II – First and Second Load Cases 91 4.28 Longitudinal Strain Profile at 32 kN, Test II – First and Second Load Cases 92 4.29 Longitudinal Strain at 230 mm from Base, Test II – First and Second Load Cases 92 4.30 Net Displacement at 0.25L, 0.5L, 0.75L and L, Test II – Third Load Case 93 4.31 Failure Modes at Load per Net Displacement Measured at L, Test II – Third Load Case 93 4.32 Compressive Rupture of the Top Strip, Test II – Third Load Case 94 4.33 Delaminating of Bottom Strips, Test II – Third Load Case 94 4.34 Buckling of Monopole, Test II – Third Load Case 94 4.35 Ruptured Strip Remains at Stiffeners, Test II - Third Load Case 94 4.36 Longitudinal Strains at 80, 150 and 230 mm from Base, Test II – Third Load Case 95 4.37 Longitudinal Strains at 460 and 1520 mm from Base, Test II – Third Load Case 95 4.38 Longitudinal Strains at 2900, 3250 and 4570 mm from Base, Test II – Third Load Case 96 4.39 Longitudinal Strain Profile at 32 and 44kN, Test II – First, Second and Third Load Cases 96 4.40 Net Displacement at 0.25L, 0.5L, 0.75L and L, Test III – First and Second Load Cases 97 4.41 Longitudinal Strains at 80, 150 and 230 mm from Base, Test III – First and Second Load Cases 97 4.42 Longitudinal Strains at 460 mm from Base, Test III – First and Second Load Cases 98 4.43 Longitudinal Strains at 1520 mm from Base, Test III – First and Second Load Cases 98 4.44 Longitudinal Strains at 3050 mm from Base, Test III – First and Second Load Cases 99 4.45 Longitudinal Strains at 4570 mm from Base, Test III – First and Second Load Cases 99 4.46 Longitudinal Strain Profile at 32 kN, Test III – First and Second Load Cases 100 4.47 Longitudinal Strains at 230 mm from Base, Test III – First and Second Load Cases 100 4.48 Location of Measured Strain at 230 mm from Base, Test III 101 4.49 Net Displacement at 0.25L, 0.5L, 0.75L and L to Loading of 55 kN, Test III –
Third Load Case 101
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Page 4.50 Gross Displacement at 0.25L, 0.5L, 0.75L and L, Test III – Third Load Case 102 4.51 Failure Modes at Load per Net Displacement Measured at L, Test III – Third Load Case 102 4.52 Delaminating of Bottom Strips, Test III – Third Load Case 103 4.53 Rupture of Top Strips, Test III – Third Load Case 103 4.54 Buckling of Monopole, Test III – Third Load Case 103 4.55 Air Voids in Epoxy, Test III – Third Load Case 103 4.56 Longitudinal Strains at 80, 150 and 230 mm from Base, Test III – Third Load Case 104 4.57 Longitudinal Strains at 460 and 1520, Test III – Third Load Case 104 4.58 Longitudinal Strains at 3050 and 4570 mm from Base, Test III – Third Load Case 105 4.59 Longitudinal Strains at 230 and 460, Test III – Third Load Case 105 4.60 Longitudinal Strain Profile at 32 and 54 kN, Test III - First, Second and Third Load Cases 106 CHAPTER 5 5.1 Illustration of Existing and Transformed Section 128 5.2 Deflection Diagram 128 5.3 Modeled and Tested Net Deflection Profiles at 32 kN, Test I – First and
Second Load Cases 129 5.4 Modeled and Tested Strain Profiles at 32 kN, Test I – First and Second Load Cases 129 5.5 Modeled and Tested Net Deflection Profiles at 32 kN, Test II – First and Second Load Cases 130 5.6 Modeled and Tested Strain Profiles at 32 kN, Test II – First and Second Load Cases 130 5.7 Modeled and Tested Net Deflection Profiles at 32 kN, Test III – First and Second Load Cases 131 5.8 Modeled and Tested Strain Profiles at 32 kN, Test III – First and Second Load Cases q 131 5.9 Modeled and Tested Net Deflection Profiles at 32 kN, Test I – Parametric Study 129 5.10 Modeled and Tested Strain Profiles at 32 kN, Test I – Parametric Study 129 5.11 Stiffness Increases vs. Reinforcement Ratios for Three Strip Specimens,
Test II 129
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CHAPTER 1 - INTRODUCTION
The wireless telecommunications industry has enjoyed significant increases in business
within the past ten years. Predictions from several financial analysts suggest the business
will only increase as the use of wireless products becomes more widespread and reliable.
The introduction of wireless fax, internet, and email are some of the more recent
advances in an industry that was previously limited to the availability of phone and fiber
optic lines. The use of cellular phones is the most powerful incentive in the wireless
industry to develop networks capable of supporting the increasing consumer demand.
Their use in the United States and abroad has created numerous revenue streams for
various businesses.
1.1 General
The advantage of wireless technology is its ability to be utilized largely at any location.
No physical lines, wires, or connections on which the electronic or laser frequencies
converge at a motherboard are required. Only strategic locating of antennas, dishes, or
receptacles are needed to transfer the information from the user to the user. These
antennas, which are the basis of the cellular networks, must be mounted at various
locations to ensure coverage capacity to the user. They are mounted on all types of
structures from water tanks to skyscrapers and in some cases even the insides of buildings
and palladiums. One of the most popular mounting devices are large steel and concrete
towers. They can be placed strategically without the requirements of large amounts of
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land and can elevate the antennas to any height desired. Only cost, local regulations, and
the availability of land hinder their construction.
Several large communication businesses have recognized the opportunity of marketing
telecommunications towers and have built numerous towers on which antennas can be
installed. It is in the best interest of each tower leasing business to have as many
antennas on their towers as possible since this equates to more capital. Towers, like all
structures, are designed and maintained in accordance with national building codes and
standards. They are designed to satisfy serviceability and strength requirements,
including weight, wind and ice loads specific to the tower’s geographic location.
Due to the demand for wireless service, there is a need to increase the number of
antennas a tower can support. Strengthening of cellular towers is required to ensure the
structure can carry the increased loading from the additional antennas. The strengthening
system must be cost effective while not interrupting service of the tower’s current
tenants. There are several alternatives for retrofitting towers but most are expensive and
cumbersome to install. A solution must be developed which significantly enhances the
overall load carry capacity of the tower without altering its overall appearance or
serviceability. It must be rugged and durable, able to withstand the forces and elements
of nature while being easy to prepare and install.
Utilization of high modulus carbon fiber reinforced polymers (CFRP) provides an
excellent potential solution which could greatly enhance the strength capacities of
3
telecommunication towers. The inherent strength qualities of CFRP offer significant load
carrying improvement, introducing itself as a prime candidate for enhancing the strength
of the tower. Installation of the CFRP can be relatively simple and completed in a
relatively short time, offering significant advantages over existing strengthening
techniques. This system eliminates the need for welding, which is a major issue in terms
of cost and function. The light weight of CFRP material also lends itself to be safer and
easy to handle material. CFRP has superior resistance to fatigue, thus enhancing the
serviceability of the tower structure. Finally, CFRP resistance to corrosion enhances its
value to future deterioration of the structure.
1.2 Objectives
The main objective of this study is to determine the effectiveness of using CFRP to
increase in the strength and stiffness of monopole steel towers. The specific aspects
considered in this study are:
1. Evaluation of the stiffness of the tower strengthened with CFRP within the steel
elastic range, with respect to the stiffness of unstrengthened towers.
2. Determination of the overall strength increases of the tower strengthened using
various types of CFRP.
3. Develop an analytical model to predict the flexural stiffness and strength of
unstrengthened steel monopole towers with the steel elastic range.
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4. Examine the different possible failures modes of the towers strengthened with
CFRP.
5. Study the various factors affecting strengthening these towers with CFRP.
6. Provide design recommendations and installation methods for the proposed
strengthening technique.
1.3 Scope
The scope of this work consists of an experimental investigation using large scale models
of the towers and development of an analytical model. The experimental program
provides detailed information on the behavior and increases of the strength and stiffness,
as well as failure modes, of towers strengthened with CFRP material. The study provides
also insight into the installation process, offering methodology of handling the material
and existing tower structure. The analytical model is proposed to predict the measured
values and validate the increase of the strength and stiffness within the steel elastic range.
The models will also be used to study various parametric factors believed to influence
strength and stiffness values used in the proposed technique.
The experimental program includes testing of three steel towers. The first test utilizes
CFRP in a sheet form. The first load case of Test I include testing of the unstrengthened
tower to 60 percent of its nominal flexural yield strength. The tower is then unloaded and
followed by installing CFRP sheets designed to increase the yield strength and stiffness
between 20 and 40 percent. The strengthened tower was loaded to reach the same
5
midspan deflection measured from the first load case at 60 percent, yield strength of the
unstrengthened tower. The strengthened tower was then loaded up to fail.
The second tower was strengthened with High-Modulus (HM) CFRP and tested in similar
manner to the first tower. The CFRP strips were manufactured using the same carbon
fiber material of the sheets. The design strengthening was also applied to increase yield
strength and stiffness approximately 20 to 40 percent.
The third tower follows the same testing procedure as Test I and II, but uses a
Intermediate-Modulus (IM) CFRP strip. The strips were manufactured using different
fiber material. The strengthening scheme was designed to increase the yield strength
approximately 20 to 40 percent.
The analytical model, used to predict the strength and stiffness of the unstrengthened and
strengthened towers within the steel elastic range, considers the linear behavior of both
the steel and CFRP. This includes modeling of the flexural strains and deflection of the
tower at various locations. The analytical model results are compared to the measured
values obtained from the experimental program results. Parametric studies are focused
on various parameters believed to affect the stiffness and strength.
The following chapters of this thesis include:
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Chapter 2: Literature review detailing the various types of towers, design loads, and
design codes. Additional review will be related to CFRP research in concrete and steel
bridge girder reinforcement, bond development and fatigue properties. Current tower
strengthening schemes will also be reviewed.
Chapter 3: Description of the experimental program, detailing the three tested steel
towers used in the experimental program. Properties of the epoxy, CFRP and steel
material qualities, tower geometry, CFRP and tower surface preparation, CFRP
installation, test instrumentation and setup will be provided.
Chapter 4: Presentation of the results of the three tests is summarized. Results include
all net strains and deflections, along with detailed account of the structure’s behavior
during each load case. Specific failure modes will be explained for each test.
Chapter 5: Presentation of the analytical models. The analytical model results are
presented with the experimental results to extend discussion into the validity of the
models and the testing procedures. The parametric studies are also listed.
Chapter 6: Summary and conclusion of the study. Recommendations for future studies
are given.
Appendices: Additional data from the experimental investigation.
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CHAPTER 2 - BACKGROUND & LITERATURE REVIEW
This chapter details the background of the proposed research. Specifically, the types of
towers used in the telecommunications industry, the type and nature of the loads
considered in design and the industry codes governing the design and maintenance of
these towers are discussed. Various alternative solutions for strengthening towers will
also be detailed. Pervious research utilizing CFRP as a strengthening technique is
discussed.
2.1 Introduction
Telecommunication antennas and dishes are installed at various heights, azimuths and
orientations on large steel and pre-stressed concrete towers. These towers are typically
located in high density population regions and along major travel routes. They are
maintained according to local and national building codes from the government and
telecommunications industry. With the growing demand for wireless services, there is a
need to strengthen towers to accommodate additional equipment. The optimum solutions
are highly dependent on the cost, space and local building restrictions.
Carbon Fiber Reinforced Polymer (CFRP) is a relatively new construction material which
is gaining widespread popularity in rehabilitating existing structures. Due to it’s light
unit weight and high strength characteristics, this material has been used for many years
with great success in the aerospace industry, where weight to strength ratio is of great
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importance. In the construction industry, CFRP is used to increase the strength capacity
and ductility of structures resisting wind, live, seismic loads. Most research in the
construction industry utilizing CFRP has been centered on strengthening bridges.
Specifically, the research has been aimed at establishing bond properties and
characteristics. Ductility and the strength of CFRP composite systems can provide has
also been investigated. Additional research within the construction industry has been
focused on fatigue resistance of CFRP.
2.2 Telecommunication Towers
Towers are fabricated with various geometries and are used for a variety of
telecommunication applications. They are largely designed to resist wind, ice, and
seismic loading in accordance with recognized structural design building codes. The
towers are made primarily categorized in three types: lattice/self-supporting, guyed and
monopoles. Each tower has a specific geometry and unique design characteristics.
2.2.1 Lattice/Self-Supporting Towers
Lattice/self-supporting towers are steel trusses constructed to form a cantilever beam
perpendicular to the ground surface. Figure 2.1 illustrates the typical size and geometry
of a self-supporting tower. They are constructed of moderate (A36) to high grade (A572)
steel with the truss members welded or bolted in place using A325 high strength bolts.
These towers typically range from 15 m to 150 m in height and 1 m to 20 m in width,
although towers with heights upward of 300 m have been erected [FWT, 2]. They are
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usually tapered with decreasing width as elevation increases and have triangular or square
cross-sections.
Lattice/self supporting towers earn the name due to their geometry and design
characteristics. The tower is a lattice-trussed structure. The truss members can be steel
pipes, solid rods, angles, flat or bent plates, channels, or cables. Each member is
designed to resist a specific load or provide support to adjoining members. The members
connect to each other to form a frame with rigid axial and lateral stiffness. Because of
this stiff frame, the term self-supporting evolved. This frame keeps the tower erect
during its lifetime as no other supports are in place.
This inherent stiffness is one of the design strengths of self-supporting towers. A
standard self-supporting tower will typically deflect laterally less than 5 percent of its
height at it’s maximum designed wind load. Operational requirements of some antennas
and dishes are very dependent on their direction and orientation, so a rigid structure is
necessary to ensure the antenna service reliability. Self-supporting towers are the most
reliable for maintaining this twist and sway serviceability. Another advantage is they can
be built on small plots of land. This is particularly advantageous in urban areas or along
roadways where space is at a premium. A key disadvantage, however, is their cost. In
order to maintain the high stiffness and strength capacity, lattice/self-supporting towers
tend to require more steel to fabricate and labor to install. Of the three tower types, they
tend to be the heaviest. Also, potentially they can be viewed as eyesores as typically they
are not designed to be aesthetically pleasing.
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2.2.2 Guyed Towers
Guyed towers are similar to self-supporting towers in that they are also lattice structures
but have a much lower lateral stiffness. Illustration of a typical guyed tower is shown in
Figure 2.2. These tower’s heights can be in excess of 450 m with shaft widths ranging
from 0.5 m to 3 m [PiRod, 4]. Their shafts are typically prismatic throughout their
elevation and composed of the same variety of structural members and connections as a
self-supporting tower.
The main difference is the attachment of guy lines at various elevations to the tower’s
shaft. These guy lines maintain the stability of the tower. Design is similar to that of a
multi-span bridge. Essentially, the guys act in tension to counteract the lateral loads
applied by the wind, like a bridge column. The trussed shaft resists lateral moments and
shears as well axial loads from its own weight, ice and the guy line tension forces, like a
bridge girder. The bases of the towers are typically designed as pinned connections to
eliminate bending, which lowers lateral stiffness in the entire truss section, allowing the
guy lines to resist the lateral wind loads. Guy lines are made of steel braided wire of
various diameters and strengths with specific stiffness and weights. The guy lines are
attached at the ground using any type of anchor foundation.
The advantage of guyed towers is the ability to build them to great heights at a lower cost
than a self-supporting tower. The members of a guyed tower do not have to be sized to
be as large because the guy lines transfer most of the lateral loads to the ground. The
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heights obtainable for a guy tower at a reasonable cost are impossible to achieve using a
self-supporting design.
The disadvantage of guyed towers is the amount of land necessary for installation.
Where self-supporting tower uses a ground area of 230 m2 or less, a guyed tower with
identical height may need in excess of 23,000 m2. The guy wire anchors typically are
installed at a radius of 75 percent of the tower height away from the tower base. Towers
over 300 m in height can require as much as 45,000 m2 (10 acres) of land. Also, because
the stiffness of a guyed tower is less, deflection and rotation of the tower can also be
much greater. Antennas requiring specific twist and sway tolerances must be placed
carefully on the tower to remain within their service limits
2.2.3 Monopole Towers
Monopoles are single circular or polygonal cross-sectioned shafts extending to heights of
up to 75 m (250 ft). The shafts can be one piece or slip fit/bolted on top of one another.
Each shaft section usually is between 6 m to 15 m long. Typically, the shafts are non-
prismatic, with decreasing diameter as elevation increases. This taper can be constant
with height or stepped inward at various elevations. Illustration of two types of
monopoles is shown in Figures 2.3 and 2.4. The shafts are made from moderate (A53) to
high grade steel (A572) or pre-stressed concrete. Their shaft thickness is typically small,
15 mm or less for steel and 120 mm for pre-stressed concrete. Average diameters range
from 200 mm to 2400 mm [FWT, 2].
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The main advantages to monopoles are their ease of installation and mounting equipment
and their general reliability to resisting natural elements. Very little land is required to
install them, as most foundations require less than 4 m2 of ground space. As they are
made up of only a few elements, monopoles can be installed very quickly, outside of the
construction the foundation. Equipment is easily mounted to a monopole as well. Also,
unlike self-support and guyed towers, there are no bolts or gussets that can be eroded or
removed, potentially causing premature failure and leading to reduced reliability.
Finally, monopoles tend to be less noticeable and can be designed to be camouflaged
with their surroundings, so the potential for resistance by zoning ordnances to allow
installation is minimized.
Some disadvantages of monopoles are the height limitations and in upgrading the
structure’s strength capacity. Monopole heights are usually limited to 60 m as heights
greater than this are usually economically unfeasible and inherently unstable structures.
Next, monopoles can be difficult to strengthen. The difficultly in strengthening a
monopole is in attaching the reinforcement. Unlike guyed or self-supporting towers,
where reinforcement is as simple as replacing a smaller, over-stressed member with a
larger, stronger one, a monopole has only one member, thus replacement means installing
a new pole. Developing a design to attach additional steel to the monopole surface is
fairly simple, but establishing a sound bond between the two surfaces typically requires
extensive construction work. Finally, monopoles have lower lateral stiffness as
compared to self-supporting or guyed towers. Monopoles typically deflect between 10 to
15 percent of their overall height laterally. Although the monopole may be structurally
13
stable, it’s lack of stiffness may exceed the twist and sway tolerances of some antenna or
dish equipment.
2.2.4 Tower Design Loads
Towers are built to withstand various loading scenarios. These differ depending on
location, functionality, and importance. Dead, live, wind, earthquake, and ice loads must
be taken into account when designing or analyzing the tower for strength and
serviceability.
Dead loads tend to stress the tower slightly as weight of the tower, antennas, mounts and
transmission lines are very small compared to the buckling capacity of the tower
elements. Typically, a tower member or cross-section as a whole will buckle at ten to
fifteen times the element or tower weight, respectively. Also, because towers are
relatively light, base shears from earthquakes tend to be mild as the acceleration exists
but large mass needed to develop significant base shears does not. Hence, seismic
concerns typically are moderate. Ice loads also tend to be slight, although their impact
must be carefully reviewed for guyed towers. Large amounts of ice can severely impact
the loads from the guy lines.
Resistance to the forces generated by wind is the primary objective when designing and
analyzing towers. Evaluating the wind effects on towers is a complex problem in
aerodynamics. Wind is considered to be a fluid impacting an immovable object at a
plane normal to the wind direction. The resulting forces are derived from wind pressure,
14
cross-sectional area of the object, shape of the object and gust effects of the wind on the
object. Wind is to be considered to be nonviscous and incompressible [4, Gaylord].
Bernoulli’s equation for streamline flow is used to calculate wind pressure. This pressure
is known as the velocity, dynamic, or stagnation pressure. As air weighs 9.82 x 10-4
kg/m3 at 15° C at sea level, the equation reduces to:
q = 0.613 x V2 (a)
• q = Wind Pressure (Pa)
• V = Wind Velocity (m/s)
From this, height above ground, wind gust and shape of the impacted object must be
considered. Most published wind speeds are measured approximately 0 - 50 feet above
the surface of the ground. As friction with the ground greatly decreases the wind’s
velocity, increase in wind pressure as height above the ground increases is accounted for
by the exposure coefficient KZ. This function, also know as the escalation factor, is
added to equation (a) when calculating pressure at specific elevations and varies
depending on the topography and roughness of the terrain. Areas with more hills, trees,
and buildings will result in more lower KZ values whereas areas that are flat, continuous,
or near or over water will have higher KZ values.
Gusting must be considered to account for the dynamic effects of the wind on the
structure. Depending on the manner in which the design wind speed is measured, gusting
effects may or may not be significant. Wind speed is measured in terms of fastest mile,
or sustained wind, or in terms of a three second, or gusting wind. A fastest mile wind
15
speed is based on the amount of time it takes one mile of wind to pass a stationary point.
A three second wind is the amount of wind that passes a stationary point in three seconds.
Thus, gusting is already accounted for when three second wind speeds are used but must
be accounted for when considering a fastest mile wind speed in a structure’s strength
design. Gusting is given lower significance for a taller structure as the likelihood of a tall
structure being entirely enveloped in a large wind gust is minimal. However, when tall
structures have low lateral stiffness, dynamic wind effects can be significant and wind
pressure is increased to account for this. The pressure is increased by a factor known as
GH, or gust effect factor, with the dynamic effects being accounted as a static load. Other
factors, including directionality and site specific increases, are typically included in code
design as they account for statistical studies revealing the likely hood a specific wind
event in specific terrains [26, ASCE].
Shape factor, CF, is the most complex issue when considering force applied to a structure
by wind. The shape factor must take into account the drag on the element as well as the
lift. These factors are not constant as wind velocity changes them continuously. Shape
factors are functions of air density (ρ), velocity (v), diameter/width and shape of the
structure (d) and viscosity of the air (µ) [5, Sachs]. The only reliable way to determine a
shape factor for a specific structure is to place the structure in a wind tunnel with
controlled wind speed and derive CF from the force applied to the structure. However,
enough testing has been completed such that all design codes offer generic shape factors
based on shape, the objects dimensions and the pressure being super or sub-critical to
gain a conservative estimate of the wind loads on a structure. Equation (b) below relates
16
the calculation of a wind force on to a specific structure due to size, drag, elevation and
wind speed.
F = q x GH x KZ x CF x A (b)
• F = Wind Force
• A = Cross-sectional Area
2.2.5 Industry Design Codes
Telecommunication towers are designed in accordance with local and national building
codes. These codes include the International Building Code, the BOCA National
Building Code and the Uniform Building Code. These national codes note towers must
resist design loads in accordance with the latest revision of ASCE7. These codes also
recognize TIA/EIA-222 (Telecommunications Industry Association/Electronic Industries
Association) as the industry standard by which telecommunication tower are designed,
built, and maintained. The standard’s official title is “Structural Standards for Steel
Antenna Towers and Antenna Supporting Structures.” This code dictates every aspect of
design of any of the three types of telecommunication towers. Design windspeed,
exposure coefficients, gust and drag factors, ice considerations, strength design and
twist/sway limits are just a few of the articles defined explicitly within this standard.
This code considers steel design per the 1989 American Institute of Steel Construction
(AISC), “Specifications for Structural Steel Buildings – Allowable Stress Design and
Plastic Design” and the American Concrete Institute (ACI) 3.18-89, “Building Code
Requirements for Reinforced Concrete” as the standard for reinforced concrete design.
17
ASCE has also published design guidelines specific to transmission structures, which is
applicable for telecommunication towers. This guideline, ANSI/ASCE 10-90, Design of
Latticed Steel Transmission Structures, reviews the procedures for determining member
strengths and stability, as well as stiffness. This standard has been adopted by TIA/EIA-
222 and most of its design parameters can be found in within the TIA/EIA-222 text.
2.3 Current Conventional Strengthening Methods for Monopoles
There are several solutions available on the market for strengthening monopoles. The
design principle is to enhance the flexural strength capacity of monopoles by adding
longitudinal steel plates, bars, tubes or fiber composites onto the outside of the existing
monopole surface. All strengthening solutions can be designed to increase strength or
stiffness to the desired capacity demands. The main difference between the various
solutions is in it’s attachment to the existing monopole and the material used for
strengthening the monopole.
Morisson Hershfield markets a solution known as the DualPole system. The concept
behind this solution is to build a new tower around the existing tower. Figures 2.5 and
2.6 illustrate the installation and cross-section of the DualPole system. Two sections of
high grade sheet steel are fabricated exactly to fit over the outside surface and encase the
existing monopole. The sections are welded onto the existing structure using low heat
welding and repair of galvanization is completed using zinc rich paint. The DualPole
18
solution utilizes low heat welding to eliminate the possible of heat or fire damage to the
monopole or other equipment on the tower [12].
Fort Worth Tower, a leading fabricator of towers within the industry, markets a solution
known as the FWT Monopole Upgrade Solution (MUS). The design concept behind this
system is to add longitudinal plates to the outside of the monopole surface. The plates
are bolted at the ends to steel bands. The steel bands and plates are then bonded to the
shaft surface using an epoxy adhesive, which is shown in Figures 2.7 and 2.8. Per FWT
documentation, actual testing has been conducted to validate the upgrade. The testing
followed ASCE Manuel 72, section 4 [FWT, 2].
The ScienTel Tower Strengthening Program (STSP), referred to as “The Boot,” is similar
to the MH DualPole System except it doesn’t conform exactly to the existing tower’s
shaft. Figures 2.9 and 2.10 illustrate the installation and completed installation of the
Boot. Two strengthened sections are bolted at each ends and longitudinally along the
shaft to create a new tower section outside the existing monopole. The strengthening
sections are installed slightly off the existing monopole surface, separated by strips of
rubber attached inside of the new shaft. New sections are bolted together at their ends
and along longitudinal seams at the shaft edge, compressing the rubber strips into the
existing tower shaft and creating a friction seal. The seal theoretically transfers load to
the strengthening system along the existing monopole shaft, creating a composite section.
High grade steel, similar to the existing tower’s shaft, is used to create the strengthening
sections [ScienTel, 13].
19
Westower Communications offers a solution uses high strength, threaded bars installed
parallel with the monopole shaft as the backbone of it’s strengthening mechanism. This
system, shown in Figures 2.11 and 2.12, is built from steel bars are manufactured by
Dywidag-Systems International. These bars range in diameter from 30 mm to 45 mm
and are fabricated from A722 steel. Normal application for the bars marketed by
Dywidag is for post-tensioning of concrete [Dywidag, 14]. Clip angles are bolted into the
existing monopole at approximately 750 mm increments and the Dywidag bars are
attached via two u-bolts through the clip angles. Bars are linked together using couplers
at each end of the bar and the bars are grouted into the existing foundation [Westower,
15].
AreoSolutions offers the AeroForce Monopole and Tower Upgrade System (AMUS).
This package is different from the above noted strengthening solutions as it uses CFRP
installed laterally along the monopole shaft. Bonded to the exterior surface of the
existing structure using epoxy adhesive, as shown in Figure 2.13, the CFRP adds
additional flexural strength. The upgrade can also be completed using high strength steel
plates, as opposed to CFRP [Aero, 16]. AeroSolutions also has completed extensive
adhesive testing, as shown in Figure 2.14, to account for the reliability of the epoxy bond.
The final monopole strengthening solution is marked by Hutter Trankina Engineering and
is listed as the HT Simplified Monopole Tower Reinforcing System (HTSMTRS).
Figures 2.15 and 2.16 illustrate the completed installation of the HTSMTRS. The design
concept for this solution is to weld continuous, high strength, flat, steel plates parallel
20
with the existing monopole shaft. The flat plates are spot-welded approximately 250 to
500 mm to create a composite structure. Each end of the flat plates is extensively welded
to assure the development of each installed plate. Stiffeners are welded at the base of the
monopole to distribute the forces throughout the baseplate.
2.4 Carbon Fiber Reinforced Polymers
The majority of research investigating composite CFRP/steel relationships deals with
establishing bond strength and durability. Specifically, surface preparation of the
adherends, role of galvanization, environmental effects, and strength of the adhesive bond
are several topics which have been researched. Additional large scale testing of bridge
girders strengthened with CFRP has also been investigated.
2.4.1 Previous Testing and Applications
Research completed by Moulds and Price suggests the width and thickness of the
adhesive, as well as the width, thickness and modulus of the adherends play the most
significant roles in establishing bond strength. Using single lap shear tests, their work
observed that wider splices and thicker adhesive bonds reduced bond stress. Increased
shear lag, however, was a by product of these changes. Their work also found when two
adherends have varying modulus, the bond stress is increased on the bonding surface with
the adherend having the lower modulus. This behavior is noted again when thickness of
the two adherends is different. Assuming the adherends have identical modulus, bond
21
stress was increased at the bonding surface with the thinner adherend. Observations also
noted maximum stresses occurring along the edges of the adhesion bond [18].
Single lap shear tests completed by Bourban studied the improvement in bond strength
when silane coupling agents are applied to metal surfaces. After prepping the steel
adherend surface by bead blasted and cleaning, a silane adhesive promoter was added to
the splice prior to epoxy application. Their work noted significant strength and durability
increases, especially when exposed to moisture [21].
Additional single lap shear tests completed by Nakazawa examining into the effects of
galvanization on adhesive bonds also indicated moisture had a negative effect on bond
strength in steel. Identical single lap shear tests were completed with the differences
being the steel surface was either untreated, galvanized or galvannealed. Unlike the tests
by Bourban, the steel surfaces were not blasted, only cleaned with degreaser. All tests
were exposed to moisture, with the results indicated adhesive failure with the untreated
and galvannealed, whereas the galvanized failed due to a combination of cohesion and
adhesive breakdown. All experienced lower durability magnitudes. Also, aside from the
moisture content, the zinc coating used for the galvanization promoted poor adhesion
between the steel surface and the epoxy [Nakazawa, 19].
Fusion bonding, which is heating of the steel prior to application of the epoxy resin, was
examined by Bourban. Results show lower bonding time and equivalent strength to
22
standard adhesives. However, only specific adhesives can be utilized as some adhesive
strengths are reduced through this curing process [20].
Changing environmental conditions have also been found to have adverse effects on bond
strengths. Wedge tests completed by Karbhari examined the effect of different moisture
types and temperatures on bond strengths during application and loading and revealed
elevated temperatures (65° C) lower bond strengths. Salt water also resulted in
significant reduction in strength, although its effects were not nearly as severe as the hot
water. Testing completed in freezing (-18° C) conditions showed greatest bond durability
and retainage of strength [22].
Bridge girders offer a unique design problem from which use of CFRP has been studied
as a potential strengthening solution. Due to corrosion and fatigue, steel bridge girders
lose much of their original design strength. According to the National Bridge Inventory
(NBI) update in 1998, over 172,000 bridges have been found to need repair
[Tavakkolizadeh, 18].
Testing by Mertz and Gillespie in 1996 and Tavakkolizadeh and Saadatmanesh in 2003
have shown steel/concrete composite beams can be strengthened significantly. The tests
utilized the CFRP as a tensile reinforcement with CFRP material adhesively bonded to
the bottom flange of the steel girder. The existing concrete superstructure is considered
to resist compressive bending loads. Tests by Mertz and Gillespie indicated average
strength increases over existing structures of 60 to 100 percent while Tavakkolizadeh and
23
Saadatmanesh tests showed improvement between 41 to 76 percent. Also, analytical
models published by Tavakkolizadeh and Saadatmanesh proved conservative,
underestimating the composite sections strength. Failure modes included CFRP
debonding and compressive crushing of concrete, with the later being dominating for the
majority of the tests [23].
Additional work by Tavakkolizadeh and Saadatmanesh also has shown fatigue strength
of a steel/concrete composite beam reinforced with CFRP can be significantly improved.
Strengthened specimens experienced useable lifespan of 2.6 to 3.4 times longer than an
unstrengthened specimen, and the total number of cycles to failure after cracking was 3.5
times longer for the strengthened specimen to the unstrengthened specimen [24].
24
Figure 2.1 Lattice/Self-Supporting Tower [Rohn, 1]
Figure 2.2 Guyed Tower [FWT, 2]
Figure 2.3 Tapered Monopole [Rohn, 1] Figure 2.4 Stepped Monopole [PiRod, 3]
25
Figure 2.5 DualPole Installation [MH, 12] Figure 2.6 DualPole Cross-section [MH, 12]
Figure 2.7 MUS Steel Band and Epoxy Application [FWT, 2]
Figure 2.8 MUS Installation [FWT, 2]
Figure 2.9 STSP Completed Installation
[ScienTel, 13] Figure 2.10 STSP Installation [ScienTel, 13]
26
Figure 2.11 WDMRS Base Installation
[Westower, 15] Figure 2.12 WDMRS Installation
Looking Up [Westower, 15]
Figure 2.13 AMUS Completed
Installation [Aero, 16] Figure 2.14 AeroSoultions Adhesive Testing
[Aero, 16]
Figure 2.15 HTSMTR Completed
Installation [Hutter, 17] Figure 2.16 HTSMTR Installation
[Hutter, 17]
27
CHAPTER 3 - EXPERIMENTAL PROGRAM
Monopole towers are one of the most appropriate structures for strengthening with CFRP.
They are the most difficult to strengthen conventionally and present the greatest need for
strengthening as a high percentage of existing monopoles are designed for a relatively
limited amount of telecommunications equipment in comparison to the current industry
demands for equipment.
3.1 Introduction
Three large scale steel monopoles were investigated to determine the effectiveness of
three types of CFRP at increasing the strength of a monopole. Due to the large size of
typical field monopoles, a reduced scale version was used for this investigation. Each
tower was tested using three specific loading cases to establish strength and stiffness
characteristics of the monopole before and after strengthening with CFRP. Each test
utilized a different CFRP strengthening system which was designed to provide nominal
increase of the elastic strength and stiffness of the monopole ranging by 20 to 40 percent.
Design considerations included fabricating a large scale monopole which mimicked the
behavior of full scale monopoles and utilizing a strengthening system which could be
used, on a larger scale, for a full sized monopole. Bond preparation and CFRP
installation techniques which can be utilized in a field application were used for installing
28
the CFRP to the monopole specimens. The applied load was designed to simulate
moments and shear forces equivalent to field conditions.
The goal of the experimental program is to determine the effectiveness of using HM and
IM CFRP in increasing the strength and stiffness in the elastic range as well as
determining the ultimate strengths and failure modes of the strengthened towers.
Included in this chapter are the material properties, design parameters, fabrication
methods, instrumentation setup and load application for the three tests. All of these
principles were designed to achieve the main goal of determining the effectiveness of the
CFRP at strengthening monopoles.
3.2 Material Characteristics
The monopole, CFRP and epoxy are fabricated using commercially available materials.
Only the CFRP contains inherently unique mechanical properties. The monopole, aside
from size, and epoxy are typically used in field applications.
3.2.1 Monopole
Three monopoles were specifically manufactured for the experimental program. Each
was identically built using the same steel grades and having the same physical
dimensions. As shown in Figure 3.1, the monopole shaft is fabricated from A572 Grade
65, 5mm thick, steel and bent at five points into two, equally sized, six-sided, semicircle
cross-sections measuring 6096 mm in length. The bend radius between flats creating the
29
semicircular cross-section measures 102 mm from the inside of the monopole shaft.
The cross-sections pieced together to create the polygonal closed shaft with the
semicircular sections welded together with a 5 mm, full length, partial penetration E80
weld. The cross-section diameter of the combined sections measures 457 mm across flats
at the largest, base end and 330 mm across flats at the smallest, free end, leading to a
tapered shape factor of 20.8 mm/m.
Figure 3.2 illustrates the baseplate and anchor bolt dimensions and orientations. The
baseplate for the monopoles was cut from 38 mm thick, A572 Grade 50 steel and cross-
sectional dimensions were 699 mm square. The baseplate was welded directly to the
twelve sided shaft using a 5 mm partial penetration E80 weld. An additional 13 mm E80
fillet weld was applied to the outside of the monopole shaft at the baseplate. Six 38 mm
diameter holes were centered on a 305 mm by 610 mm bolt square and six, A325
specification, 32 mm diameter, anchor bolts connected the entire monopole to the
reaction wall of the Constructed Facilities Laboratory (CFL) for testing.
Six coupon tests, one from every other flat, of the shaft steel were tested to determine the
yield, ultimate strengths and the elastic modulus. Figure 3.4 provides graphical
representation of the measured values. Measured results from the six coupon tests reveal
the steel yielding stress to be 414 MPa at 0.23 percent strain with ultimate stress reaching
628 MPa at 10 percent strain. Based on these results, an elastic modulus of 182 GPa was
determined. The elastic modulus calculation was determined using the six coupons with
a strain range from 0.0 to 0.23. The standard deviation of the elastic modulus was 6 GPa.
30
Design of the test monopoles is based on diameter (d) to shaft thickness (t) and flat width
(w) to shaft thickness ratios along with yield stress (Fy). These ratios are the limits at
which the monopole shaft remains stable at high stress. A monopole design exceeding
these ratios will experience local buckling or plastic deformation prior to steel yielding
whereas a monopole section designed under these ratios will continue to develop strength
beyond the initial yielding of the steel. AISC accounts for the elastic limit for circular
tubes in axial compression and flexure as shown in equation (1):
d / t ≤ 8,970 / Fy (1)
If the d/t ratio exceeds the limitation provided by equation (1), the tube cross-section will
deform prior to the steel reaching its nominal design yield stress. However, a section
with d/t ratio less than equation 1 will develop the full yield capacity of the section.
A second, more precise set of specifications is provided by the TIA/EIA-222 Standard.
These ratios are taken specifically from testing completed by D.D. Cannon, Jr. and R.A.
LeMaster [11]. Their work considered the w/t ratio influence on the development of
stress on the various polygonal cross-sections. After substantial amounts of testing, their
work provided specific equations based on w/t ratios from which the maximum yield
stress (Fcr) prior to local buckling could be calculated. The ratio for a dodecagonal cross-
section is given in equations 2, 3, and 4:
If Fy(w/t) < 240 (2)
31
• Fcr = Fy
If 240 ≤ Fy(w/t) ≤ 365 (3)
• Fcr = 1.45 Fy (1.0 – 0.00129 √ Fy (w/t))
If Fy(w/t) ≥ 365 (4)
• Monopole shaft is unstable
Units for Equations 1, 2, 3, and 4 are ksi for stress and inches for d, w, and t.
The tested monopoles have a d/t ratio less than 8,970 / Fy and a w/t ratio less than 240.
They were designed within these limits to assure the section could reach yield before
local buckling would occur.
Other factors considered in the design were the steel grade and polygonal shape. The
majority of commercially available polygonal cross-sectioned monopoles are fabricated
from high grade steel with yield stress typically ranging from 60 to 65 ksi, so identical
material was used in the design. Most existing multi-sided monopoles are twelve sided,
thus the test design simulated common industry design. The baseplate, anchor bolts and
base welds were specifically oversized to avoid premature failure of the connections.
Based on the ultimate anticipated design moment, the factor of safety of the baseplate
was a minimum of 1.33 and while a factor of safety of 1.67 was designed for the welds
and anchor bolts. Other parameters, including shaft bend radius and full length shaft
weld, follow typical fabrication standards used by PiRod on their monopole production
line.
32
3.2.2 Carbon Fiber
The CFRP strengthening the monopoles was manufactured by Mitsubishi Chemical and
the material types were supplied in two forms for the three tests. The first type is Dialead
K63712 carbon fiber and it was manufactured in sheet and strip forms. Figure 3.5 shows
the two different forms. The other material was Dialead K63312 and it was produced in
strip form only. The main difference in the two Dialead product makeups is the K63712
has a higher modulus and lower compressive rupture strain than the K63312. Mechanical
properties of the two material types are detailed in Table 3.1. Dialead K63712 (high-
modulus) sheets were used for Test I and Dialead K63712 strips were used for Test II.
Test III was completed using Dialead K63312 (low-modulus) strips.
The difference in sheet and strip form is the injection of the resin Resolution Performance
Products Epon 9310 with Ancamine 9360 curing agent. CFRP in sheet form was not
injected with the resin. They are bundles of individual carbon fibers fit together with
transverse stitching to maintain their shape and order. However, the stitching provides no
transverse strength, thus the sheets only have stiffness and strength along their length.
Due to their lateral flexibility, the sheets were shipped in rolled bundles in lengths in
excess of 30 m. The strips, being supplied with cured resin, developed into a hardened,
solid material, with consistency similar to planks used for hardwood floors. Each strip
cross-section was a solid mixture of carbon fiber and epoxy resin, creating a solid, void
free matrix. Strips are shipped in 6096 mm lengths.
33
3.2.3 Epoxy
Two epoxies were used in the experimental program. The first epoxy was utilized for the
high-modulus sheets used in Test I and is a solvent free, cold cure, adhesive named
SikaDur 330. This adhesive is manufactured by Sika Corporation and the epoxy is two
parts, one part resin and one part hardener. The two parts were mixed using a slow speed
drill with paddle mixer. Pot life for this epoxy is between 30 to 60 minutes with almost
full curing strength being developed in less than 24 hours. Published tensile strength is
30 MPa and flexural modulus is 3800 MPa, per Sika technical documentation.
The second epoxy was utilized for the high and intermediate-modulus strips used for Test
II and III, respectively, and is a cold cure, two part adhesive called Spabond 345. This
epoxy is manufactured by SP Systems is a two part, one part resin, one part hardener.
Mixing was completed using a custom mixing gun available from SP Systems to ensure
the proper two to one ratio of adhesive to hardener combination. Pot life for this epoxy is
less than 30 minutes with full curing strength being developed in 24 hours. Published
tensile strength from SP systems is approximately 38 MPa.
3.3 Design of the Test I, II and III
The purpose of the experimental program is to determine the increase of the strength and
stiffness of the monopoles strengthened with CFRP. The test specimens were designed to
increase the strength and stiffness of the monopole by 20 to 40 percent in comparison to
the strength of the unstrengthened monopole. Test I was used to examine the
34
effectiveness of the high-modulus sheets, Test II determined the effectiveness of the
high-modulus strips and Test III strengthened the monopole with intermediate-modulus
strips to evaluate their effectiveness. Similarity between the three tests was met by
designing each test to have similar CFRP axial stiffness (AE) at the base of the
monopole.
3.3.1 Test I Design – High-Modulus Sheets
Test I strengthened the monopole with high-modulus sheets. Two sets of sheets were cut
into lengths of 1370, 1980, 2590 and 3200 mm. One set of the four sheets was centered
directly on the top flat of the monopole and the other total was installed directly below on
the bottom flat, as shown in Figure 3.6. The sheets overlapped on adjoining flats as
required for the installation. The sheets were installed longitudinally along the monopole
shaft beginning at the base with 152 mm of each pressed adhered to the baseplate,
perpendicular to the monopole shaft. This 152 mm of sheet was held into place using
clip angles secured through the anchor bolt connections, as shown in Figure 3.7. Purpose
of the clip angles was to immobilize the sheets at the base to fully develop their strength
and stiffness. Each sheet was then rolled out along the shaft, beginning with the 3200
mm length sheet. The next longest sheet was then applied atop the previous one. The
1370 mm length sheet was installed last.
Additional transverse sheets, also as shown in Figure 3.6, were wrapped around the
monopole after the longitudinal sheets were installed. The purpose of the transverse
sheets was to enhance the bond between the longitudinal sheets and monopole shaft and
35
provide confinement to reduce possible local buckling of the sheets. Each sheet was
initially split in half, along its width, leaving two 165 mm width half sheets and cut into
lengths of 600 to 720 mm. The sheets were then wrapped across the top or bottom half of
monopole section. Seven transverse sheets were wrapped continuously from the base to
1224 mm length from the base. From this distance to 2444 mm from the base, the six
more transverse sheets were installed at 200 mm on center. The last two transverse
sheets were installed at 2749 and 3054 mm from the base.
The design provided the highest amount of sheets in locations of highest stress. The
highest stresses from the applied lateral load were found at the base of the tower, thus
greater amounts of sheets were installed at the base. Stress levels were lowered as
moment due to the applied lateral load was shrank near the tip of the monopole, therefore
amount of sheets installed was reduced accordingly. The monopole sections from 3050
mm from the base to the tip were strong enough to carry the applied loads, therefore sheet
installation was not extended to this section of the monopole. The transverse sheet
installation followed the same principle. With greater numbers of longitudinal sheets
installed near the base, more transverse sheet wrapping was applied. As the sheet
numbers were reduced, so were the transverse sheet wrappings.
3.3.2 Test II Design – High-Modulus Strips
Test II strengthened the monopole with high-modulus strips. Twelve strips were cut into
lengths of four 1830, two 2440, four 3050 and two 3660 mm plys and were installed as
shown in Figure 3.8. The two 3660 mm strips were installed directly on the top and
36
bottom monopole flats and were followed by the installation of the two 2440 mm strips to
the top surfaces the 3660 mm strips. The four 1830 and four 3050 mm strips were
installed at the top and bottom of the monopole on the flats next to the 3660 mm strips.
The 3050 mm strips were installed first and followed by the 1830 mm strips which were
installed on the top surface of the 3050 mm strips. All strips were centered on the width
of the flats and installation of each strip began at the base of the monopole.
Four stiffeners were added at the edges of top and bottom three flats. Figure 3.9
illustrates the dimensions and orientation of the stiffeners at the base. The stiffeners were
added to overlap the development length of the high-modulus strips within 200 mm of the
base. The stiffeners allowed full development length of the high-modulus strips,
therefore mobilizing their strength and stiffness. As with the connections and base welds,
the stiffeners were significantly oversized to minimize stresses even as applied load
reached failure of the strengthened monopole.
The design again was based on locating the largest amount of strips in locations of
highest stresses in the monopole. Unlike Test I, no transverse strengthening was used to
enhance the bond between the strips and monopole shaft. Each strip length was increased
610 mm over the lengths used for the sheets to allow for additional development length
of the strips.
37
Test III Design –Intermediate-Modulus Strips
Test III strengthened the monopole with intermediate-modulus strips. Twelve strips were
cut into lengths of four 1830, two 2440, four 3050 and two 3660 mm as shown in Figure
3.8. The width of the two 2440 and two 3660 mm strips were 76 mm as provided by the
manufacturer while the four 1830 and four 3050 mm strip widths were reduced to 51 mm.
As with Test II, the two 2440 and 3660 mm strips were installed on top and bottom flat of
the monopole with the 3660 mm strips being installed first, followed by the 2440 mm
strips. The 2440 mm strips were installed on top of the 3660 mm strips. The four 1830
and 3050 mm strips were installed the flats on each side of the top and bottom flat. The
four 3050 mm strips were installed first and the 1830 mm strips were installed to the top
surface of the 3050 mm strips.
As with Test II, four stiffeners were installed at the edges of top and bottom three flats.
Figure 3.9 shows the locations of the eight stiffeners at the base. The stiffeners were
installed to reduce the stresses in the monopole within 200 mm of the base. The
stiffeners allowed full development of the intermediate-modulus strips at the base,
therefore mobilizing their strength and stiffness. The stiffeners were significantly
oversized to ensure low stresses up to the failure load of the monopole.
The design rational was to add the largest amount of strips to the locations of highest
stresses. The purpose of reducing the width of the 1830 and 3050 mm strips was to
maintain a similar axial stiffness (AE) at the base to Test II. The thickness of the
intermediate-modulus strips was double the thickness of the high-modulus strips.
38
Although the modulus of the intermediate-modulus strips was lower, the additional
thickness would increase the AE ratio and therefore strengthened the monopole
significantly higher than the design used in Test II. To ensure similar AE ratios, the
width of the 1830 and 3050 mm strips was reduced.
3.4 Fabrication of the Specimens
Installation of the sheets and strips followed a regulated, controlled process. Specifically,
both the monopole and the CFRP material had to be prepared for installation by roughing
the surfaces of the adherends and cleaning to remove any loose residue. The sheets and
strips could then be installed following specific measures to attain a sound bond between
the adherends.
3.4.1 Monopole Surface Preparation and Cleaning
Surface preparation of the steel monopole was completed in three steps for Tests I, II and
III. Figure 3.10 illustrates each of the three steps. The initial step was to sand blast the
entire outer surface of the monopole shaft and baseplate. Thirty to fifty mesh sand was
used to give the surface an even, slightly rough textured finish, as shown in Figure 3.10 a.
The sand blasting created a slightly uneven surface which increased surface bond area as
well as disrupting the epoxy failure planes. Also, mill scale had to be removed from the
surface to enhance the bond between the epoxy and steel. Presence of the mill scale
could reduce the bond between the steel and CFRP or would simply rip off as the
39
strengthened monopole was loaded. Thirty to fifty mesh sand proved to be the suitable
size to remove the mill scale without significantly reducing cross-sectional area.
Following the sand blasting, the monopole was washed in Acetone cleaning solution.
Acetone was applied liberally over the surfaces were the CFRP was to be bonded. The
Acetone was poured onto the required surfaces and then washed away using cotton cloths
and nylon brushes, as shown in Figure 3.10 b. In all tests, the scrubbing started at the
base and pushed loose debris away from the base of the pole towards the tip of the
monopole.
The final step was air brushing with a high pressure air compressor to rid the bonding
area of any debris from the brushes or cotton rags, which is shown in Figure 3.10 c. The
pressured air was applied at an even rate over the surface and pushed the debris from the
baseplate away towards the tip of the monopole.
3.4.2 CFRP Preparation
Preparation of the sheets and strips followed a similar progression as preparing the
monopole surface. Prior to any sanding or cleaning, the CFRP sheets and strips were cut
into the sizes needed for the test. The sheets were sliced using a cutting knife while the
strips were cut using a handsaw. The sheets were then wiped clean with Acetone before
installation. No sanding was required as the carbon fiber surface had no chemical or
sealing agent requiring removal and the sheets already had a textured finish. The strips,
however, had to be lightly sanded on the surfaces which would be bonded before
40
installation. One hundred eighty grit sandpaper was used to slightly roughen the surface,
as well as remove the sealing agent applied to the strips at the manufacturing process.
These surfaces were then cleaned with water and paper towels, as shown in Figure 3.10 d
until all carbon residues had been removed. Final cleansing involved wiping Methanol
and paper towels completely across the strip’s surface to remove any remaining loose
dust or moisture from the sanding process or atmosphere.
3.4.3 Installation
Two different approaches were used for the sheets and strips. For the sheets, the epoxy
was applied directly to the monopole steel surface. Epoxy was applied thoroughly across
the steel shaft of the pole where the sheet would be installed. Care was taken to assure
the entire surface was covered with a sufficient thickness of epoxy. The sheets were then
rolled onto the monopole surface, as shown in Figure 3.11 a and 3.11 b, to ensure no
creasing or bunching of the sheets occurred. Installation started at midspan of the
monopole and worked towards the base. After a sheet was rolled though it’s full length,
pressure was applied to the sheets starting at the base, working out towards the tip, to
force the epoxy through the voids in the sheets. Purpose of starting at the base was to
maintain an even, straight positioning of the sheets as any crookedness, creasing or
bunching of the would be detrimental to the strength. The pressure was applied using
metal and plastic dry wall/painter spatulas.
Pressure was applied to the sheets until the epoxy had completed penetrated the layer,
leaving the epoxy seeping through the creases between the fibers. Additional epoxy then
41
was added to the surface, encasing the sheets in epoxy. The shorter sheets were then
installed atop the longer sheets following the same process. With all the longitudinal
sheets were applied to the surface of the monopole and baseplate, the clip angles were
bolted tightly to the ends of the sheets to complete the anchorage, as shown in Figure
3.11 c. Installation of the transverse sheets completed the retrofit, as shown in Figure
3.11 d. They were wrapped around the circumference of the strengthened monopole and
bonded with additional epoxy. The monopole was given seven days to cure before
testing was initiated.
The strips were bonded by applying the epoxy directly to their own surface. The entire
surface of the strip was covered with adhesive with sufficient thickness of approximately
1 mm. Adhesive thickness was made greatest at the center of the strip and tapered
thinner to the edges, as shown in Figure 3.12 to eliminate potential air voids in the epoxy.
The strip was then pressed onto the monopole surface, as shown in Figure 3.13, and
pressure was applied to the strips using spatulas and rollers. The base end was bonded
first with the rest of the strip following towards the tip of the monopole. The strip end
bonded at the base was pushed completely to the weld attaching the monopole to the
baseplate but not onto it. Additional adhesive was added at the edges of the sheets to seal
the crease between the steel shaft and strip. For sections of the pole where additional
strips were stacked atop another, the same procedure was followed.
42
3.5 Test Setup
Instrumentation was placed so as to gain an envelope of longitudinal strain and deflection
along the length of the monopole as well as measure the rotation at the base. With this
measured data, the strain and deflection profiles could be examined with respect to
applied load. Location of the instrumentation for the three tests mirrored each other
closely to ensure opportunity to have direct correlation of results between the load cases.
The applied loads were designed to simulate the moments and shears that a prototypical
monopole in the field would experience.
3.5.1 Instrumentation
The testing procedure of all the monopoles observed in loading had two main goals. The
first was to monitor the strains along the top and bottom of the shaft and locate the points
of highest stress over the course of the loading. The second concerned itself with
recording deflection with respect to load upon the monopole at the time. Rotation and
slip of the baseplate was monitored as well.
Pi and strain gauges were used to capture the strain profile. Both types of gauges are
illustrated in Figure 3.14. Three different lengths of pi gauges were used for the three
tests. One hundred (100) mm pi gauges were located on the monopole where large strain
was expected while 300 mm pi gauges were placed where small strain was expected to be
measured. The 200 mm pi gauges were used where moderate strain measurements were
expected to be measured. The pi gauges were installed directly onto either the steel or
43
CFRP surface using welding studs. These studs were bonded using high strength
adhesive. Figure 3.15 illustrates a typical pi gauge layout for one of the tests.
Strain gauges were used to measure strains in locations where pi gauges could not be
mounted. Like the larger pi gauges, strain gauges are extremely sensitive to the surfaces
on which they are mounted, thus capturing very accurate strain measurements. Strain
gauges were installed directly on the surface they were measuring.
Figure 3.17 illustrates typical pi and strain gauge locations for the three tests. Most of the
pi and strain gauges were installed to measure longitudinal strains. Aside from Test I,
gauges were not set up to measure transverse strain. The highest stress rates were
expected to be concentrated near the base of the monopole, thus this portion of the
monopole was more heavily instrumented. Also, the highest longitudinal stresses would
occur at the top and bottom of the monopole cross-section, so most gauges were installed
on the top and bottom flats. Each of these gauges was centered on the flat of the cross-
section.
Displacement was measured through use of potentiometers located at quarter points along
the pole shaft. Figure 3.18 and 3.19 illustrates the locations of the potentiometers. Each
potentiometer was installed beneath the monopole and connected to the bottom steel flat.
They were centered vertically so as to measure deflection of the monopole shaft.
Potentiometers measuring deflections between 150 to 760 mm were utilized.
44
Rotation of the base was also measured by potentiometers. Potentiometers with
deflection limits of less than 25 mm were used as displacement of the monopole at the
base would be very slight. Specific locations of the base potentiometers are shown in
Figure 3.16 and 3.18. One potentiometer was centered 25 mm above top flat and another
was centered 25 mm below the bottom flat, placing the instrument between the monopole
shaft and anchor bolt. The purpose of these instruments was to measure the displacement
of the baseplate relative to the mounting wall. By measuring the exact displacements
with respect to the center of the monopole, the exact rotation at the base could be
determined. Another 25 mm potentiometer was centered directly on top of the baseplate.
Its purpose was to measure displacement of the baseplate with respect to the mounting
wall in the direction of loading. As the anchor bolts were 6.4 cm smaller in diameter than
their mounting holes in the baseplate, some slippage of the section was expected.
3.5.2 Load Application
Three static load cases were completed for each test. The initial load case loaded the
monopole prior to strengthening with CFRP to 60 percent of it’s nominal design strength
and then unloaded. The purpose of this load case was to measure the monopole’s elastic
strength and stiffness prior to strengthening with CFRP. The second load case was
completed after the monopole had been strengthened with CFRP and the strengthened
monopole was loaded until the midspan deflection was the same as measured in the first
loading case. The monopole was then unloaded and inspected for damage. The purpose
of this load case was to measure the monopole’s elastic strength and stiffness after
strengthening with CFRP. The third load case loaded the monopole until failure of the
45
CFRP and buckling of the monopole shaft occurred. Once significant strength had been
measured, the monopole was unloaded and inspected for damage. The purpose of this
load case was to establish ultimate strength values and determine failure modes of the
composite systems.
The loading applied for the experimental program was designed to simulate design loads
in field structures. Monopoles in the field are designed to resist specific wind loads in
accordance with recognized industry standards. However, it was impractical to
assimilate the distributed wind loads field monopoles experience, so a single, transverse,
static load was applied at the tip of the monopole to generate the equivalent moments and
shear forces. Figure 3.19 and 3.20 illustrates the location and setup of the monopole
loading. The load was applied at 5790 mm from the base of the monopole for Test I and
II and at 5740 mm from the base of the monopole for Test III. Slight shifting of the steel
frame between the Test II and III caused the location of the applied load to be altered.
The specific location of the applied load remained constant for all loading cases of each
test. The applied load centered directly over the monopole shaft for the three tests to
eliminate any torsional loading effects.
The load was applied at a rate 2.5 mm/min. The loads were transferred from actuator to
the monopole through large, nylon straps during the first two load cases and with heavy,
steel chains for the third load case. The woven straps were chosen for the first and
second load cases because the centroid of the applied load could be accurately located.
46
The steel chains were necessary for the third load case as excessive elongation of the
nylon straps prevented achieving failure of the strengthened monopoles.
Strengthened monopoles were allowed a minimum of one week to cure before any testing
was completed. Unstrengthened monopoles were tested as soon as the instrumentation
had been installed and calibrated.
47
Figure 3.1 Monopole Shaft Dimensions and Fabrication Method
Figure 3.2 – Baseplate Dimensions
and Anchor Bolt Orientation Figure 3.3 Monopole Specimen
0
200
400
600
800
0.00 3.00 6.00 9.00 12.00 15.00 18.00
Strain (%)
Str
ess
(MP
a)
Coupon 1 Coupon 2 Coupon 3 Coupon 4 Coupon 5 Coupon 6
Figure 3.4 – Stress/Strain Coupon Test Results
48
(a)
(b)
Figure 3.5 – CFRP Sheets (a) and Strips (b - Strip Width = 25 mm)
Material Properties of CFRP Dialead K63312 – Intermediate-Modulus
Dialead K63712 – High-Modulus
Sheets
Tensile Strength (MPa) 2600 2600 Tensile Modulus (GPa) 440 640 Ultimate Elongation (%) 0.6 0.4 Density (g/cm3) 2.06 2.12 Thickness (mm) - 0.192 Width (mm) - 330
Strips
Tensile Strength (MPa) 1500 1500 Tensile Modulus (GPa) 270 370 Ultimate Tensile Elongation (%) 0.6 0.4 Compressive Strength (MPa) 520 440 Compressive Modulus (GPa) 200 330 Ultimate Elongation (%) 0.3 0.1 Thickness (mm) 3.0 1.5 Width (mm) 76.2 76.2 Fiber Volume (%) 60 60
Table 3.1 – Material Properties of Dialead K63312 and Dialead K63712
49
Figure 3.6 – Longitudinal and Transverse Sheet Layout, Test I
Figure 3.7 – Clip Angles at Base, Test I
50
Figure 3.8 – Longitudinal Strip Layout, Test II & III
Figure 3.9 – Stiffener Placement, Dimension and View, Test II & III
51
(a)
(b)
(c)
(d)
Figure 3.10 Surface Preparation Sandblasted Surface (a), Monopole Surface Cleaning with Acetone (b),
Air Brushing (c) and Strip Cleaning with Methanol (d)
(a)
(b)
(c)
(d)
Figure 3.11 Longitudinal (a, b & c) and Transverse Sheet Installation (d)
52
Figure 3.12 – Adhesive Application Figure 3.13 Strip Installation
(a)
(b) Figure 3.14 Pi Gauge (a) and Strain Gauge (b)
Figure 3.15 Pi Gauge Layout Figure 3.16 Base Potentiometer Layout
53
Figure 3.17 Typical Pi and Strain Gauge Locations
(Above Dimensions Denote Strain Gauges, Below Dimensions Denote Pi Gauges)
Figure 3.18 Potentiometer Locations Along Monopole Shaft and Baseplate
55
CHAPTER 4 - TEST RESULTS
This chapter discusses the results of the experimental program which includes testing of
the three steel monopoles strengthened with high-modulus sheets, high-modulus strips
and intermediate-modulus strips. These results include measurements of the load
deflection relationship and strain profiles. The tests are used to determine the stiffness,
ultimate flexural strength and failure modes of each system. Through comparison of the
measured data from the three tests, effectiveness of the sheets and strips as strengthening
systems is evaluated.
Each monopole was tested under the effect of three static loading conditions with specific
loading objectives. The first load case applied a load equivalent to 60 percent of the
monopole’s nominal flexural yield strength capacity before strengthening with CFRP.
The purpose of this load case was to measure the load deflection relationship and strain
profiles within the serviceability range and use these results as control data to evaluate
the effectiveness of the strengthening system. The second load case was conducted after
the monopole was strengthened with the sheets or strips. Loading was applied until the
midspan deflection of the monopole strengthened with CFRP was equal to the midspan
deflection of the unstrengthened monopole from the first load case. The monopole was
then unloaded and examined for any possible permanent deformation of the steel or any
sign of possible delaminating of the CFRP. Purpose of the second loading was to
evaluate the effect of the CFRP through measurements from the load deflection
relationship and strain profiles within the serviceability range. The third load case was
56
used to determine the ultimate flexural strength capacity and mode of failure of the
strengthened monopole. Loading was increased until failure due to rupture of the CFRP,
delaminating of the CFRP, or buckling of the steel cross-section occurred. Once
significant strength loss was measured, the applied load was removed. The purpose of
this loading was to monitor the strengthened monopole’s deflection and strain profiles
after yielding of the steel cross-section, determine the failure mode of the CFRP and
determine the ultimate flexural strength capacity of the strengthened monopole.
Using the load data measured from the three tests, effectiveness of using the sheets and
strips for increasing the flexural capacity of the monopoles can be evaluated. Through
comparison of the load deflection relationship of the strengthened monopole in
comparison to the control data, development of the additional stiffness due to the
installation of the CFRP can be determined. Finally, through study of the longitudinal
strains measured at various locations along the monopole’s length, behavior of the
monopole was evaluated.
4.1 Test I – Monopole Strengthening with High-Modulus Sheets
The monopole used for Test I was strengthened with high-modulus sheets. Measured
results from this test include load deflection relationships and strain profiles for all three
load cases. Possible shift of the neutral axis, with respect of the original centroid of the
monopole, was based from the strain measurements. Failure modes of the strengthened
system were examined at the measured failure load.
57
4.1.1 Stiffness and Strength
Measured locations of the deflection and strain of the tested monopole was measured at
quarter points along the length, as illustrated in Figure 4.1 and 4.2, for all three load
cases. Measurements of the deflection at 0.25L, 0.5L, 0.75L and L from the first load
case are shown in Figure 4.3. The figure shows linear deflection behavior with
increasing load at all quarter points. Based on the slope of the load deflection
relationship, measured stiffness values at 0.25L, 0.5L, 0.75L and L from the base were
4.33, 1.18, 0.60 and 0.38 kN/mm, respectively. The measured load corresponding to 60
percent of the yield strength was 32 kN. Measured deflection for the second load case
after strengthening with high-modulus sheets is also shown in Figure 4.3. The measured
deflections of the strengthened monopole depict linear deflection as the load increased at
all the quarter. Measured stiffness values for the strengthened monopole were 4.93, 1.48,
0.72 and 0.44 kN/mm at the 0.25L, 0.5L, 0.75L and L locations, respectively. The
maximum applied load during the second loading was 41 kN, which produced midspan
deflection equal to the midspan deflection measured for the unstrengthened monopole.
Comparison of the measured stiffness from the first two load cases reflect stiffness
increases of 13, 25, 20 and 17 percent at 0.25L, 0.5L, 0.75L and L locations, respectively.
Longitudinal strain was measured at several locations on the monopole as shown in
Figure 4.2. The measured results from the first and second load cases are shown in
Figures 4.4 through 4.8. Also, a profile of the strain measured along the length of the
monopole at an applied load of 32 kN for load cases I and II are shown in Figure 4.9.
The sign convention for the measured strain in Figures 4.4 though 4.9 is negative for
58
compressive strain and positive for tensile strain. Strain measured from the base to 3000
mm, which is the strengthened length of the monopole, showed an average reduction of
20 percent due to the use of the strengthening system. For the remaining unstrengthened
length of the monopole, the strains for both loading cases were similar in magnitudes.
All strains exhibited linear behavior after load greater than 20 kN was applied to the
monopole.
Location of the neutral axis, based on the measured strain profile shown in Figure 4.9,
reflected insignificant shift from the center of the cross-section for the monopole for the
first and second load cases. The compressive and tensile strains for each load case were
largely equivalent to each other in magnitude and the maximum shift at any location
along the shaft was less than 30 mm which is less than 6 percent of the diameter of the
monopole at the base. The slight measured shifts was towards the tension side of the
monopole. This behavior could reflect slightly greater efficiency of the strengthening
system in tension zones in comparison to the compression zones.
The measured deflections from the third load case are shown in Figure 4.10. At the
quarter points along the monopole prior to loading exceeding 45 kN, the measured
stiffness were 4.92, 1.51, 0.72 and 0.45 kN/mm at 0.25L, 0.5L, 0.75L and L locations,
respectively. Measurements of the deflections after loading exceeded 45 kN reflected
non-linear behavior with gradual measured stiffness loss. Minor localized debonding of
the sheets near the base occurred at 75 kN, as shown in Figure 4.11, resulting in a slight
drop of the strength, which can be noted in the deflection results at all quarter points
59
shown in Figure 4.10. Additional slight, localized crack propagation of the debonding
near the base occurred at 86 kN, which again resulted in a slight drop in strength as
shown in Figure 4.10. Loading was stopped at 91.5 kN, due to limitation of the stroke in
the actuator caused by excessive elongation of the nylon loading straps, which are shown
in Figure 4.12. At this stage, the monopole was unloaded and the nylon straps were
replaced with steel chains, shown in Figure 4.13. The monopole was then loaded up to
failure.
Deflection measurements for the third load case with chains, illustrated in Figure 4.14,
shows linear deflection behavior with increasing applied load up to the previously
maximum load of 91.5 kN using the nylon straps. After passing this load, the deflection
behavior becomes non-linear and is accompanied with significant loss in stiffness.
Complete rupture of the sheets in tension occurred at 200 mm from the base at a load of
95 kN. Rupture of the sheets coincided with buckling of the monopole 200 mm from the
base. Figure 4.15 illustrates the rupture of the sheets and buckling of the monopole.
Significant strength loss was measured after the rupture of the sheets and buckling of the
monopole shaft. The ultimate strength capacity of the monopole was 95 kN.
Examination of the measured compressive strain shown in Figure 4.16 for the third load
case with nylon straps indicate non-linear behavior at 200, 460 and 1520 mm from the
base as the applied loading exceeded 45 kN. Measurements of the tension strains at 200
and 460 mm were linear up to an applied load of 75 kN, but are not representative of the
strain behavior after this load. The pi gauges were affected by the localized debonding
60
which caused movement and loss of calibration. The neutral axis at these locations
shifted towards the tension strains by a magnitude of approximately 10 percent the
diameter of the cross-section. Strain values measured at 2900, 3250 and 4570 mm,
shown in Figure 4.17, reflect linear behavior and neutral axis shift was less than 5 percent
the diameter of the cross-section at all locations measured.
Similar to the measured deflection behavior, the measured strain values from the third
load case with steel chains show linear behavior up to 90 kN. Graphed illustration of the
measured results is shown in Figure 4.18 and 4.19. The compression strains rapidly
increase after initiation of the local buckling of the monopole near the base. Shift of the
neutral axis beyond 90 kN was significant towards the tension strains. The shift was
approximately 103 mm from the center of the monopole which is 23 percent of the
monopole’s diameter, based on strain measurements taken at 460 mm from the base. The
shift was less predominant as measurements were taken away from the base. As a result
of the local buckling which began at the end of third load case with straps, the pi gauge
expecting to measure tension strain at 200 mm from the base did not work.
Figure 4.20 shows the measured strain profile along the length of the monopole at an
applied load of 95 kN along with the strain profiles from the first and second load cases.
This graph reveals the significant shift of the neutral axis towards the tension strains
within the strengthened section of the monopole. The graph also shows the neutral axis
return to its center location for the unstrengthened length of the monopole. Compression
61
strains are significantly higher near the base as the monopole tension fiber ruptured and
the monopole buckled at the 95 kN load.
Transverse strain was measured at locations of 610 and 1210 mm from the base of the
monopole on the 30, 60 and 90 degree flats, as shown in Figure 4.2. Measurements were
recorded for three load cases and results of the third load case with chains are shown in
Figure 4.21. Ultimately, the transverse strains measured were insignificant, with
maximum strain at the failure load of 95 kN reaching 0.1 percent. The additional strain
shown in the graphs exceeding 0.1 percent was measured as the monopole was being
unloaded after failure.
4.1.2 Discussion of Test Results
The results suggest several noticeable characteristics of strengthened monopole behavior.
First, there will be slight non-linear behavior as the loading initially applied to the
monopole. The initial non-linear behavior noted in the second load case was likely due to
slackness of the sheets at the base prior to loading. Once the slack was removed, the
fibers in the sheet were aligned to provide linear resistance to the applied load.
The linear behavior measured in the first and second load cases infers the bond is
transferring force into the sheets. Any shift, slip, or delaminating of the bond would
create non-linear deflections as load was applied. Not withstanding the slight non-linear
behavior at the early stages of loading, the measured behavior is linear up to yielding of
the steel. Aside from the observed localized delaminating of the sheets near the base
62
observed during the third load case with nylon straps, the adhesive bond was found to be
in excellent condition throughout the sheets even after the monopole reached it’s failure
load.
Examination of the strain behavior and corresponding shift of the neutral axis indicates
the contribution of the high-modulus sheets in tension and compression is almost equal
up to yielding of the steel. The slight shift in the neutral axis prior to the steel yielding
indicates also that the high-modulus sheets have an approximately equal tension and
compression modulus. Significant neutral axis shift is noted when the compressive
strains exceed the yield strain of the steel, so this shift is likely due to the drop in the steel
modulus as the steel yielded or reduction in strength of the high-modulus sheets in
compression. As no rupture or delaminating of the sheets in compression was observed,
the sheet compression modulus likely continued to provide stiffness up failure of the
monopole. However, without further testing of the sheet/adhesive matrix in compression
beyond strain of 0.2 percent, possible reduction in the compression modulus cannot be
confirmed.
Measurements of the transverse strains measured during the third loading with the steel
chains suggest the monopole cross-section maintained its shape up to buckling and
rupture of the sheets. The transverse strains proved to be insignificant as compared to the
longitudinal strains at the same locations on the monopole. However, their magnitudes
almost doubled after the monopole buckled. This data concludes the cross-sectional
shape was unchanged up to buckling of the monopole and rupture of the sheets. The
63
transverse sheets, while providing no additional strength or stiffness, may have provided
resistance to ovalizing of the monopole shaft.
The sudden buckling occurring simultaneously with the rupture of the sheets indicates the
sheets were providing significant strength capacity to the monopole up to failure. The
measured strains in Figure 4.16 and 4.18 show significant non-linear compression strain
increases as the monopole approached its failure load. This indicates a softening of the
steel in compression near the base and the impending buckling of the shaft. However, as
the transverse strains results show, the monopole cross-section remained in place. Thus,
rupture of the sheets caused redistribution of the load, inducing the buckling of the shaft.
4.2 Test II – Monopole Strengthened with High-Modulus Strips
The monopole used in Test II was strengthening with the high-modulus strips. For this
test, stiffeners were welded to the base of the monopole prior to the first load case. Load
deflection relationships, strain profiles and neutral axis locations were measured from the
three load cases for Test II. These results are presented both graphically and through
discussion. Determination of the ultimate flexural strength capacity and failure modes of
the strengthening system are also discussed.
4.2.1 Stiffness and Strength
Deflection was measured at the quarter points of the monopole all three load cases.
These locations are illustrated in Figure 4.1. The measured deflection from the first load
64
case is shown in Figure 4.22. The unstrengthened monopole was loaded to 32 kN, which
is 60 percent of it’s nominal flexural yield strength, and then unloaded. Linear deflection
was measured at all quarter points. Based on the load deflection slope, stiffness values of
4.72, 1.33, 0.65 and 0.40 kN/mm at 0.25L, 0.5L, 0.75L and L, respectively, were
measured for the unstrengthened monopole. The deflection results from the second load
case are also shown in Figure 4.22 with linear deflection being measured at all quarter
points. The measured stiffness at 0.25L, 0.5L, 0.75L and L were 7.07, 1.91, 0.91 and
0.57 kN/mm, respectively, based on the load deflection slope. The second load case was
stopped at 43 kN and then unloaded as midspan deflection of the strengthened monopole
matched the midspan deflection measured from the unstrengthened monopole.
Comparison of the measured stiffness values from the first and second load cases
indicates stiffness increases of 50, 43, 40 and 41 percent at 0.25L, 0.5L, 0.75L and L,
respectively.
The locations of the measured longitudinal strain are illustrated in Figure 4.3 and the
graphed results for the first and second load cases are shown in Figures 4.23 through
4.27. A profile of the strain measured along the length of the monopole at an applied
load of 32 kN for the first and second load cases is shown in Figure 4.28. Sign
convention of compressive strain is shown as negative and tension strain is shown as
positive. The measured strains from both the unstrengthened and strengthened monopole
show largely linear behavior at all locations. Strain measured from the unstrengthened
monopole at 1520 mm through 4570 mm from the base initially exhibited slightly non-
linear behavior, but the strain measurements assumed linear behavior after the applied
65
load exceeded 20 kN. The tension strains measured at 460 mm from the base for the
strengthened monopole are unreliable after applied load exceeded 40 kN as calibration
was lost on the pi gauge at this point.
The strain profile for the first and second load cases, shown in Figure 4.28, shows the
neutral axis shift from the centroid of the monopole to be insignificant. Neutral axis
shifts for the unstrengthened and strengthened monopole were less than 5 percent the
diameter of the monopole along all sections of the shaft. The neutral axis shifts were
concentrated towards the compression strains for the unstrengthened monopole and
towards the tension strains for the strengthened monopole. The measured strains taken
from the second load case from the base to 3660 mm were found to be an average of 31
percent less as compared to the measured strains from the first load case. For strains
measured from 3660 mm to the tip of the monopole for the first and second load cases,
strain reductions due to the strengthening system were found to be negligible.
Figure 4.29 illustrates the strain measurements taken at 230 mm from the base during the
second load case. At this location, strain was measured at the center of the strip and on
the adjoining steel surface directly to the side of the strip. The results reveal the steel
strains to be smaller in compression and higher in tension as compared to the strip strains.
Steel tension strains were 30 percent larger in magnitude than steel compression strains
and the steel tension strains became slightly non-linear as applied load exceeded 38 kN.
Strip compression and tension strain magnitudes were equivalent to each other and
exhibited linear behavior throughout the loading.
66
Deflection results from the third load case are shown in Figure 4.33. The measured
stiffness based on the load deflection slope at an applied load of 48 kN was 6.58, 1.86,
0.88 and 0.53 kN/mm at 0.25L, 0.5L, 0.75L and L, respectively. At 48 kN, complete
crushing of the top, 2440 and 3660 mm strips in compression occurred 230 mm from the
base, or at the ends of the stiffeners. All other strips on the top and bottom of the
monopole remained intact. The crushing with respect to the load deflection profile is
shown in Figure 4.31 and illustration of the crushing is shown in Figure 4.32. Specific
installation location of the top, 2440 and 3660 mm strips is shown in Figure 3.7.
Additional crushing of the top 1830 and 3050 mm strips was observed periodically at
various locations as applied load increased from 48 kN to 63 kN. At 63 kN, all of the top
strips had completely crushed in compression at the ends of the stiffeners. The monopole
continued to exhibit linear deflection during this loading period. However, stiffness
values measured from loading of 48 kN to 63 kN dropped to 3.99, 1.32, 0.64 and 0.40
kN/mm at 0.25L, 0.5L, 0.75L and L, respectively. This equated to a loss of stiffness of
roughly 30 percent at each location as compared to the stiffness measured prior to initial
compressive crushing.
The bottom, 1830 and 2440 mm strips delaminated from the monopole shaft at 63 kN.
Strength capacity of the monopole, shown in Figure 4.30 and 4.31, dropped to 59 kN
following the delaminating of the bottom strips. After the drop in strength, load
increased to 61 kN when the bottom, 3050 and 3660 mm strips ruptured and delaminated
simultaneously at the end of the stiffeners. Illustration of the tensile rupture and
67
delaminating of the strips is shown in Figure 4.33. Loading continued with non-linear
deflection measured at all locations. Buckling of the monopole shaft at the ends of the
stiffeners, shown in Figure 4.34, started at 77 kN and continued until loading was stopped
and the monopole was unloaded. The ultimate load capacity of the monopole was 79 kN.
The buckling of the monopole developed over a short period of time and was very ductile
as gradual loss in stiffness was measured.
Measured strains from the third loading case are shown in Figures 4.36 through 4.38.
Most strains measured within the strengthened section of the monopole were inaccurate
after rupture or delaminating of the strips. The pi gauge at 150 mm mounted to the
bottom of the monopole also measured strain incorrectly, so measurements from the
location were not included in the results. The figures show measured strain results up to
the loss of calibration the gauges. The measurements taken from locations of 2900 mm
and greater from the base remained accurate after delaminating and rupture of the strips.
Measured strain behavior was similar to the behavior observed during the second load
case as all strain remained mostly linear up to the rupture of the strips at 48 kN. Initial
compressive crushing of the strips at 48 kN was measured at a strain of approximately
0.15 percent. Tensile rupture of the strips at 63 kN was measured at a strain of
approximately 0.18 percent. Neutral axis shifts were also insignificant, moving less 5
percent the diameter of the monopole away from the monopole centroid at any location.
All measured locations noted neutral axis shifts towards the tension strains.
68
A strain profile from the third load case is shown in Figure 4.39, along with strain
profiles from the first and second load cases. The results illustrate a significant strain
reduction and fairly equal strain magnitude in locations where the monopole is
strengthened. Unstrengthened locations show linearly proportioned strain as
measurements are taken away from the tip of the monopole.
Post failure inspection found the strips bonded between the stiffeners experienced no
delaminating or rupture, except at the end of the stiffeners. Figure 4.35 shows the post
failure remains of the strips between the stiffeners. The top strips which experienced
compressive crushing did not debond until the monopole shaft buckled. Figure 4.33 also
illustrates the surface of the epoxy bond after delaminating from the strips. Post failure
examination of the delaminated strips revealed significant air voids throughout the
adhesive layer. Air voids were also found in the adhesive layer bonding the strip layers
together. In several locations, the air voids within the epoxy bond encompassed over 50
percent of the width of the adhesive cross-section.
4.2.2 Discussion of Test Results
The most important observation of Test II was the rupture of the strips in tension at low
strain. The strip stiffness did not change throughout the loading as linear deflection prior
to rupture was measured at all locations. However, the rupture at a measured strain of
0.15 to 0.18 percent is significantly less than the expected value. Earlier tests completed
by the manufacturer on the strip laminate found rupture at strains of approximately 0.15
percent in compression and 0.4 percent in tension. The low compressive strain was
69
anticipated, but much greater ductility was expected of the strips in tension. The low
rupture strain was likely caused by the poor bond development between the adhered
surfaces. Air voids found in the post failure examination of the strips likely caused stress
concentrations during loading within the strip, creating non-uniform stress distribution.
Because the strips are an orthotropic material, load is not evenly distributed throughout
the strip if bonding irregularities exist. The stress concentrations would eventually
overload and rupture, causing failure to the entire strip as the remaining cross-section
could not carry the additional load.
Bond preparation is extremely critical for this strengthening system. Post failure
examination of the delaminated strips revealed the epoxy remained bonded to both the
steel and strips. Despite the air voids within the adhesive, a rigid bond was established
between the epoxy and adhered surface. Thus, the roughing and cleaning methods used
for bond preparation to the steel and strips were satisfactory.
The multiple air voids found in epoxy likely lies in the mechanical properties of the
adhesive. The pot life of the adhesive was 30 minutes. This amount of time for installing
each strip proved to be too short to apply the appropriate amount of pressure to eliminate
air voids in the epoxy. As the viscosity of the adhesive was low, significant pressure had
to be applied to ensure a consistent, void free bond. Due to the rapid curing and low
viscosity of the epoxy, needed pressure could not be applied to the strips.
70
The stiffeners installed at the base provided excellent immobilization of the shaft section
for developing the strip strength. The strength and stiffness of each strip was completely
developed within the 200 mm length from the base, minimizing stress in the bond at the
ends of the strips. The initial rupture of the strips in both tension and compression
occurred at the stiffener ends, which indicates the highest point of stress along the strip
length. Delaminating and rupture of the strips was not found between the stiffeners, even
after buckling of the monopole occurred.
The epoxy used most likely is adequate to develop the full strength of the strips, both in
compression and tension if a consistent, void free adhesive bond is developed. In
locations where a solid layer of adhesive had cured, delaminating occurred at
significantly higher loading. Also, the strips between stiffeners remained rigidly attached
throughout the final loading, so adequate adhesive bond strength was developed in
specific locations. Assuming a consistent, solid bond had cured on the entire surface of
the strips, material failure and ultimately buckling of the monopole shaft would have
likely been the collapse mode of the system.
4.3 Test III – Monopole Strengthened with Intermediate-Modulus Strips
The monopole used for Test III was strengthened with intermediate-modulus strips.
Stiffeners were welded at the base of the monopole prior to the first load case. Strain and
load deflection relationships were measured for all three load conditions during this test.
Neutral axis locations are calculated based on the measured strains. Measured results
71
from this test are presented graphically and through discussion. The ultimate flexural
strength capacity and failure modes of the monopole strengthened with intermediate-
modulus strips are also determined.
4.3.1 Stiffness and Strength
Deflection was measured for the three load cases at quarter point locations along the
monopole’s shaft which are illustrated in Figure 4.1. Measurements of the deflection
taken during the first load case are shown in Figure 4.40. The applied load reached 33
kN, or approximately 60 percent of the monopole’s yield strength capacity with linear
deflection being measured throughout the loading. The measured load deflection slope
revealed stiffness values of 3.83, 1.29, 0.63 and 0.39 kN/mm at 0.25L, 0.5L, 0.75L and L
from the base, respectively. Measured deflection from the second load case after
strengthening with low-modulus sheets is also shown in Figure 4.40. The measured
deflections of the strengthened monopole depict linear deflection with increasing load to
41 kN, where loading was stopped as midspan deflection reached approximately the same
magnitude as measured from the first load case. Measured stiffness values based on the
load deflection slope from the strengthened monopole were 7.18, 2.11, 0.93 and 0.57
kN/mm at the 0.25L, 0.5L, 0.75L and L locations, respectively. Comparison of the
measured stiffness results indicates the strengthened monopole’s stiffness was 86, 64, 48
and 44 percent at 0.25L, 0.5L, 0.75L and L locations, respectively.
Figure 4.2 illustrates the locations of measured longitudinal strain along the length of the
monopole. Figures 4.41 through 4.45 show the measured longitudinal strain results from
72
the monopole before and after it was strengthened with the intermediate-modulus strips.
Figure 4.46 illustrates a profile of the longitudinal strain measured along the length of the
monopole at an applied load of 32 kN from the first and second load cases. Figure 4.47
shows the results of strain measured at 230 and 460 mm on the strips and monopole steel
shaft adjoining the strip. Sign convention of the measured strains shows compressive
strain as negative and tensile strain as positive. Comparison of the strains measured from
the monopole shows the strains dropped roughly 52 percent from the base to 3660 mm
from the base after strengthening with the intermediate-modulus strips. For the
remaining 3660 mm to tip section of the monopole, the strains for both load cases were
almost identical. Strain measured at all locations from the first and second load cases
exhibited linear behavior throughout the loading. Slight deformation was measured after
the monopole was unloaded at 150 mm from the base, as shown in Figure 4.41. This is
likely due to residual stress from the welding of the stiffeners to the base of the monopole
and is considered an abnormality.
The neutral axis location based on the measured strain profile shown in Figure 4.46
reflected almost no shift from the center of the cross-section for the monopole for the first
and second load cases. The measured compressive and tensile strains from each load
case were approximately equivalent in magnitude. The maximum measured shift along
the shaft was less than 30 mm or 6 percent the diameter of the monopole at the base and
most measured shifts were less 3 percent. The slight neutral axis shifts that did occur
were towards the tension strains for both the unstrengthened and strengthened
monopoles.
73
Figure 4.47 shows strain measured from the monopole steel and strip surface. Instrument
orientation of the measured strains at 230 mm is shown in Figure 4.48. Instrumentation
at 460 mm was mounted identically as shown in Figure 4.48. The tensile strains have
magnitude equivalent to the compressive strains measured on the monopole steel and
strips surfaces. However, the steel strains are not specifically equivalent to the strip
strains measured at same distances from the base. Strains measured 230 mm from the
base revealed the steel strains to be 18 percent larger when compared to the strip strains.
Strains taken 460 mm from the base revealed strains measured from the strips to be 26
percent larger than strain measured from the steel.
The measured deflections from the third load case are illustrated in three figures. Figure
4.49 shows the net deflection measured to 55 kN and reveals linear deflection with
increasing load at all quarter points. Based on the load deflection slope, the measured
stiffness at 0.25L, 0.5L, 0.75L and L is 7.02, 2.06, 0.93 and 0.56 kN/mm, respectively.
Figure 4.50 shows the measured gross deflection from the entire loading. The first graph
is shown without measured deflection from the entire loading because instrumentation
measuring rotation and deflection at the base was lost at 55 kN. At this load, almost all
of the strips installed underneath the monopole completely delaminated. Figure 4.52
illustrates the delaminating of the strips from the bottom of the monopole. One bottom
3050 mm strip and all of the top strips remained intact and undamaged. All of the
instruments attached to the bottom strips were lost and many other instruments, including
the potentiometers measuring rotation at the base lost calibration with the delaminating of
the bottom strips.
74
Figure 4.51 illustrates the failure modes of the strips with respect to the load deflection
relationship measured at the tip of the monopole. After the initial delaminating of most
of the bottom strips, strength capacity of the monopole dropped to 50 kN. Loading
continued and measured deflection exhibited slightly non-linear behavior from loading of
50 to 85 kN. At 85, almost all of top strips fully delaminated and crushed simultaneously
at the ends of the stiffeners, which is shown in Figure 4.53. One top, 3050 mm strip
remained intact. Strength capacity of the monopole dropped from 85 to 78 kN. Applied
loading increased to 80 kN when the remaining top, 3050 mm strip fully delaminated and
crushed simultaneously at the end of the stiffeners. After a minor drop in strength,
loading continued to 83 kN when the final remaining bottom, 3050 strip delaminated and
ruptured at the end of the stiffeners. Strength capacity dropped to 79 kN with this final
loss of the strips and the monopole finally buckled near the base at 83 kN. Illustration of
the buckling is shown in Figure 4.54. Loading was stopped once strength capacity of the
buckled monopole began to drop significantly. The ultimate strength capacity of the
monopole was 85 kN.
The measured strains taken from the third load case are shown in Figures 4.56 through
4.59. Results are shown up to the loss of instrumentation. All gauges measuring strain
on the bottom of the monopole were lost after the strips delaminated at 55 kN.
Immediate increased strain was measured at all locations were the gauges remained intact
directly following the strip debonding. Aside from strain measured at 80, 150 and 460
mm and the offsets at 55 kN, linear behavior was measured at all locations throughout the
third load case. The non-linear behavior was noted at 80 and 150 mm is likely due to
75
stress concentrations within the strips inside the stiffeners and the behavior measured at
460 mm is due to the tower steel yielding. The stress concentrations are also the likely
cause of accelerated strain measured at 150 mm. Crushing strain of the strips in
compression was measured between 0.2 and 0.25 percent.
The strain profile of the third load case is shown in Figure 4.60, along with the strain
profiles from the first and second load cases. The results show slight neutral axis shift
towards the tension strains from the base to 1500 mm, but no neutral axis shift from 1500
to the tip of the monopole. The profile also shows the stress concentrations within the
section of the monopole were the stiffeners were installed. Strain magnitudes were
highest near the base and remained constant from 1500 to 3000 mm. From 3000 mm to
the location of the applied load, strain measurements were reduced at a linear rate with no
longitudinal strain being measured at the tip.
Similar to Test II, post failure inspection found the strips to be largely intact between the
stiffeners. Rupture and delaminating of the strips was found at the ends of the stiffeners
and no rupture or delaminating was found between the end of the stiffeners and the base
of the monopole. Also as found in Test II, inspection of the delaminated strips found
significant air voids in the adhesive bond. Figure 4.55 shows the adhesive bond
irregularities on the strip surface. The poor bonding was found on the strip to steel
adhesions and the strip to strip adhesions.
76
4.3.1 Discussion of Test Results
The most important observation of Test III is the crushing of the strips in compression at
strain values of 0.2 to 0.25 percent. Testing completed by the manufacturer listed
ultimate compressive rupture strain as approximately 0.25, thus almost the full strength of
the strip was achieved. The load deflection relationship shown in Figure 4.50 also
reveals significant strength loss was observed after the strips crushed in compression and
delaminated from the monopole surface. The strength loss was observed during
advanced stages of loading and with minor contribution from the bottom. Based on these
observations, significant strength and stiffness was provided by the strips in compression.
The non-uniform strain distribution measured around the stiffeners is likely due to poor
quality of the welds used for the stiffeners. Different welding machines were used for
welding the stiffeners to the test monopole specimens used for Test II and III. The
welding machines used for welding the stiffeners Test III did not provide consistent spark
or heat when applying the weld. This created an inconsistent weld line along each side of
the stiffeners which likely altered the stress distribution through the monopole shaft near
the base. The strips were bonded to the shaft surface, thus they would experience the
non-uniform stress distribution. Also, due to the inconsistent heat applied to the steel,
additional heat stress likely was added in the steel near the stiffeners.
The strain differences, shown in Figure 4.45, between the steel and strips are likely due to
boundary conditions and residual stresses. The strain, measured at 230 mm from the base
and shown in Figure 4.46, was taken at the center of the flat and directly in front of the
77
stiffener for the strip and steel, respectively. The presence of the stiffener likely acted as
a stress enhancer as the heat stress added from the weld concentrated additional stress.
Due to this, the strain measured in the steel was higher than measured in the strip. With
properly applied welds, the stiffener actually would have an opposite effect of lowering
stress as the strength of the stiffeners would have influenced the monopole shaft strength
positively near its boundaries. This behavior was noted in Test II, which had
significantly higher quality welds. Measurements taken from the third load case and
shown in Figure 4.59 reveal the strain measured from the strips eventually have larger
magnitude than strain measured form steel, thus eventually the strength of the stiffeners
has the desired effect of lowering strain in the steel around the base.
The steel strain measured at 460 mm from the base was located on the bend linking the
flats. The monopole shaft is bent without heat, thus some residual stress and strain
hardening was imparted during the manufacturing process. The strain hardening likely
caused greater strain toward the center of the flats as opposed to the stiffer edges, thus
higher strain was found on the strips.
As with Test II, air voids in the adhesive bond were the main problem behind the strips
developing there full strength in tension. In several locations, the less than 50 percent of
the width of the strip was adhered to the steel with adhesive. The low viscosity and short
pot life of the adhesive likely resulted in the irregular adhesive bond. Significant
pressure needs to be applied to ensure the air voids are removed the adhesive bond due to
the low viscosity of the epoxy. As curing of the epoxy was very fast, installation of the
78
strips had to be completed quickly, thus the needed pressure could not be applied to the
strip surfaces.
Also as with Test II, the stiffeners provided excellent support for developing the strips at
the base. Rupture of the strips in tension and compression all occurred just within or
outside the edge of the stiffeners, thus the strips within the stiffeners were fully
developed. The epoxy used for Test III is also likely adequate, as it developed the strip
strengths within the stiffeners. However, a consistent epoxy surface must be applied and
cured to the adhered surfaces before the adhesive bond can be considered reliable.
79
Figure 4.1 Measured Deflection Locations for Tests I, II and III
Figure 4.2 Strain Measurement Locations For Tests I, II and III
Typical Measured Strain Locations – Above and Beneath Monopole
80
0
10
20
30
40
50
0.0 30.0 60.0 90.0 120.0
Displacement (mm)
Lo
ad
(k
N)
Before Strengthening After Strengthening
0.25L 0.5L 0.75L L
Figure 4.3 Net Displacement at 0.25L, 0.5L, 0.75L and L
Test I – First and Second Load Cases
0
10
20
30
40
50
-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15
Strain (%)
Lo
ad
(k
N)
Unstrengthened Strengthened
150 mm
200 mm
Figure 4.4 Longitudinal Strain at 150 – 200 mm from Base
Test I – First and Second Load Cases
81
0
10
20
30
40
50
-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15
Strain (%)
Lo
ad
(k
N)
Unstrengthened Strengthened
Figure 4.5 Longitudinal Strain at 460 mm from Base Test I – First and Second Load Cases
0
10
20
30
40
50
-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15
Strain (%)
Lo
ad
(k
N)
Unstrengthened Strengthened
Figure 4.6 Longitudinal Strain at 1520 mm from Base Test I – First and Second Load Cases
82
0
10
20
30
40
50
-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15
Strain (%)
Lo
ad
(k
N)
Unstrengthened Strengthened
2900 mm
3050 mm
3250 mm
Figure 4.7 Longitudinal Strain at 2900, 3050 and 3250 mm from Base
Test I – First and Second Load Cases
0
10
20
30
40
50
-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15
Strain (%)
Lo
ad
(k
N)
Unstrengthened Strengthened
Figure 4.8 Longitudinal Strain at 4570 mm from Base Test I – First and Second Load Cases
83
-0.15
-0.10
-0.05
0.00
0.05
0.10
0.15
0 1500 3000 4500 6000
Length from Base (mm)
Str
ain
(%
) Unstrengthened
Strengthened
Figure 4.9 Longitudinal Strain Profile at 32 kN (Base = 0 mm) Test I – First and Second Load Cases
0
25
50
75
100
0 50 100 150 200 250 300 350
Displacement (mm)
Lo
ad
(k
N)
0.25L 0.5L 0.75L L
Figure 4.10 Net Displacement at 0.25L, 0.5L, 0.75L and L Test I – Third Load Case with Nylon Straps
84
Figure 4.11 Minor Localized Debonding of Sheets at 75 kN
Test I – Third Load Case with Nylon Straps
Figure 4.12 Monopole Load Application
with Nylon Straps Figure 4.13 Monopole Load Application
with Steel Chains
85
0
25
50
75
100
0 50 100 150 200 250 300 350
Displacement (mm)
Lo
ad
(k
N)
0.25L 0.5L 0.75L L
Figure 4.14 Net Displacement at 0.25L, 0.5L, 0.75L and L
Test I – Third Load Case with Steel Chains
(a)
(b)
Figure 4.15 Buckling of Monopole Shaft (a) and Rupture of Sheets (b) Test I – Third Load Case with Steel Chains
86
0
25
50
75
100
-1.00 -0.75 -0.50 -0.25 0.00 0.25 0.50 0.75 1.00
Strain (%)
Lo
ad
(k
N)
200 mm
460 mm
1520 mm
4.16 Longitudinal Strain at 200, 460 and 1520 mm from Base
Test I – Third Load Case with Nylon Straps
0
25
50
75
100
-1.00 -0.75 -0.50 -0.25 0.00 0.25 0.50 0.75 1.00
Strain (%)
Lo
ad
(k
N)
2900 mm
3250 mm
4570 mm
4.17 Longitudinal Strain at 2900, 3250 and 4570 mm from Base
Test I – Third Load Case with Nylon Straps
87
0
25
50
75
100
-1.50 -1.25 -1.00 -0.75 -0.50 -0.25 0.00 0.25 0.50 0.75 1.00
Strain (%)
Lo
ad
(k
N)
200 mm
460 mm
1520 mm
4.18 Longitudinal Strain at 200, 460 and 1520 mm from Base
Test I – Third Load Case with Steel Chains
0
25
50
75
100
-1.00 -0.75 -0.50 -0.25 0.00 0.25 0.50 0.75 1.00
Strain (%)
Lo
ad
(k
N)
2900 mm
3250 mm
4570 mm
4.19 Longitudinal Strain at 2900, 3250 and 4570 mm from Base
Test I – Third Load Case with Steel Chains
88
-1.20
-0.80
-0.40
0.00
0.40
0.80
1.20
0 1500 3000 4500 6000
Length from Base (mm)
Str
ain
(%
)
32 kN - Unstrengthened
32 kN - Strengthened 95 kN - Strengthened
Figure 4.20 Longitudinal Strain Profile at 32 and 95 kN (Base = 0 mm) Test I – First, Second, and Third Load Cases
0
25
50
75
100
-0.20 -0.10 0.00 0.10 0.20
Strain (%)
Lo
ad
(k
N)
90 Degrees 60 Degrees 30 Degrees
(a)
0
25
50
75
100
-0.20 -0.10 0.00 0.10 0.20
Strain (%)
Lo
ad
(k
N)
90 Degrees 60 Degrees 30 Degrees
(b) Figure 4.21 Vertical Strains at 610 mm (a) and 1220 mm (b)
from Base of Monopole Test I – Third Load Case with Steel Chains
89
0
10
20
30
40
50
0 30 60 90 120
Displacement (mm)
Lo
ad
(k
N)
Unstrengthened Strengthened
0.25L 0.5L 0.75L L
Figure 4.22 Net Displacement at 0.25L, 0.5L, 0.75L and L
Test II – First and Second Load Cases
0
10
20
30
40
50
-0.30 -0.20 -0.10 0.00 0.10 0.20 0.30
Strain (%)
Lo
ad
(k
N)
Unstrengthened Strengthened
80 mm
150 mm
200 mm
230 mm
Figure 4.23 Longitudinal Strains at 80, 150, 200 and 230 mm from Base
Test II – First and Second Load Cases (Strain Measurements as Monopole is Unloaded Not Included)
90
0
10
20
30
40
50
-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15
Strain (%)
Lo
ad
(k
N)
Unstrengthened Strengthened
Figure 4.24 Longitudinal Strains at 460 mm from Base Test II – First and Second Load Cases
0
10
20
30
40
50
-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15
Strain (%)
Lo
ad
(k
N)
Unstrengthened Strengthened
Figure 4.25 Longitudinal Strains at 1520 mm from Base Test II – First and Second Load Cases
91
0
10
20
30
40
50
-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15
Strain (%)
Lo
ad
(k
N)
Unstrengthened Strengthened
2900 mm
3050 mm
3250 mm
Figure 4.26 Longitudinal Strains at 2900, 3050 and 3250 mm from Base
Test II – First and Second Load Cases
0
10
20
30
40
50
-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15
Strain (%)
Lo
ad
(k
N)
Unstrengthened Strengthened
Figure 4.27 Longitudinal Strains at 4570 mm from Base Test II – First and Second Load Cases
92
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0 1500 3000 4500 6000
Distance from Base (mm)
Str
ain
(%
)
Unstrengthened
Strengthened
Figure 4.28 Longitudinal Strain Profile at 32 kN (Base = 0 mm)
Test II – First and Second Load Cases
0
10
20
30
40
50
-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15
Strain (%)
Lo
ad
(k
N)
Steel CFRP
Figure 4.29 Longitudinal Strains at 230 mm from Base Test II – First and Second Load Cases
93
0
20
40
60
80
100
0 100 200 300 400
Displacement (mm)
Lo
ad
(k
N)
0.25L 0.5L 0.75L L
Figure 4.30 Net Displacement at 0.25L, 0.5L, 0.75L and L Test II – Third Load Case
0
20
40
60
80
100
0 100 200 300 400
Displacement (mm)
Lo
ad
(k
N)
Compressive Crushing of Top Strips
Delaminating of Bottom Strips
Buckling of Monopole
Figure 4.31 Failure Modes at Load per Net Displacement Measured at L Test II – Third Load Case
94
Figure 4.32 Compressive Crushing
of Top Strip Test II – Third Load Case
Figure 4.33 Delaminating of Bottom Strips Note: Black Pockets are Air Voids in Bond
Test II – Third Load Case
Figure 4.34 Buckling of Monopole
Test II – Third Load Case
Figure 4.35 Ruptured Strip Remains at Stiffeners
Test II – Third Load Case
95
0
25
50
75
100
-0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 0.20
Strain (%)
Lo
ad
(k
N)
80 mm
150 mm
230 mm
230 mm - Steel
Figure 4.36 Longitudinal Strains at 80, 150 and 230 mm from Base Test II – Third Load Case
0
25
50
75
100
-0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 0.20
Strain (%)
Lo
ad
(k
N)
460 mm
1520 mm
Figure 4.37 Longitudinal Strains at 460 and 1520 mm from Base Test II – Third Load Case
96
0
25
50
75
100
-0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 0.20
Strain (%)
Lo
ad
(k
N)
2900 mm
3250 mm
4570 mm
Figure 4.38 Longitudinal Strains at 2900, 3250 and 4570 mm from Base
Test II – Third Load Case
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0 1500 3000 4500 6000
Distance from Base (mm)
Str
ain
(%
)
32 kN - Strengthened
32 kN - Unstrengthened
44 kN - Strengthened
Figure 4.39 Longitudinal Strain Profile at 32 and 44 kN (Base = 0 mm)
Test II – First, Second and Third Load Cases
97
0
10
20
30
40
50
0 30 60 90 120
Displacement (mm)
Lo
ad
(k
N)
Unstrengthened Strengthened
0.25L 0.5L 0.75L L
Figure 4.40 Net Displacement at 0.25L, 0.5L, 0.75L and L
Test III – First and Second Load Cases
0
10
20
30
40
50
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3
Strain (%)
Lo
ad
(k
N)
Unstrengthened Strengthened
80 mm
150 mm
230 mm
Figure 4.41 Longitudinal Strains at 80, 150 and 230 mm from Base
Test III – First and Second Load Cases
98
0
10
20
30
40
50
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3
Strain (%)
Lo
ad
(k
N)
Unstrengthened Strengthened
Figure 4.42 Longitudinal Strains at 460 mm from Base Test III – First and Second Load Cases
0
10
20
30
40
50
-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15
Strain (%)
Lo
ad
(k
N)
Unstrengthened Strengthened
Figure 4.43 Longitudinal Strains at 1520 mm from Base Test III – First and Second Load Cases
99
0
10
20
30
40
50
-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15
Strain (%)
Lo
ad
(k
N)
Unstrengthened Strengthened
Figure 4.44 Longitudinal Strains at 3050 mm from Base Test III – First and Second Load Cases
0
10
20
30
40
50
-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15
Strain (%)
Lo
ad
(k
N)
Unstrengthened Strengthened
Figure 4.45 Longitudinal Strains at 4570 mm from Base Test III – First and Second Load Cases
100
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0 1500 3000 4500 6000
Distance from Base (mm)
Str
ain
(%
)
Unstrengthened
Strengthened
Figure 4.46 Longitudinal Strain Profile at 32 kN (Base = 0 mm)
Test III – First and Second Load Cases
0
10
20
30
40
50
-0.30 -0.20 -0.10 0.00 0.10 0.20 0.30
Strain (%)
Lo
ad
(k
N)
Steel CFRP
230 mm
460 mm
Figure 4.47 Longitudinal Strains at 230 mm from Base
Test III – First and Second Load Cases
101
Figure 4.48 Location of Measured Strain at 230 mm from Base
Test III
0
10
20
30
40
50
60
0 30 60 90 120
Displacement (mm)
Lo
ad
(k
N)
0.25L 0.5L 0.75L L
Figure 4.49 Net Displacement at 0.25L, 0.5L, 0.75L and L, to Loading of 55 kN Test III – Third Load Case
102
0
20
40
60
80
100
0 100 200 300 400
Displacement (mm)
Lo
ad
(k
N)
0.25L 0.5L 0.75L L
Figure 4.50 Gross Displacement at 0.25L, 0.5L, 0.75L and L Test III – Third Load Case
0
20
40
60
80
100
0 100 200 300 400
Displacement (mm)
Lo
ad
(k
N)
Initial Delaminating of Bottom Strips
Delaminating and Rupture of the Strips
Buckling of the Monopole
Final delaminating and rupture of the remaining strips.
Figure 4.51 Failure Modes at Load per Net Displacement Measured at L Test III – Third Load Case
103
Figure 4.52 Delaminating of Bottom Strips
Test III – Third Load Case Figure 4.53 Crushing of Top Strips
Test III – Third Load Case
Figure 4.54 Buckling of Monopole
Test III – Third Load Case
Figure 4.55 Air Voids in Epoxy Note: Black Pockets are Air Voids in Bond
Test III – Third Load Case
104
0
25
50
75
100
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
Strain (%)
Lo
ad
(k
N)
80 mm
150 mm
230 mm
Figure 4.56 Longitudinal Strains at 80, 150 and 230 mm from Base
Test III – Third Load Case
0
20
40
60
80
100
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
Strain (%)
Lo
ad
(k
N)
460 mm
1520 mm
Figure 4.57 Longitudinal Strains at 460 and 1520 mm from Base Test III – Third Load Case
105
0
20
40
60
80
100
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
Strain (%)
Lo
ad
(k
N)
3050 mm
4570 mm
Figure 4.58 Longitudinal Strains at 3050 and 4570 mm from Base
Test III – Third Load Case
0
20
40
60
80
100
-0.4 -0.2 0.0 0.2 0.4
Strain (%)
Lo
ad
(k
N)
Steel CFRP
230 mm 460 mm
Figure 4.59 Longitudinal Strains at 230 and 460 mm from Base
Test III – Third Load Case
106
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0 1500 3000 4500 6000
Distance from Base (mm)
Str
ain
(%
)
Unstrengthened at 32 kN
Strengthened at 32 kN
Strengthened at 54 kN
Figure 4.60 Longitudinal Strain Profile at 32 and 54 kN (Base = 0 mm) Test III – First, Second and Third Load Cases
107
CHAPTER 5 - ANALYTICAL MODEL
This chapter discusses three analytical models designed to predict the strain and
deflection behavior measured from Tests I, II and III of the monopoles strengthened with
the sheets and strips. An elastic stiffness model is designed to account for lateral flexural
stiffness of the monopoles both with and without the sheets and strips. Specific modeling
parameters used for building this model include the monopole and CFRP geometry and
mechanical properties, design load and application, and the finite amount of elements
which make up the monopole. The model is designed to predict the behavior under the
effect of the first two load cases of Test I, II and III. Inelastic steel deformation, non-
linear strain and deflection behavior, rupture and delaminating of the CFRP and buckling
of the monopole included in the model design. The results from the first two load cases
of Test I, II and III are compared to the model’s results to validate the proposed model.
Two parametric studies reviewing the effect of the additional layers of sheets and strips to
the strengthening solutions used in Test I and II complete this chapter.
To evaluate the accuracy of the proposed analytical model with respect to the measured
results from the experimental program, the term difference error is used. This term will
give a numerical percentage of the differences between the prediction and the measured
values as shown in equation 1:
1001Values Measured
Prediction AnalyticalError Difference ×−=
(1)
108
5.1 Elastic Flexural Stiffness Model
The flexural stiffness model predicting the behavior is based on the Moment-Area
Method and the Transformed-Section Method [Gere, 25]. The Moment-Area Method is
based on two theorems. The first moment-area theorem is related to curvature (θ) of a
beam and states the angle (θB/A) between two tangential points is equal in magnitude to
the area (A) of the moment (M) divided by flexural modulus (EI) between points A and
B, as shown in equation 2:
dxEIMB
AAB ∫=/θ (2)
The second moment-area is related to deflection (δ) and states the deflection (δB/A)
between two tangential points is equal in magnitude to the moment of the area of the
M/EI diagram between points A and B, as shown in equation 3:
dxEIMx
B
AAB ∫=/δ (3)
Deflection is analytically measured by summing the results from a boundary condition to
the location of the defection.
The transformed-section method is a procedure for converting the cross-section of a
composite beam into beam having the mechanical properties of only one the composite
materials. Specific limits to this theory are the composite materials must be linearly
109
elastic and the neutral axis and moment-resisting capacity of the transformed beam must
be identical to the composite beam. The modular ratio (N) is given in equation 4:
2
1
EEN = (4)
E1 and E2 are the flexural modulus of material 1 and 2 comprising the composite section.
The composite cross-section is transformed by multiplying either the height or the width
of the one of the composite materials by N to generate an equivalent EA product which
does not alter the location of the neutral axis, as shown in equation 5:
21112211 AENAEAEAE +=+ (5)
The transformed-section method applies to the strengthened monopole as shown in
Figure 5.1. The mechanical properties of the monopole steel are used as the control
material and the CFRP mechanical properties are adapted to the steel. The sheets and
strips are isolated on three top and bottom flats and the thickness of each sheet and strip
is multiplied by N to achieve a new, equivalent EA cross-section. The centroid of each
sheet and strip is considered to remain at its original location. Using this model, the
inertia (I) is calculated at each cross-section.
The moment-area method was applied to the monopole as shown in Figure 5.2. The
stiffness model used for the monopoles was made up of 240 elements 25.4 mm in length
(l). Each individual element has specific cross-section geometry and mechanical
properties, based on the results from the transformed-section method. Material properties
110
of the monopole steel were based on tested data from the steel coupon tests and CFRP
material properties were based on material properties from the manufacturer. Material
property values are listed in sections 3.2.1 and 3.2.2. Moment was calculated based on
shear on the element and distance of the element from the applied load. Strain was
calculated based on transformed section mechanical properties and moments derived
from both methods as shown in equation 6:
EIMy
=ε (6)
Several assumptions were included in the model to predict the load deflection and strain
behavior of the unstrengthened and strengthened monopoles. These assumptions are:
1. Strains varying linearly across the depth of the cross-section.
2. Perfect composite action was considered. No bond slippage or failure between
the monopole shaft and CFRP surfaces was assumed to occur.
3. Linear elastic behavior for the steel. Elastic modulus taken from the coupon tests
was used in all models.
4. Linear elastic behavior for the CFRP.
5. Shear deformation calculations were not included in the model.
6. Perfect boundary conditions, i.e. no rotation or slippage at the base and complete
moment resisting, fixed connection to the mounting wall.
7. Loading in the model was limited to the rupture strain of the fiber.
8. Development length was not considered in the model. The sheets and strips were
assumed to be fully developed at all points along there length.
111
9. Only the cross-section of the CFRP was used in calculating strength and stiffness.
The adhesive thickness was ignored.
10. Sign convention for the calculated strain is negative for compressive strain and
positive for tensile strain.
5.2 Test I Model
The high-modulus sheet modulus in compression was not supplied by the manufacturer.
However, both tensile and compressive modulus was supplied by the manufacturer for
the high-modulus strips, thus the compressive modulus of the sheet was calculated as
given in Equation 7:
TSheetTStrip
CStripCSheet E
EE
E = (7)
The resulting magnitude of the modified sheet compressive modulus is 569 GPa. The
adhesive was assumed to provide infinite resistance to buckling of the sheets, so a pure
compressive resistance would be achieved.
The clip angles used at the base to anchor the sheets were considered to have no
contribution towards the lateral strength and stiffness of the strengthened monopole. The
transverse sheets installed along the circumference of the monopole were also considered
to have no impact to the lateral strength and stiffness of the structure. This assumption is
due to the strength of the sheet wrappings being in the transverse, not longitudinal
direction. Their assumed impact was only to support the bond between the sheets and
112
steel surface, immobilizing the sheets to develop full stiffness in compression and
tension.
5.2.1 Deflection, Stiffness and Strain
Applied loads of 32 and 40 kN were used to examine the prediction capability of the
unstrengthened and strengthened monopole models, respectively. These loads are
identical to the loads applied from the first and second load cases of Test I of the
experimental program. Figure 5.3 illustrates the load deflection profile measured during
the first and second load cases of Test I and the calculated from the models simulating the
first and second load cases of Test I at 32 kN. Based on the predicted load deflection
slope, the stiffness values of the monopole before strengthening with high-modulus
sheets at 0.25L, 0.5L, 0.75L and L were 4.93, 1.28, 0.60 and 0.37 kN/mm, respectively.
The stiffness values predicted for the model after the monopole was strengthened
increased to 7.04, 1.76, 0.78 and 0.46 kN/mm at 0.25L, 0.5L, 0.75L and L, respectively.
Comparison of the stiffness values at the respective locations shows the strengthened
monopole stiffness increased 43, 38, 31 and 26 percent at 0.25L, 0.5L, 0.75L and L,
respectively in comparison to the unstrengthened monopole stiffness.
Figure 5.4 shows the strain profiles measured from the experimental program and
predicted values from the analytical model at 32 kN per Test. Neutral axis shift away
from the centroid of the monopole were towards the tensile strains, however varied less
than 10 mm from the base to the end for the strengthened monopole and no neutral axis
shift away from the centroid of the monopole was noted for the unstrengthened
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monopole. Comparison of the predicted longitudinal strain of the monopole before and
after strengthening shows that the strains were reduced by an average of 31 percent from
the base to 3050 mm due to the strengthening system. Strain predicted at 3050 mm to the
tip had equivalent magnitude for both the strengthened and unstrengthened monopole.
5.2.2 Discussion of Tested vs. Modeled Results
Based on review of Figure 5.3, the analytical model predicts deflection for the
unstrengthened monopole very accurately. Comparison of the modeled vs. measured
deflection at 0.25L, 0.5L, 0.75L and L found difference error of 14, 8, 1 and 2 percent,
respectively. Predicted values for the strengthened monopole were not as accurate as the
unstrengthened monopole model. Difference error in the modeled vs. measured
deflection was 32, 17, 9 and 5 percent at 0.25L, 0.5L, 0.75L and L, respectively. Due to
the lack of conformance of the modeled strengthened results to the measured
strengthened deflection results, the modeled stiffness increases was significantly higher
than measured in the experimental program. Predicted strain, shown in Figure 5.4,
conformed fairly closely on average to the measured strain from the experimental
program, although predicted strain reductions from the base to 3050 mm were
significantly different. The neutral axis characteristics found by the experimental
program were predicted very accurately by the analytical model.
The results of the analytical model suggest that this approach was valid for modeling the
strains of both the unstrengthened and strengthened monopole and for calculating
deflection from 0.5L to the tip. However, conservative prediction of the strain and
114
deflection was found inside the midspan. The discrepancy may be attributed to the
inability of simulating the boundary conditions used in the model and the assumption of
full development of all the high-modulus sheets in compression. Development of an
infinitely stiff, moment resisting joint is impossible to achieve in testing, but good
conformance can be achieved if a rigid joint is supporting a tested member whose
stiffness is significantly less in comparison. The monopole shaft near the base was very
stiff, therefore poor conformance was noted near the base. The deflection measurements
taken at 0.25L likely reflected shear deflection as well as flexural deflection, which were
not captured in the model. Also, predicted results tend to improve as modeled results are
examined at greater distances away from the boundary condition, due to its lowered
influence. This was found with the monopole model as deflections calculated at 0.75L
and L showed great conformance to the measured deflections.
Lack of full development of the 1220 and 2440 mm strips in compression and tension is
the other likely cause for the inconsistent comparison of the measured vs. modeled
results. Examination of the strain profile revealed the strains measured from the base to
1500 mm decreased as they were measured away from the base. However, strains
measured from 1500 to 3000 mm remained almost constant. Examination of the
predicted strains reveals the strains to be largely constant from the base to 3000 mm.
This observation indicates the 1220 and 2440 mm strips are not providing strength
capacity to the monopole as expected in the calculated results. This observation is
investigated further in a parametric study in section 5.5.1.
115
Bond failure and sheet rupture did not occur during the first or second loading case of
Test I, so assuming no impact from these issues is acceptable. During the first two
loadings, no strains measured along the axis of the monopole exceeded the steel yield
strain and, after unloading, the monopole showed no effects of permanent deformation,
so the assumption of all materials remaining linearly elastic is also validated.
5.3 Test II Model
Specific characteristics of this model included modeling of the stiffeners part of the
monopole cross-section. The stiffeners were included in the inertia calculations, forming
a complete composite section. Although the stiffeners were made from steel with a lower
yield stress, the section with stiffeners never approached its yield stress during the first or
second loading, thus no permanent deformation of the was occurred. The tested elastic
modulus from the coupons was used for the stiffener elastic modulus.
5.3.1 Deflection, Stiffness and Strain
An applied load of 32 kN and 42 kN was used to examine the predicting capabilities of
the analytical model for the unstrengthened and strengthened monopoles. The load
deflection profile at 32 kN measured from the first and second load cases of Test I and
the predicted from the models simulating the first and second load cases of Test I are
shown in Figure 5.5. The stiffness values, based on the load deflection slope, of the
monopole before strengthening with high-modulus strips was 5.90, 1.40, 0.64 and 0.39
kN/mm at 0.25L, 0.5L, 0.75L and L, respectively. The stiffness values predicted from
116
the model after the monopole was strengthened at 0.25L, 0.5L, 0.75L and L were to 8.73,
2.08, 0.91 and 0.53 kN/mm, respectively. Comparison of the predicted stiffness values
shows the stiffness of the monopole strengthened with high-modulus strips indicate an
increase of 48, 48, 42 and 37 percent at 0.25L, 0.5L, 0.75L and L, respectively in
comparison to the unstrengthened monopole stiffness.
The strain profiles measured from the experimental program and predicted values are
shown in Figure 5.6. The illustrated strains are the results or calculations of the
monopole loaded to 32 kN. Shift of the neutral axis from the centroid of the monopole
was less than 5 mm from the base to 3660 mm and no shift was found at 3660 mm to the
tip. The shifts were towards the tensile strains from the base to 3660 mm. Average strain
reduction due to the strengthening system was 39 percent from the base to 3660 mm.
Strain calculated from 3660 mm to the tip had equivalent magnitude for the
unstrengthened and strengthened monopole.
5.3.2 Discussion of Tested vs. Modeled Results
Examination of the deflection and strain profiles reveal the modeled results accurately
predict the deflection, stiffness and strain values at load. The deflection difference error
was less than 10 percent at 0.5L, 0.75L and L for the strengthened monopole and the
deflection difference error of the unstrengthened monopole was less than 5 percent at
these locations. The modeled stiffness increases conformed closely to the measured
stiffness increases all quarter points, accurately predicting the measured stiffness found in
the experimental program. The calculated strains showed greatest conformance to the
117
measured results of the experimental program. Aside from the calculated strain variance
from the measured strains near the base from the unstrengthened monopole, difference
error between the calculated vs. measured strains was negligible.
Data on the specific material characteristics of the strips in compression and the
monopole shaft steel is the likely the main justification of the good conformity of the
predicted results from this model to the measured results from Test II. The modulus of
the strips in compression was supplied by the manufacturer, so exact stiffness, as opposed
to a modified assumption which was used in section 5.2.1, could be used to complete the
model. An additional influence which would positively impact the model was the strip
layering. The strips were installed in two layers, therefore load transfer between the
strips could be easily developed. Shear lag or lack of development of the outside strip
was minimized. Additional strip layers added to the strengthening solution would likely
have compromised the design assumption of strains varying linearly across the cross-
section. Finally, the stiffeners at the base aided in creating the theoretical infinitely stiff
fixed boundary condition. An infinitely stiff, moment resisting base cannot be
completely attained in the manufacturing process, but a rigid base supporting a flexible
structure can closely simulate this boundary condition. By adding stiffeners, the base
was made significantly more rigid, positively influencing the flexural behavior of the
monopole. The analytical model is purely on flexural behavior, so the stiffeners would
cause the model to adhere closely with the measured deflection results near the base.
118
5.4 Test III Model
Specific characteristics of this model include the stiffeners as a design parameter for
calculating the flexural strength and stiffness and the location of the applied load to 5740
mm from the base, as opposed to the 5790 mm used for the Test I and II. Section 3.5.2
explains the purpose of relocating of the applied load used in testing this monopole. The
stiffeners were manufactured from a lower grade steel than the monopole shaft and
baseplate, but as the yield stress of the section was never reached during the first two
loading cases of Test III, the section remained elastic. The elastic modulus of the
stiffeners was assumed to be identical to the elastic modulus based on the results of the
coupon tests of the monopole shaft steel.
5.4.1 Deflection, Stiffness and Strain
An applied load of 33 and 42 kN was used in the analysis for the unstrengthened and
strengthened monopoles for the first and second load cases. The predicted load
deflection profiles from the models, based on an applied load of 32 kN, is shown in
Figure 5.7, along with the measured load deflection profiles form the first and second
load cases from the experimental program. The predicted stiffness values based on the
modeled load deflection relationship of the unstrengthened monopole is 3.78, 1.29, 0.62
and 0.39 kN/mm at 0.25L, 0.5L, 0.75L and L, respectively. Predicted stiffness values
from the modeled monopole strengthened with the intermediate-modulus strips at 0.25L,
0.5L, 0.75L and L were 7.88, 2.21, 0.98 and 0.59 kN/mm, respectively. The resulting
119
monopole stiffness increase due to the installation of the intermediate-modulus strips is
68, 68, 60 and 52 percent at 0.25L, 0.5L, 0.75L and L, respectively.
Figure 5.8 illustrates the strain profiles measured at 32 kN from the first and second load
cases from the experimental program and the predicted strain profiles based on models of
the monopole before and after strengthening. Shift of the neutral axis from the centroid
of the monopole was less than 10 mm from the base to 3660 mm and no shift was
calculated from 3660 mm to the tip. The shifts calculated from the base to 3660 mm
were towards the tensile strains. Comparison of the monopole strain before and after
strengthening revealed a 55 percent decrease in strain from the base to 3660 mm due to
the intermediate-modulus strip installation. Strains calculated at 3660 mm to the tip from
the model before and after the monopole was strengthened were equivalent in magnitude.
5.4.2 Discussion of Tested vs. Modeled Results
Model prediction of the deflection behavior and the reduction of the measured strain from
the experimental program of Test III were very accurate. Deflection and stiffness
difference error of both the unstrengthened and strengthened monopole were less than 10
percent at 0.5L, 0.75L and L. Difference error between the calculated and measured
deflection and stiffness values at 0.25L varied between 26 and 58 percent. Average strain
reduction due to the intermediate-modulus strip installation was also very similar, as the
calculated average strain reduction from the base to 3660 mm from the base was within 3
percent of the measured average strain reductions. The model was not accurate in
predicting the tensile strains of the strengthened monopole from Test III from the base to
120
3000 mm from the base. The model also did not accurately predict the unstrengthened
monopole strains measured at the base to 1000 mm from the base.
Specific detailed data regarding the material properties of the intermediate strips and the
monopole shaft steel is the likely the reason for the high conformance of the calculated
values from the analytical model to the measured values from the experimental program.
Specific knowledge of the intermediate-modulus strips in compression was especially
important as this eliminated an unknown variable, as opposed to assuming a modified
value as discussed in section 5.2.2. Installing the strips in only two layers also aided in
conforming the calculated values to the measured values. Installation of the strips in
layers greater than two would have likely compromised the assumption of strain varying
linearly throughout the cross-section. Additional layers of strips would possible have
experienced some shear lag and would not allow full development of the strength of the
material. The use of the stiffeners also likely aided in developing the design assumption
of a fixed base. Through installing the stiffeners at the base, the connection became
much stiffer, especially in comparison to the monopole shaft. Flexural stiffness models
using fixed boundary conditions are most accurate when the tested boundary condition
stiffness is significantly greater than the beam stiffness, thus installing the stiffeners
would have contributed to the boundary condition stiffness.
The lack of conformance of the predicted tension strains in comparison to the measured
tension strains may also be attributed to the irregular bonding surfaces found during the
post failure examination of the intermediate-modulus strips. The irregular bonding
121
surface, as discussed in section 4.3.2, is likely the cause of stress concentrations within
the strips which were not uniform. The stress concentrations would have resulted in
greater stiffness from the strips, which were accurately predicted by the model, but not
lower strains, which were predicted by the model. The flexural model assumed full
development of the strips at all locations, thus the presence of stress concentrations
compromises the design assumption. With a more uniform bond applied to the adherend
surfaces, the measured strain likely would have been much closer in magnitude to the
predicted strains.
The lack of conformance of the predicted strains in comparison to the measured strains
near the base of the unstrengthened monopole could also be due to the stress
concentrations induced by welding of the stiffeners to the base. Heat stress from the
welds, which is also discussed in section 4.3.2, likely imparted additional stress into the
monopole of which was not accounted in the model. Use of smaller stiffeners and welds,
along with better quality welding, would have likely resulted in measured strains which
would have conformed to the predicted values.
5.5 Parametric Study Using the Proposed Analytical Model
Two parametric studies are considered to study the influence of specific parameters on
behavior of the strengthened monopoles. The first parametric study will review the
influence of the quantity of high-modulus sheet layers used in Test I. The design
parameters of this study are identical to the analytical model designed to predict the
122
measured results of Test I with the exception of various sheet layers being removed from
the model to alter the reinforcement ratio. Comparison of the resulting predicted stiffness
increases will be made with the original model results. The purpose of this study is to
reveal the potential impacts to the tower’s composite behavior assuming limited amounts
of layers are installed for strengthening. The second parametric study will review the
influence of the strip compressive and tensile elastic modulus based on modeled results.
The analytical model designed to predict the behavior measured from the second load
case of Test II will be used to show the effect modulus has on monopole stiffness. The
unique design parameters of this study are three strip specimens having specific
compressive and tensile modulus. Each specific strip will be considered with varying
reinforcement ratios, or cross-sectional strip area divided by cross-sectional steel area at
the base of the monopole. The purpose of this study is to illustrate the stiffness increases
with respect to modulus and the reinforcement ratio.
5.5.1 Effect of Reinforcement Ratio - Test I Model
Specific detail into the analytical model used for the first parametric study is described in
sections 5.1 and 5.2.1. The only significant difference between the model used to predict
the results from Test I and the model used for the first parametric study is the quantity of
high-modulus sheets considered. The reinforcement ratio was changed by varying the
number of layers of high-modulus sheets on the monopole. The first model considered
the 3050 mm sheets only installed on the top and bottom of the monopole. The second
considered the 2440 and 3050 mm sheets and the third model considered the 1230, 2440
and 3050 mm sheets installed on the top and bottom of the monopole. The purpose of
123
this parametric study is to study the impact the additional sheets towards the stiffness of
the monopole at the quarter points.
Figure 5.9 shows the stiffness increases at the quarter points along the monopole shaft for
the one, two and three sheet layer calculations along with the stiffness increases found in
the four sheet layers detailed in section 5.2.1. At 0.25L (1525 mm), each additional sheet
doubles the percent increase in stiffness, which results in each sheet adding the same
amount of stiffness as the sheet preceding it. However, at L (6100 mm), the impact is
diminished as each additional sheet applies only 2/3rd the stiffness as the sheet preceding
it. Results calculated from 0.5L and 0.75L fall in between the 2/3rd and double stiffness
increase of each additional sheet.
These results illustrate that the shorter, additional layers of sheets provide greatest
stiffness enhancement near the base. Their effect, although significant, is becomes
diminished as the stiffness is measured near the tip. Exact stiffness increase becomes
more difficult to predict near the tip as the additional increases become very similar as
the reinforcement ratios are increased. However, the results also prove that with
significant stiffness increases due to higher reinforcement, greater strength can also be
expected near the base. Therefore, as the largest stresses are typically found near or at
the base of a monopole, a reinforcing design can adequately satisfy the strength
requirements without using excess sheets, assuming stiffness is not a design
consideration.
124
The first parametric study results also offers insight into the lack of complete conformity
of the analytical model to the tested results. The tested result percent stiffness increases
fell in between the stiffness increases predicted by the two and three high-modulus sheet
layer models. Based on the parametric study results, only marginal development of the
third and fourth sheet layers would have significantly limited stiffness. Therefore, the
parametric study lends evidence of the third and fourth sheets layers never fully
developing, only partially reaching their full strengths. This possibility cannot be
concluded without further testing, but does offer insight behind the Test I stiffness
increases not reaching the predicted values.
5.5.2 Effect of Strip Modulus - Test II Model
The model designed to predict the strength and stiffness results from the first and second
load cases of Test II was used to complete the second parametric study. Sections 5.1 and
5.3.1 detail the design parameters and assumptions used to build the model. Specific
design parameters of the strip strengthening system, including orientation, number of and
length of each strip on the monopole shaft was identical to the strip orientation and
installation for Test II. The design parameters altered for this study were the tensile and
compressive modulus and the cross-section area of the strip. Three types of strips were
used. The first was the high-modulus strips used in Test II. The second strip used was
the intermediate-modulus strip utilized for Test III. Figure 3.1 details the tensile and
compressive modulus of the high and intermediate-modulus strips used for the second
parametric study. The third strip used is a generic CFRP strip with modulus in
compression and tension of 100 and 140 GPa, respectively. Cross-sectional area of the
125
strips was evaluated in terms of a reinforcement ratio. The reinforcement ratio (RR) was
calculated as given in equation 8:
Monopole
Strips
AA
RR = (8)
To achieve the varied reinforcement ratios, the thickness of the strips was changed
uniformly throughout their lengths. Reinforcement ratios would then be linearly
proportionate along the length of the monopole. For listing of the results, the
reinforcement ratio was calculated from the base of the monopole. The reinforcement
ratio was evaluated based on a range varying from 0 (no strips installed) to 0.5 (strip
cross-sectional area = half of steel cross-sectional area). Stiffness increase was based on
the calculated stiffness at the tip (L) of the monopole strengthened with a specific total of
strips divided by the calculated stiffness at the tip (L) of the monopole prior to
strengthening with strips. The purpose of the parametric study is to illustrate the potential
stiffness increases by using the strips with higher tensile and compressive modulus over
the standard low modulus strips.
The calculated results of the second parametric study are shown in Figure 5.11. The
results indicate linear increases in stiffness at the tip for the three strips based on
increasing reinforcement ratios. The results also show significant increases in stiffness
based on the magnitude of the modulus of the strips. Specifically, installation of identical
volumes of low, intermediate and high-modulus strips onto the monopole shows
intermediate and high-modulus strips provide 2 and 3 times the stiffness of the low-
modulus strip installation, respectively. The high-modulus strips also provide
126
approximately 50 percent greater stiffness than the intermediate-modulus strips at
identical reinforcement ratios.
Conclusions to the second parametric study show the potential increases in stiffness due
to installation of the three strips are linearly related to the reinforcement ratio.
Specifically, significant savings in strip volumes can be found by using the intermediate
or high-modulus strips. Through reduction in volume of strips needed for strength and
stiffness requirements, installation time and cost of material is reduced. The uncertainty
due to layering of the strips is also reduced few layers will be needed to attain equivalent
strength and stiffness to the lower modulus strips. Significantly greater strength and
stiffness can be attained by using the higher modulus strips. The result is higher factors
of safety for the strength and serviceability design can be utilized with approximately the
same material.
128
0
25
50
75
100
0 1525 3050 4575 6100
Length from Base (mm)
Dis
pla
ce
me
nt
(mm
)
Unstrengthened - Modeled
Unstrengthened - Tested
Strengthened - Modeled
Strengthened - Tested
Figure 5.3 Modeled and Tested Net Deflection Profiles at 32 kN Test I – First and Second Load Cases
-0.15
-0.10
-0.05
0.00
0.05
0.10
0.15
0 1500 3000 4500 6000
Length from Base (mm)
Str
ain
(%
)
Unstrengthened-Tested
Strengthened-Tested
Unstrengthened-Modeled
Strengthened-Modeled
Figure 5.4 Modeled and Tested Strain Profiles at 32 kN
Test I – First and Second Load Cases
129
0
25
50
75
100
0 1525 3050 4575 6100
Length from Base (mm)
Dis
pla
ce
me
nt
(mm
)
Unstrengthened - Modeled
Unstrengthened - Tested
Strengthened - Modeled
Strengthened - Tested
Figure 5.5 Modeled and Tested Net Deflection Profiles at 32 kN
Test II – First and Second Load Cases
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0 1500 3000 4500 6000
Distance from Base (mm)
Str
ain
(%
)
Unstrengthened - Tested
Strengthened - Tested
Unstrengthened - Modeled
Strengthened - Modeled
Figure 5.6 Modeled and Tested Strain Profiles at 32 kN
Test II – First and Second Load Cases
130
0
25
50
75
100
0 1525 3050 4575 6100
Length from Base (mm)
Dis
pla
ce
me
nt
(mm
)
Unstrengthened - Modeled
Unstrengthened - Tested
Strengthened - Modeled
Strengthened - Tested
Figure 5.7 Modeled and Tested Net Deflection Profiles at 32 kN
Test III – First and Second Load Cases
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0 1500 3000 4500 6000
Distance from Base (mm)
Str
ain
(%
)
Unstrengthened - Tested
Strengthened - Tested
Strengthened - Modeled
Unstrengthened - Modeled
Figure 5.8 Modeled and Tested Strain Profiles at 32 kN Test III – First and Second Load Cases
131
0
10
20
30
40
50
0 1525 3050 4575 6100
Length from Base (mm)
Sti
ffn
ess
In
cre
ase
(%
)
1 Sheet Layer
2 Sheet Layers
3 Sheet Layers
4 Sheet Layers
Tested Results
Figure 5.9 Stiffness Increases per Reinforcement Ratio at Quarter Points
Test I – First Parametric Study
0
25
50
75
100
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Area of CFRP / Area of Steel
Sti
ffn
es
s I
nc
rea
se
(%
)>
High-Modulus Strips
Intermediate -Modulus Strips
Low-Modulus Strips
Figure 5.10 Stiffness Increases vs. Reinforcement Ratios for Three Strip Specimens
Test II – Second Parametric Study
132
CHAPTER 6 - SUMMARY AND CONCLUSIONS
This chapter summarizes the measured results of the experimental program and the
analytical models. Specifically, the ultimate strength capacity and failure modes of each
strengthening solution are listed. The measured strength and stiffness increases taken
from Test I, II and III, along with the calculated strength and stiffness increases from the
analytical model is included. Conclusions and evaluations to the effectiveness of each
system are provided. Recommendations towards further research complete this chapter.
6.1 Summary
Listed are the significant results from the experimental program and the analytical
models:
1. The ultimate strength capacity of the monopoles used in Test I (High-Modulus
Sheets), Test II (High-Modulus Strips) and Test III (Intermediate-Modulus
Strips) was 95, 79, and 85 kN, respectively.
2. The failure mode observed from Test I was simultaneous rupture of the high-
modulus sheets in tension and buckling of the monopole 200 mm from the base.
Aside from minor localized debonding near the base, no deformation of the high-
modulus sheets or monopole occurred before the coinciding rupture/buckling.
3. The failure mode observed from Test II ultimately was buckling of the monopole
near the base. Prior to buckling, the high-modulus strips installed on top of the
133
monopole crushed in compression at 0.15 percent strain. Following this rupture,
the bottom high-modulus strips simultaneously delaminated and ruptured in
tension at 200 mm from the base of the monopole. The rupture strain in tension
was measured at approximately 0.18 percent. All high-modulus strips had
ruptured or delaminated prior to the buckling of the monopole.
4. The failure mode observed from Test III ultimately was buckling of the
monopole. Prior to buckling, the intermediate-modulus strips installed on the
bottom of the monopole delaminated and ruptured 200 mm from the base.
Following the delaminating, the intermediate-modulus strips crushed in
compression at 200 mm from the base. The measured rupture strain was between
0.2 and 0.25 percent. All intermediate-modulus strips had ruptured or
delaminated prior to buckling of the monopole. However, buckling of the
monopole followed shortly after the rupture of the intermediate-modulus strips in
compression.
5. The stiffness of the monopoles before and after strengthening with CFRP was
measured at the quarter (0.25L), mid (0.5L), three quarter (0.75L) and full span
(L) or tip for each test. The stiffness increases due to the CFRP installation
within the steel elastic zone for Test I was 13, 25, 20 and 17 percent at 0.25L,
0.5L, 0.75L and L, respectively. The stiffness increases measured from Test II at
the same quarter points was 50, 43, 40, and 41 percent, respectively. The
stiffness increases measured from Test III at these quarter points was 86, 64, 48
and 44 percent, respectively.
134
6. The stiffness increases per the analytical model due to the CFRP while the steel
remained elastic was calculated for each test. Calculated results from Test I were
43, 38, 31 and 26 percent at 0.25L, 0.5L, 0.75L and L, respectively. Calculated
results from Test II predicted stiffness increases of 48, 48, 42 and 37 percent at
0.25L, 0.5L, .75L and L, respectively. Stiffness increases calculated from Test
III were 68, 68, 60, and 52 at 0.25L, 0.5L, 0.75L and L, respectively.
7. The measured strain reductions due to the CFRP installation from Test I, II and
III were 20, 31 and 52 percent, respectively, from the base to the midspan (0.5L)
of the monopole. Strains measured from 0.5L to the tip (L) showed no reduction
due to the strengthening system for all three tests.
8. The calculated strain reduction due to the CFRP from the analytical models of
Test I, II and III was 31, 39 and 52 percent, respectively from the base to 0.5L of
the monopole. Strains calculated from 0.5L to L were not reduced due to the
strengthening system for all three tests.
6.2 Conclusions
Listed are the conclusions of this investigation based on the results measured, observed
and calculated from the experimental program and the analytical models.
1. High and intermediate-modulus CFRP can significantly enhance the strength and
stiffness of a monopole tower, especially while the design loads are within the
monopole steel’s elastic range.
135
2. The high-modulus sheets provide the greatest reliability for sustaining strength
during increasing load and provide the largest strength increases, but are the least
efficient of the three tested CFRP for increasing stiffness. The greater strength
and reliability is due to the excellent adhesion between the sheets and monopole
steel surface. Lack of stiffness as compared to the results from the high and
intermediate-modulus strips is likely due to the inability to properly develop the
additional layers needed to promote greater stiffness.
3. The high-modulus strips provide the greatest stiffness of the three tested CFRP
but provide the lowest strength. The high-modulus eliminates the need to add
many layers of strips to the monopole and reduces the thickness of the strips.
Thus, greater conformance to the anticipated results was found and can be
expected. Due to its low crushing strain in compression, the high-modulus strips
have the lowest ductility of the three CFRP. Therefore, it is considered to be the
least efficient for increasing strength of the monopole.
4. The intermediate-modulus strips provide a good compromise between the
advantages and disadvantages of the high-modulus sheets and strips. The
intermediate-modulus strips can be manufactured to a larger thickness to achieve
similar axial stiffness to the high-modulus strips but still retain the higher
crushing strain in compression, leading to greater strength capacity. The stiffness
increases can also be calculated with more accuracy than found with the high-
modulus sheets as the layers needed to generate the necessary stiffness can be
reduced.
136
5. The clip angles clamping the sheets to the base plate and monopole shaft and the
stiffeners welded to the base plate and monopole shaft provide excellent
immobilization of the shaft section. This immobilization allows the CFRP to
develop its entire strength and stiffness at their ends at the base of the monopole.
The highest stresses are developed at the base of the tower, thus full development
of the CFRP strengthening system is essential at this location.
6. The neutral axis does not shift significantly while the monopole loading is
within the steel elastic range for the sheets and strips. Therefore, a compressive
modulus equivalent to the tension modulus is developed for all three CFRP
tested. Continued development of the compressive modulus after yielding of the
monopole steel cannot be confirmed for the high-modulus sheets but is believed
to have continued to contribute stiffness to the monopole. The high-modulus
strips ruptured prior to yielding of the monopole shaft, thus no contribution to
strength or stiffness was measured from them. Significant loss of strength and
stiffness after the intermediate-modulus strips ruptured in compression was
measured, thus continued development of the compressive modulus was
confirmed.
7. Significant surface pressure must be applied to the strips during installation to
ensure a uniform adhesive bond. Installation must be completed quickly as well.
Failure to apply pressure and complete rapid installation of the strips leads to
significant air voids within the adhesive bond which creates stress concentrations
in the strips, leading to premature failure.
137
8. The high-modulus strips can be accurately designed for strength and stiffness
assuming one or two layers is used. Additional layers likely do not provide same
strength and stiffness increases that the first two layers of high-modulus sheets
provide. This is based on the results from the first parametric study. However,
use of adhesives with higher strengths and modulus likely can develop the full
strength of sheets installed in layers greater than two.
9. Due to its low compressive crushing strain, the high-modulus strips should not be
used for increasing the strength of monopoles. They can be used effectively for
providing stiffness assuming an appropriate factor of safety is applied to the
rupture stress of the strip.
10. The intermediate-modulus strips can be used effectively for increasing the
strength and stiffness of a monopole. However, the design strength must be
limited to the yield strain of steel and a uniform bond between the strips and steel
must be applied.
11. The moment area method and transformed section method produces very accurate
deflection and stiffness calculations of the monopole before and after
strengthening as compared to the tested results. These methods are most accurate
when compared to the tested results from the strips, which is due to the greater
control of the material properties of the strips. The transformed section method
also accurately predicts the strain behavior of the monopole before and after
strengthening, especially with the strips. The inability of the additional high-
modulus sheets to develop their tensile and compressive strength and stiffness
limits the conformance of the model to the tested results. The calculated
138
deflections and strains showed very good conformance to the tested
measurements and conformance to the tested results is greater as comparisons are
made along the monopole shaft away from the base.
6.3 Recommendations for Further Research
Listed are suggestions for further research on this topic.
1. The effectiveness of layering the sheets for increasing strength and stiffness
should be evaluated to determine the development of the additional sheets.
2. Coupon tests aimed at determining the compressive modulus of the sheets should
be examined to evaluate the behavior of the sheets at high strain.
3. Addition work can be completed to determine optimum bonding conditions for
the strips and monopole surface and for the strip to strip surface. Specifically, the
amount of applied pressure to the strip surface to form a uniform, void free
adhesive bond can be investigated further.
4. Application of CFRP onto the individual elements which make up self-supporting
and guyed tower can be investigated to determine potential strength and stiffness.
5. The effect of pre-stressing the high and intermediate modulus strips can be
investigated to determine potential strength and stiffness increases for the
monopole.
139
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Structures. 3rd Edition. B.J. Clark and David A. Damstra, McGraw-Hill, Inc. Boston. 1992. pgs. 15-21.
[6] Sachs, Peter. Wind Forces in Engineering. 2nd Edition. Pergamon Press, Inc.
Maxwell House, Fairview Park, Elmsford, NY. 1978. [7] Mosallam, A.S., P.R. Chakrabarti & E. Spencer. “Experimental Investigation on
the Use of Advanced Composites & High-Strength Adhesives in Repair of Steel Structures.” 43rd International SAMPE Symposium. May 31-June 4, 1998. pgs. 1826-1837.
[8] McKnight, Steven H., Pierre E. Bourban, John W. Gillespie, Jr., and Vistasp M.
Karbhari. “Surface Preparation of Steel for Adhesive Bonding in Rehabilitation Applications.” Infrastructure Repair Methods: Steel for Adhesive Binding. Center for Composite Materials and the Materials Science Program, University of Delaware. Newark, DE. pgs. 1148-1155.
[9] Price, A. and R.J. Moulds. “Repair and Strengthening of Structures Using Plate
Bonding.” Construction and Building Materials. Volume 5, Number 4. Butterworth-Heinemann, Ltd. December 1991. pgs. 189-192.
[10] Moriarty, Jim. “The Use of Carbon Fiber Composites in the London Underground
Limited Civil Infrastructure Rehabilitation Program.” SAMPE Journal. Volume 34, Number 2. March/April 1998. pgs. 23-28.
[11] Cannon, Jr., D.D. and R.A. LeMaster. “Local Buckling Strength of Polygonal
Tubular Poles.” American Society of Civil Engineers: Manuel 72, Reference 210. [12] Morrison Hershfield Group, DualPole Monopole Reinforcing Solution http://www.dualpole.com/index.htm. April 9, 2004.
140
[13] ScienTel Tower Strengthening Program http://www.scientech.com/scientel/solutions/tower.html. April 9, 2004. [14] Dywidag-Systems International Threaded Post-tensioning Bars. http://www.dywidag-systems.com/docs/dsi_index.php#. April 9, 2004. [15] Westower Dywidag Monopole Reinforcing System. http://www.dywidag-systems.com/docs/dsi_index.php#. April 9, 2004. [16] AeroSolutions, LLC. AeroForce Systems, Monopole and Tower Upgrades. http://www.aerosolutionsllc.com/index.html. June 19, 2003. pgs 1-8. [17] Hutter Trankina Simplified Monopole Tower Reinforcing. http://www.htedesign.com/tower_reinforcing.htm. April 9, 2004. [18] Moulds, R.J. and A. Price. “Repair and Strengthening of Structures Using Plate
Bonding.” Construction and Building Materials. Volume 5, Number 4. Butterworth-Heinemann, Ltd. December 1991. pgs. 189-192.
[19] Nakazawa, M. “Mechanism of Adhesion of Epoxy Resin to Steel Surface.”
Nippon Steel Technical Report. Number 63, October 1994.
[20] Bourban, P. E., S. H. McKnight, S. B. Shulley, V. M. Karbhari, and J. W. Gillespie, Jr. “Infrastructure: New Materials and Methods of Repair. Durability of Steel/Composites Bonds for Rehabilitation of Structural Components.” Third Materials Engineering Conference. Materials Engineering Division of the American Society of Civil Engineers. San Diego, Ca. November 13-16, 1994. pgs. 295-302.
[21] Bourban, P.E., S.H. McKnight, J. W. Gillespie, Jr., and V. M. Karbhari. “Surface Preparation of Steel for Adhesive Bonding in Rehabilitation Applications.” Infrastructure Repair Methods: Steel for Adhesive Binding. Center for Composite Materials and the Materials Science Program, University of Delaware. Newark, DE. pgs. 1148-1155.
[22] Karbhari, V.M. and S.B. Shulley. “Use of Composites for Rehabilitation of Steel Structures – Determination of Bond Durability.” Journal of Materials in Civil Engineering. Volume 7, Number 4. November 1995. pgs. 239-245.
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[24] Tavakkolizadeh, M and H. Saadatmanesh. “Fatigue Strength of Steel Girders Strengthened with Carbon Fiber Reinforced Polymer Patch” Journal of Structural Engineering. February 2003. pgs. 186-196.
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142
APPENDIX
Included are the gross deflection and base rotation measurements from all load cases of
Test I, II and III. Also included are the transverse strain measurements from the first and
second load cases of Test I. All locations noted in the following graphs are with respect
to the base, thus the base of the monopole is 0.0 or 0L. Graphed results include
measurements taken throughout the entirety of each loading case. All other pertain
information is listed with the respective graphed measurements.
143
0
10
20
30
40
50
0 30 60 90 120
Displacement (mm)
Lo
ad
(k
N)
Unstrengthened Strengthened
0.25L 0.5L 0.75L L
Figure A1 - Gross Displacement at 0.25L, 0.5L, 0.75L and L Test I – First and Second Load Cases
0
10
20
30
40
50
0.00 0.05 0.10 0.15 0.20
Rotation (°)
Lo
ad
(k
N)
Unstrengthened Strengthened
Figure A2 - Base Rotation Test I – First and Second Load Cases
144
0
10
20
30
40
50
0.0 0.5 1.0 1.5 2.0
Displacement (mm)
Lo
ad
(k
N)
Unstrengthened Strengthened
Figure A3 - Base Slip Test I – First and Second Load Cases
0
10
20
30
40
50
0.00 0.02 0.04 0.06
Strain (%)
Lo
ad
(k
N)
90 Degrees 60 Degrees 30 Degrees
(a)
0
10
20
30
40
50
0.00 0.02 0.04 0.06
Strain (%)
Lo
ad
(k
N)
90 Degrees 60 Degrees 30 Degrees
(b) Figure A4 Transverse Strains at 610 (a) and 1220 (b) mm
Test I – First and Second Load Cases
145
0
25
50
75
100
0 100 200 300 400
Displacement (mm)
Lo
ad
(k
N)
0.25L 0.5L 0.75L L
Figure A5 - Gross Displacement at 0.25L, 0.5L, 0.75L and L
Test I – Third Load Case with Nylon Straps
0
20
40
60
80
100
0.0 0.1 0.2 0.3 0.4
Rotation (°)
Lo
ad
(k
N)
Figure A6 - Base Rotation
Test I – Third Load Case with Nylon Straps
146
0
20
40
60
80
100
0.0 1.0 2.0 3.0 4.0 5.0
Displacement (mm)
Lo
ad
(k
N)
Figure A7 - Base Slip
Test I – Third Load Case with Nylon Straps
0
20
40
60
80
100
-0.20 -0.10 0.00 0.10 0.20
Strain (%)
Lo
ad
(k
N)
90 Degrees 60 Degrees 30 Degrees
(a)
0
20
40
60
80
100
-0.20 -0.10 0.00 0.10 0.20
Strain (%)
Lo
ad
(k
N)
90 Degrees 60 Degrees 30 Degrees
(b) Figure A8 Transverse Strains at 610 (a) and 1220 (b) mm
Test I – Third Load Case with Nylon Straps
147
0
25
50
75
100
0 100 200 300 400
Displacement (mm)
Lo
ad
(k
N)
0.25L 0.5L 0.75L L
Figure A9 - Gross Displacement at 0.25L, 0.5L, 0.75L and L
Test I – Third Load Case with Chains
0
20
40
60
80
100
0.0 0.1 0.2 0.3 0.4
Rotation (°)
Lo
ad
(k
N)
Figure A10 - Base Rotation
Test I – Third Load Case with Chains
148
0
20
40
60
80
100
0.0 1.0 2.0 3.0 4.0 5.0
Displacement (mm)
Lo
ad
(k
N)
Figure A11 - Base Slip Test I – Third Load Case with Chains
0
20
40
60
80
100
-0.20 -0.10 0.00 0.10 0.20
Strain (%)
Lo
ad
(k
N)
90 Degrees 60 Degrees 30 Degrees (a)
0
20
40
60
80
100
-0.20 -0.10 0.00 0.10 0.20
Strain (%)
Lo
ad
(k
N)
90 Degrees 60 Degrees 30 Degrees (b)
Figure A12 Transverse Strains at 610 (a) and 1220 (b) mm Test I – Third Load Case with Chains
149
0
10
20
30
40
50
0 30 60 90 120
Displacement (mm)
Lo
ad
(k
N)
Unstrengthened Strengthened
Figure A13 - Gross Displacement at 0.25L, 0.5L, 0.75L and L Test II – First and Second Load Cases
0
10
20
30
40
50
0.00 0.05 0.10 0.15 0.20
Rotation (°)
Lo
ad
(k
N)
Unstrengthened Strengthened
Figure A14 - Base Rotation Test II – First and Second Load Cases
150
0
10
20
30
40
50
0.0 0.5 1.0 1.5 2.0
Displacement (mm)
Lo
ad
(k
N)
Unstrengthened Strengthened
Figure A15 - Base Slip Test II – First and Second Load Cases
0
20
40
60
80
100
0 100 200 300 400
Displacement (mm)
Lo
ad
(k
N)
0.25L 0.5L 0.75L L
Figure A16 - Gross Displacement at 0.25L, 0.5L, 0.75L and L
Test II – Third Load Case
151
0
20
40
60
80
100
0.0 0.1 0.2 0.3 0.4
Rotation (°)
Lo
ad
(k
N)
Figure A17 - Base Rotation Test II – Third Load Case
0
20
40
60
80
100
0.0 1.0 2.0 3.0 4.0 5.0
Displacement (mm)
Lo
ad
(k
N)
Figure A18 - Base Slip Test II – Third Load Case
152
0
10
20
30
40
50
0 30 60 90 120
Displacement (mm)
Lo
ad
(k
N)
Unstrengthened Strengthened
0.25L 0.5L 0.75L L
Figure A19 - Gross Displacement at 0.25L, 0.5L, 0.75L and L
Test III – First and Second Load Cases
0
10
20
30
40
50
0.00 0.05 0.10 0.15 0.20
Rotation (°)
Lo
ad
(k
N)
Unstrengthened Strengthened
Figure A20 - Base Rotation Test III – First and Second Load Cases
153
0
10
20
30
40
50
0.0 0.5 1.0 1.5 2.0
Displacement (mm)
Lo
ad
(k
N)
Unstrengthened Strengthened
Figure A21 - Base Slip Test III – First and Second Load Cases
0
20
40
60
80
100
0 100 200 300 400
Displacement (mm)
Lo
ad
(k
N)
0.25L 0.5L 0.75L L
Figure A22 - Gross Displacement at 0.25L, 0.5L, 0.75L and L
Test III – Third Load Case