Development and Structural Investigation of Monocoque Fibre Composite … · 2010. 6. 9. · 3.3.2...
Transcript of Development and Structural Investigation of Monocoque Fibre Composite … · 2010. 6. 9. · 3.3.2...
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Development and Structural Investigation of Monocoque Fibre
Composite Trusses
By
Matthew Humphreys BEng Civil (Hons), MIEAust, CPEng, RPEQ
A thesis submitted to the School of Civil Engineering Queensland University of Technology
in partial fulfilment of the requirements for the degree of
Doctor of Philosophy
December 2003
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Development and Structural Investigation of Monocoque Fibre Composite Trusses iii
Abstract
Fibre composite materials are gaining recognition in civil engineering applications as
a viable alternative to traditional materials. Their migration from customary
automotive, marine, aerospace and military industries into civil engineering has
continued to gain momentum over the last three decades as new civil engineering
applications develop. The use of fibre composite materials in civil engineering has
now evolved from non-structural applications, such as handrails and cladding, into
primary structural applications such as building frames, bridge decks and concrete
reinforcement. However, there are issues which are slowing the use of fibre
composite materials into civil engineering. Issues include high costs, difficulties in
realising potential benefits, general lack of civil engineers’ familiarity with the
material and relatively little standardisation in the composites industry. For
composites to truly offer a viable alternative to traditional construction materials in
the civil engineering marketplace, it is essential that these issues be addressed. It is
proposed that this situation could be improved by demonstrating that potential
benefits offered by composites can be achieved with familiar civil engineering forms.
These forms must be well suited to fibre composite materials and be able to produce
safe and predictable civil engineering structures with existing structural engineering
methods.
Of the numerous structural forms currently being investigated for civil engineering
applications, the truss form appears particularly well suited to fibre composites. The
truss is a familiar structural engineering form which possesses certain characteristics
that make it well suited to fibre composite materials. In this research a novel
monocoque fibre composite truss concept was developed into a working structure
and investigated using analytical and experimental methods. To the best of the
author’s knowledge the research presented in this thesis represents the first doctoral
research into a structure of this type. This thesis therefore presents the details of the
development of the monocoque fibre composite (MFC) truss concept into a working
structure.
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Development and Structural Investigation of Monocoque Fibre Composite Trusses iv
The developed MFC truss was used as the basis for a detailed investigation of the
structural behaviour of the MFC truss elements and the truss as a whole. The static
structural behaviour of the principal MFC truss elements (tension members,
compression members and joints) was investigated experimentally and analytically.
Physical testing required the design and fabrication of a number of novel test rigs.
Well established engineering principles were used along with complex finite element
models to predict the behaviour of the tested truss elements and trusses. Results of
the theoretical analysis were compared with experimental results to determine how
accurately their static structural behaviour could be predicted.
It was found that the static structural behaviour of all three principal truss elements
could be accurately predicted with existing engineering methods and finite element
analysis. The knowledge gained from the investigation of the principal truss elements
was then used in an investigation of the structural behaviour of the MFC truss. Three
full-scale MFC trusses were fabricated in the form of conventional Pratt, Howe and
Warren trusses and tested to destruction. The investigation included detailed finite
element modelling of the full-scale trusses and the results were compared to the full-
scale test results. Results of the investigation demonstrated that the familiar Pratt,
Howe and Warren truss forms could be successfully manufactured with locally
available fibre composite materials and existing manufacturing technology. The
static structural behaviour of these fibre composite truss forms was accurately
predicted with well established engineering principles and finite element analysis.
A successful marriage between fibre composite materials and a civil engineering
structure has been achieved. Monocoque fibre composite trusses have been
developed in the familiar Pratt, Howe and Warren truss forms. These structures
possess characteristics that make them well suited to applications as primary load
bearing structures.
KEYWORDS: civil engineering, construction, structural engineering, finite element
analysis, fibre composites, truss structures, Pratt truss, Howe truss, Warren truss,
glass fibre, carbon fibre, epoxy resin, particulate filled resin, fibre composites in civil
engineering.
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Development and Structural Investigation of Monocoque Fibre Composite Trusses v
Publications
Humphreys, M.F., van Erp, G.M., and Tranberg, C.T. (1999), “Monocoque Fibre
Composite Truss Joints”, ACUN 1 – Proceedings of the First International
Composites Meeting, University of New South Wales, Australia, pp 247 - 251.
Humphreys, M.F., van Erp, G.M., and Tranberg, C.T. (1999), “An Investigation into
the Structural Behaviour of Monocoque Fibre Composite Truss Joints”, Proceedings
of ICCM12 – International Conference on Composite Materials, International
Committee on Composite Materials, Paris, France, Paper 274.
Humphreys, M.F., van Erp, G.M., and Tranberg, C.T. (1999), “Structural Behaviour
of Monocoque Fibre Composite Trusses”, Mechanics of Structures and Materials,
Edited by Bradford M. A., Bridge, R. Q., Foster, S. J., Balkema, Rotterdam, The
Netherlands, pp 501 - 506.
Humphreys, M.F., van Erp, G.M., and Tranberg, C.T. (1999), “The Structural
Behaviour of Monocoque Fibre Composite Truss Joints”, Advanced Composite
Letters, Vol 8 No. 4, Adcotec, London, UK, pp 173 - 180.
Humphreys, M.F., (2003)”, “Extending the Service Life of Buildings and
Infrastructure With Fibre Composites”, PRRES9 - Proceedings of the Ninth Pacific
Rim Real Estate Society Conference, Brisbane, Australia.
Humphreys, M.F., (2003), “The Use of Polymer Composite in Construction”,
SASBE2003 – Proceedings of the Smart and Sustainable Built Environment
conference, Brisbane, Australia, pp 585 - 593.
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Development and Structural Investigation of Monocoque Fibre Composite Trusses vi
Contents
Statement of original authorship i
Acknowledgements ii
Abstract iii
Publications v
Contents vi
List of Figures xii
List of Tables xviii
Notation xxi
Chapter 1 – Introduction
1.1 Background 1.2 Aims 1.3 Scope 1.4 Thesis structure
1 - 1
1 - 2
1 - 6
1 - 7
1 - 8
Chapter 2 – Fibre Composite in Civil Engineering
2.1 Introduction 2.2 Fibre composite materials in construction and civil
engineering
2.2.1 Rehabilitation and retrofit 2.2.2 Concrete structures reinforced with fibre
composites
2.2.3 New fibre composite civil structures 2.3 Issues affecting the use of fibre composites in civil
engineering applications
2.3.1 Cost 2.3.2 Structural performance 2.3.3 Durability 2.3.4 Familiarity and education 2.3.5 Specification and standardisation 2.3.6 Compatibility 2.3.7 Temperature and fire performance
2 - 1
2 - 1
2 - 2
2 - 3
2 - 4
2 - 7
2 - 8
2 - 9
2 - 15
2 - 24
2 - 29
2 - 31
2 - 32
2 - 33
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Development and Structural Investigation of Monocoque Fibre Composite Trusses vii
2.4 The need for a new approach 2.4.1 Lessons from history 2.4.2 Current approach
2.5 Summary
Chapter 3 –Monocoque Fibre Composite Trusses
3.1 Trusses 3.1.1 Truss definition 3.1.2 Brief history of trusses 3.1.3 Characteristics of trusses suited to fibre
composite materials
2 - 36
2 - 36
2 - 38
2 - 40
3 - 1
3 - 2
3 - 2
3 - 3
3 - 6
3.2 FRP trusses 3.2.1 Concrete filled CFRP tubes 3.2.2 Experimental transmission towers with serrated
joints
3.2.3 CFRP roof truss 3.2.4 Expandable space trusses 3.2.5 Pultruded section pedestrian bridge 3.2.6 Areas for potential improvement of existing
approaches to FRP trusses
3 - 8
3 - 8
3 - 9
3 - 11
3 - 11
3 - 12
3 - 13
3.3 The monocoque fibre composite (MFC) truss 3.3.1 Configuration 3.3.2 Form 3.3.3 Materials 3.3.4 Fabrication 3.3.5 Static load response
3.4 Adopted configuration of the MFC truss
Chapter 4 – Static Structural Behaviour of MFC Truss Tension
Elements
4.1 Introduction 4.2 Preliminary investigation of tension elements
4.2.1 Material properties
3 - 15
3 - 17
3 - 31
3 - 38
3 - 51
3 - 56
3 - 63
4 - 1
4 - 1
4 - 3
4 - 4
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Development and Structural Investigation of Monocoque Fibre Composite Trusses viii
4.2.2 Specimen geometry 4.2.3 Prediction of static structural behaviour 4.2.4 Fabrication of test specimens 4.2.5 Testing 4.2.6 Results 4.2.7 Discussion 4.2.8 Summary
4.3 Detailed investigation of tension elements 4.3.1 Proposed approach 4.3.2 Specimen design 4.3.3 Prediction of stiffness and strength 4.3.4 Fabrication 4.3.5 Test setup 4.3.6 Results 4.3.7 Comparison of prediction with test results 4.3.8 Discussion
4.4 Conclusions
Chapter 5 – Static Structural Behaviour of MFC Truss Compression
Elements
5.1 Preliminary investigation of compression elements 5.1.1 Specimen details 5.1.2 Analysis and prediction of behaviour 5.1.3 Testing 5.1.4 Results and discussion
5.2 Detailed investigation of compression elements 5.2.1 Member design 5.2.2 Analysis 5.2.3 Fabrication 5.2.4 Test setup 5.2.5 Results 5.2.6 Discussion
5.3 Summary and conclusions
4 - 4
4 - 5
4 - 7
4 - 8
4 - 9
4 - 11
4 - 16
4 - 17
4 - 17
4 - 18
4 - 20
4 - 35
4 - 36
4 - 37
4 - 41
4 - 45
4 - 50
5 - 1
5 - 1
5 - 2
5 - 3
5 - 7
5 - 8
5 - 11
5 - 12
5 - 16
5 - 22
5 - 26
5 - 29
5 - 34
5 - 39
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Development and Structural Investigation of Monocoque Fibre Composite Trusses ix
Chapter 6 – Monocoque Fibre Composite Truss Joints
6.1 Joint strength 6.1.1 Calculating joint strength 6.1.2 Preliminary experimental investigation 6.1.3 Joint testing
6.2 Joint rigidity 6.2.1 Secondary stresses 6.2.2 Truss deflection
6.3 Simplified joint analysis 6.4 Conclusions
6 - 1
6 - 1
6 - 2
6 - 12
6 - 20
6 - 33
6 - 34
6 - 39
6 - 42
6 - 43
Chapter 7 – Static Structural Behaviour of MFC Trusses
7.1 Design of truss specimens 7.2 Analysis
7.2.1 Approach 7.2.2 Elements and mesh 7.2.3 Material properties 7.2.4 Loading and restraints 7.2.5 Results of finite element analysis 7.2.6 Discussion of finite element analysis
7.3 Fabrication of truss specimens 7.4 Testing of MFC trusses
7.4.1 Test description 7.4.2 Test results
7.5 Comparison of predictions with test results 7.5.1 Stiffness 7.5.2 Strength
7.6 Summary
7 - 1
7 - 1
7 – 6
7 - 6
7 - 7
7 - 9
7 - 29
7 - 30
7 - 36
7 - 37
7 - 44
7 - 44
7 - 46
7 - 54
7 - 54
7 - 56
7 - 57
Chapter 8 – Conclusions and Recommendations
8.1 Conclusions 8.1.1 Development of truss concept into working
structure
8 - 1
8 - 1
8 - 2
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Development and Structural Investigation of Monocoque Fibre Composite Trusses x
8.1.2 Investigation of structural behaviour of principal truss elements and demonstration of accuracy
with which structural behaviour can be predicted
8.1.3 Design and construction of prototype Pratt, Howe and Warren trusses
8.1.4 Evaluation of the structural performance of the three prototype trusses to characterise their
behaviour in terms of stiffness, strength, failure
mode, predictability and warning of failure
8.2 Recommendations for future research
8 - 4
8 - 9
8 - 9
8 - 10
Appendix A – Material Properties
A - 1
Appendix B – Tension Specimen Graphs
B - 1
Appendix C – Compression Specimen Graphs
C - 1
Appendix D – T-joint Graphs
D - 1
References
R - 1
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Development and Structural Investigation of Monocoque Fibre Composite Trusses xi
List of Figures
Figure 1.1 – Applications of fibre composites in civil engineering
structures
1 - 4
Figure 1.2 – Examples of fibre composite trusses 2 - 5
Figure 2.1 – Simply supported beam with UDL 2 - 18
Figure 2.2 –Behaviour of hybrid FRP 2 - 22
Figure 2.3 – a) Veirendeel load path b) “Long” truss form 2 - 24
Figure 3.1 – Truss terminology 3 - 2
Figure 3.2 – Early truss structures 3 - 3
Figure 3.3 – Nail-plate 3 - 6
Figure 3.4 – Reinforced concrete jointed CFRP tube bridge (Karbhari,
1998)
3 - 9
Figure 3.5 – Serrated joints (Goldsworthy, 1998) 3 - 10
Figure 3.6 – CFRP roof truss (Agematzu, 1998) 3 - 11
Figure 3.7 – Telescopic space truss (NASA, 2002) 3 - 12
Figure 3.8 – Pultruded section pedestrian bridge (Milcovich, 2002) 3 - 13
Figure 3.9 – Conceptual configuration of monocoque truss 3 - 17
Figure 3.10 – Truss joint showing web member lapped onto bottom chord
at panel point
3 - 17
Figure 3.11 – Lapping fibres from adjacent members 3 - 18
Figure 3.12 – Pratt truss fibre architecture with arbitrary fill layers 3 - 19
Figure 3.13 – Pratt truss fibre architecture with aligned fill layers 3 - 20
Figure 3.14 – Joint layup types 3 - 22
Figure 3.15 – Type 1 fibre architecture 3 - 23
Figure 3.16 – Type 2 fibre architecture 3 - 24
Figure 3.17 – Joint type 3 fibre architecture 3 - 25
Figure 3.18 – Typical MFC truss cross-sections 3 - 27
Figure 3.19 – Frame analysis truss configurations 3 - 33
Figure 3.20 – Typical frame analysis truss 3 - 34
Figure 3.21 – Typical displaced shape 3 - 35
Figure 3.22 – Typical combined stress 3 - 36
Figure 3.23 –Buckled shape of polystyrene foam core MFC truss from 3 - 44
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Development and Structural Investigation of Monocoque Fibre Composite Trusses xii
FEA
Figure 3.24 – Truss test configuration 3 - 44
Figure 3.25 – Foam and balsa truss core configuration 3 - 45
Figure 3.26 – Foam truss failure mode 3 - 45
Figure 3.27 – Load-displacement graph for polystyrene foam truss member 3 - 46
Figure 3.28 – Initial balsa investigation 3 - 47
Figure 3.29 – Initial balsa investigation 3 - 48
Figure 3.30 – PFR 3 - 50
Figure 3.31 – Use of pre-fabricated sandwich panel to produce MFC truss 3 - 53
Figure 3.32 – Fabrication of PFR core 3 - 55
Figure 4.1 – Typical MFC truss member 4 - 2
Figure 4.2 – Representative tension element 4 - 5
Figure 4.3 – Bi-linear load versus strain curve 4 - 7
Figure 4.4 – Representative tension element specimens 4 - 8
Figure 4.5 – Typical test configuration 4 - 9
Figure 4.6 – Typical load - displacement curves 4 - 9
Figure 4.7 – Type 1 specimen failure modes 4 - 12
Figure 4.8 – Transverse cracks in PFR specimens 4 - 14
Figure 4.9 – Transverse cracks in PFR specimens 4 - 15
Figure 4.10 – Tension specimen geometry and fibre architecture 4 - 19
Figure 4.11 – Two-dimensional FE model 4 - 24
Figure 4.12 – Principal stress distribution at maximum load 4 - 26
Figure 4.13 – Deformed FE model of RVE 4 - 27
Figure 4.14 – Two-stage prediction of load vs strain behaviour (PFR core) 4 - 27
Figure 4.15 – Incremental prediction of load vs strain behaviour (PFR core) 4 - 29
Figure 4.16 – Predicted load vs strain curve for plaster core specimens 4 - 30
Figure 4.17 – Predicted load vs strain curve for foam core specimens 4 - 31
Figure 4.18 – Predicted load vs strain curve for neat resin core specimens 4 - 32
Figure 4.19 – σ11 stress distribution of RVE 4 - 34
Figure 4.20 – Typical tension specimens 4 - 36
Figure 4.21 –Test set-up 4 - 37
Figure 4.22 – Typical tension specimen failure modes 4 - 37
Figure 4.23 – Predicted and Experimental load vs strain graphs for tension 4 - 43
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Development and Structural Investigation of Monocoque Fibre Composite Trusses xiii
elements
Figure 4.24 – SEM of typical transverse crack 4 - 45
Figure 5.1 – Location of cut line 5 - 2
Figure 5.2 – Typical preliminary test specimens 5 - 3
Figure 5.3 – Typical MFC truss compression member section geometry 5 - 5
Figure 5.4 - Test set-up 5 - 8
Figure 5.5 – Typical compression load vs axial shortening curves 5 - 8
Figure 5.6 – Typical failure modes 5 - 9
Figure 5.7 – Compression member failure zones 5 - 12
Figure 5.8 – Proportions of stocky specimens with a non-compact cross
section
5 - 14
Figure 5.9 – Stocky / non-compact member section 5 - 15
Figure 5.10 – Typical slender member cross section 5 - 16
Figure 5.11 – First critical buckling mode (λ = 77.9, applied load = 4 kN) 5 - 19
Figure 5.12 – Slender member (symmetric about x-x neutral axis) cross
section
5 - 21
Figure 5.13 – Locations of extracted stocky compression specimens with
compact cross section
5 - 22
Figure 5.14 – 5 mm thick PFR core 5 - 23
Figure 5.15 – Stocky specimen with non-compact cross section 5 - 24
Figure 5.16 – Surface profile measurement of stocky specimen with a non-
compact cross section
5 - 24
Figure 5.17 – Production of slender specimen cores 5 - 25
Figure 5.18 – Out-of-straightness measurement of slender specimens 5 - 26
Figure 5.19 – Finished slender specimens 5 - 26
Figure 5.20 – Test rig for stocky member with a compact cross section 5 - 27
Figure 5.21 – Test setup for stocky member with non-compact cross
section
5 - 28
Figure 5.22 – Test setup for slender specimens 5 - 28
Figure 5.23 – Typical load vs strain graph for Pratt and Warren specimens 5 - 30
Figure 5.24 – Typical Pratt specimen failure modes 5 - 31
Figure 5.25 – Typical Warren specimen failure modes 5 - 31
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Development and Structural Investigation of Monocoque Fibre Composite Trusses xiv
Figure 5.26 – Strain gauge and dial gauge readings 5 - 32
Figure 5.27 – Typical slender specimen load vs strain measurements 5 - 33
Figure 5.28 – Typical flexural test specimen 5 - 37
Figure 5.29 – Revised local buckling model (applied load = 4000 N, λ =
161.
5 - 38
Figure 6. 1 – Sensitivity of lap strength to lap length 6 - 5
Figure 6. 2 – Failure loads of lap specimens 6 - 6
Figure 6.3 – Simplified Goland and Reissner model 6 - 6
Figure 6.4 – Stress distribution of Goland and Reissner model 6 - 7
Figure 6.5 – Stress distribution of Goland and Reissner model 6 - 8
Figure 6.6 – Section through truss joint 6 - 9
Figure 6.7 – Section A-A 6 - 9
Figure 6.8 – FE model of joint section 6 - 10
Figure 6.9 – Stress distribution along bond plane of model 6 - 11
Figure 6.10 – Modified lap-shear specimen 6 - 13
Figure 6.11 – Modified lap-shear test configuration 6 - 13
Figure 6.12 – Average tensile failure stress of modified lap specimens 6 - 14
Figure 6.13 – Test specimen configuration 6 - 15
Figure 6.14 – “T” joint specimens 6 - 16
Figure 6.15 – Test fixtures 6 - 17
Figure 6.16 – “T” joint test set-up 6 - 17
Figure 6.17 – Typical failure mode of Type 1 (lap joints) 6 - 18
Figure 6.18 – Typical load vs displacement curve for Type 1 (lap joints) 6 - 18
Figure 6.19 – Typical failure mode of Type 2 (loop joints) 6 - 19
Figure 6.20 – Typical load vs displacement curve for Type 2 (loop joints) 6 - 19
Figure 6.21 – Extracted Pratt joint 6 - 21
Figure 6.22 – Joint geometry (solid areas hatched) 6 - 21
Figure 6.23 – Full casting mould 6 - 23
Figure 6.24 – Finished PFR core 6 - 24
Figure 6.25 – Finished Type 1 joint 6 - 24
Figure 6.26 – Finished Type 2 joint 6 - 25
Figure 6.27 – Actual PFR core section 6 - 25
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Development and Structural Investigation of Monocoque Fibre Composite Trusses xv
Figure 6.28 – Finished Type 4 joint 6 - 26
Figure 6.29 – Schematic test rig 6 - 27
Figure 6.30 – Test set-up 6 - 27
Figure 6.31 – Typical failure mode of Type 1 joint 6 - 28
Figure 6.32 – Typical failure mode of Type 2 joint 6 - 29
Figure 6.33 – Typical failure mode of Balsa core 6 - 29
Figure 6.34 – Failed PFR core 6 – 30
Figure 6.35 - First noise location 6 – 32
Figure 6.36 – Finite element mesh, loading and restraints 6 – 34
Figure 6.37 – Finite element model section geometry and material
properties
6 - 35
Figure 6.38 – Bending moment distribution in simply supported, four-
panel, rigid jointed Pratt truss
6 - 36
Figure 6.39 – Member stress distribution with secondary stresses 6 - 37
Figure 6.40 – Effect of stress reversal on member strain in primary tensile
member
6 - 38
Figure 6.41 – Relaxation of primary stress 6 - 39
Figure 6.42 – Deflection of fixed-end beam 6 - 40
Figure 6.43 – Deflection of truss panel 6 - 40
Figure 6.44 – Four-panel MFC Pratt truss frame model with UDL 6 - 41
Figure 6.45 – Truss deflection 6 - 41
Figure 6.46 – Typical 2D Plate element Pratt joint model 6 - 42
Figure 7.1 – Truss geometries and dimensions 7 - 2
Figure 7.2 – Typical joint details 7 - 3
Figure 7.3 – Nominal truss core cross section dimensions 7 - 3
Figure 7.4 – Void extents 7 - 4
Figure 7.5 – Fibre architecture 7 - 5
Figure 7.6 – Typical truss member cross section 7 - 5
Figure 7.7 – Strain measurement locations 7 - 7
Figure 7.8 – Strand 7 plate element types used in finite element analysis 7 - 8
Figure 7.9 – Final finite element meshes for Pratt, Howe and Warren
trusses
7 - 8
Figure 7.10 – Number of layers per face in MFC truss tension members 7 - 11
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Development and Structural Investigation of Monocoque Fibre Composite Trusses xvi
Figure 7.11 – Typical MFC truss member RVE 7 - 12
Figure 7.12 – Symmetric FEA model of RVE 7 - 12
Figure 7.13 – Deformed RVE, εeq = 6.3E-5mm/mm 7 - 13
Figure 7.14 – Typical material properties for tension members (Pratt truss
shown)
7 - 16
Figure 7.15 – Number of layers per face in MFC truss compression
members
7 - 18
Figure 7.16 – Typical material properties for compression members (Pratt
truss shown)
7 - 19
Figure 7.17 – Material properties and orientations in a typical Pratt truss
joint
7 - 21
Figure 7.18 – Typical detailed and simplified truss joints 7 - 23
Figure 7.19 – Typical detailed and simplified truss joints 7 - 24
Figure 7.20 - ε11 Max for different joint forms 7 - 25
Figure 7.21 – Typical displacement points (Pratt truss shown) 7 - 27
Figure 7.22 – Typical isotropic material properties used in truss joints 7 - 29
Figure 7.23 – Typical external loading and constraints (Pratt truss shown) 7 - 30
Figure 7.24 – Truss midspan top chord deflections 7 - 31
Figure 7.25 – Pratt truss FE model – fibre direction strain distribution 7 - 33
Figure 7.26 – Pratt truss local compressive strain concentration 7 - 34
Figure 7.27 – Howe truss FE model – fibre direction strain distribution 7 - 34
Figure 7.28 – Howe truss local tensile strain concentration 7 - 35
Figure 7.29 – Warren truss FE model – fibre direction strain distribution 7 - 35
Figure 7.30 – Warren truss local tensile strain concentration 7 - 36
Figure 7.31 – Typical mould configuration 7 - 38
Figure 7.32 – Typical void former restraint 7 - 39
Figure 7.33 – Typical finished MFC trusses 7 - 40
Figure 7.34 – Typical MFC member cross section 7 - 42
Figure 7.35 – Typical MFC truss joints 7 - 42
Figure 7.36 – Peel ply at strain gauge locations 7 - 45
Figure 7.37 – Typical strain gauge locations 7 - 45
Figure 7.38 – Typical test setup (Pratt truss shown) 7 - 46
Figure 7.39 – MFC truss load versus displacement graphs 7 - 47
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Development and Structural Investigation of Monocoque Fibre Composite Trusses xvii
Figure 7.40 – Strain versus load curves for Pratt, Howe and Warren trusses 7 - 49
Figure 7.41 – Failure zone of Pratt truss 7 - 51
Figure 7.42 – Location of first noise in Pratt truss 7 - 51
Figure 7.43 – Failure zone of Howe truss 7 - 52
Figure 7.44 – Location of first noise in Howe truss 7 - 52
Figure 7.45 – Failure zone of Warren truss 7 - 53
Figure 7.46 – Location of first noise in Warren truss 7 - 53
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Development and Structural Investigation of Monocoque Fibre Composite Trusses xviii
List of Tables Table 2.1 – Material cost comparison between traditional materials and
fibre composites
2 - 10
Table 2.2 – Indicative Strength/dollar ratio for different materials 2 - 11
Table 2.3 – Comparison of specific strength and specific elastic modulus
between traditional materials and fibre composite materials
2 - 16
Table 2.4 – Issues of emerging materials 2 - 36
Table 3.1 – Common characteristics of mechanical and adhesive joints 3 - 14
Table 3.2 – Member length and number of joints 3 - 34
Table 3.3 – Truss midspan deflection 3 - 35
Table 3.4 –Maximum combined stress in each truss 3 - 36
Table 3.5 – Percent increase in stresses due to joint rotation in each truss 3 - 37
Table 3.6 – Mechanical properties of ADR246 / West Systems 105 3 - 40
Table 3.7 – Mechanical properties of LB100 polystyrene foam typical 3 - 43
Table 3.8 – Mechanical properties of end-grain balsa (Diab, 2002) 3 - 47
Table 3.9 – PFR (44% SL150 filler by vol) typical mechanical properties 3 - 49
Table 3.10 – Summary of materials adopted 3 - 51
Table 3.11 – Contribution to member compressive stiffness by core
materials
3 - 57
Table 4.1 – Material properties for preliminary tension element tests 4 - 4
Table 4.2 – Tension test results 4 - 10
Table 4.3 – Additional tension test observations for Type 1 specimens 4 - 11
Table 4.4 – Core and hardpoint materials 4 - 19
Table 4.5 – Four core / laminated face systems 4 - 20
Table 4.6 – Material properties for prediction of PFR tension member pre-
cracking behaviour
4 – 23
Table 4.7 – Material properties of RVE used in FEA 4 - 25
Table 4.8 – Incremental equivalent elastic modulus 4 – 28
Table 4.9 – Material properties for prediction of plaster core tension
member behaviour
4 - 30
Table 4.10 – Material properties for foam core tension member behaviour 4 - 31
Table 4.11 – Material properties for neat resin core tension member 4 - 32
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Development and Structural Investigation of Monocoque Fibre Composite Trusses xix
behaviour
Table 4.12a – PFR core specimen results 4 - 39
Table 4.12b – Additional PFR core specimen results 4 - 40
Table 4.13 – Plaster core specimen results 4 – 40
Table 4.14 – Foam core specimen results 4 - 41
Table 4.15 – Neat resin core specimen results 4 - 41
Table 4.16(a) – Comparison of predicted results with experimental results 4 - 42
Table 4.16 (b) – Comparison of predicted results with observed continued 4 - 42
Table 4.17 – Difference in average ultimate properties of PFR and plaster /
foam core specimens
4 - 49
Table 5.1 – Mechanical properties of 720gsm UD Colan E-glass and BASS
Pacific 420gsm heatset UD E-glass
5 - 5
Table 5.2 – Adopted material properties 5 - 6
Table 5.3 – Predicted capacity of compression specimens 5 - 7
Table 5.4 – Comparison of test results with predictions 5 - 10
Table 5.5 – Slenderness ratio of test specimens 5 - 13
Table 5.6 – Calculation of predicted failure load for specimen W1 5 - 17
Table 5.7 – Specimen predicted failure loads 5 - 17
Table 5.8 – Material properties used in FEA of stocky member with non-
compact cross section
5 - 18
Table 5.9 – Test results for stocky specimens with a compact cross section 5 - 29
Table 5.10 – Slender specimen results summary 5 - 33
Table 5.11 – Comparison of predicted and test values 5 - 34
Table 5.12 – Flexural elastic modulus 5 - 37
Table 6.1 – Estimated joint and member strengths 6 - 3
Table 6.2 – FE model properties (refer Figure 6.7) 6 - 11
Table 6.3 – Stress / capacity ratio 6 - 12
Table 6.4 – Lap lengths 6 - 13
Table 6.5 – Failure loads of Type 1 (lap joints) 6 - 18
Table 6.6 – Failure loads of Type 2 (loop joint) 6 - 19
Table 6.7 – Test joint materials 6 - 22
Table 6.8 – Joint fibre architecture 6 - 22
Table 6.9 – Joint ultimate capacity 6 - 28
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Development and Structural Investigation of Monocoque Fibre Composite Trusses xx
Table 7.1 – RVE FEA model properties 7 - 13
Table 7.2 – Gross tension member tensile elastic moduli 7 - 14
Table 7.3 – Apparent tension member core tensile elastic moduli 7 - 15
Table 7.4 – RVE FEA model properties 7 - 18
Table 7.5 – Comparison of actual joint strains with simplified joint strains 7 - 26
Table 7.6 – Comparison of IP deflection 7 - 28
Table 7.7 –Truss deflections and strain gauge readings at 100kN load 7 - 32
Table 7.8 – Truss stiffness results from FE models 7 - 32
Table 7.9– Derived load carrying capacity 7 - 36
Table 7.10 – MFC truss weight, first noise load and crack spacing 7 - 48
Table 7.11 – Truss deflections at intermediate loads and ultimate load,
stiffness at failure and span-to-deflection ratio at failure
7 - 48
Table 7.12 – Summary of member strains at ultimate load 7 - 50
Table 7.13 – Truss stiffness comparison 7 - 54
Table 7.14 – Comparison of deflection and strain for Pratt Truss1 7 - 54
Table 7.15 – Comparison of deflection and strain for Howe Truss1 7 - 55
Table 7.16 – Comparison of deflection and strain for Warren Truss1 7 - 55
Table 7.17 – Strength / failure mode comparison summary (Pratt) 7 - 56
Table 7.18 – Strength / failure mode comparison summary (Howe) 7 - 56
Table 7.19 – Strength / failure mode comparison summary (Warren) 7 - 57
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Development and Structural Investigation of Monocoque Fibre Composite Trusses xxi
Notation
Abbreviations
AFRP = aramid fibre reinforced plastic (polymers)
CERF = Civil Engineering Research Fund
CFRP = carbon fibre reinforced plastic (polymers)
COV = coefficient of variation
CTE = coefficient of thermal expansion
DB = double bias fibres
FCDD = Fibre Composites Design and Development
FEA = finite element analysis
FRP = fibre reinforced plastic (polymers)
GFRP = glass fibre reinforced plastic (polymers)
gsm = grams per square metre
HFS = high failure strain
HSHF = high strength / high failure strain
HSLF = high strength / low failure strain
LFS = low failure strain
LSHF = low strength / high failure strain
LSLF = low strength / low failure strain
MFC = monocoque fibre composite
NASA = National Aeronautics and Space Administration
PFR = particulate filled resin
QUT = Queensland University of Technology
RVE = representative volume element
UD = unidirectional fibres
UDL = uniformly distributed load
USQ = University of Southern Queensland
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Development and Structural Investigation of Monocoque Fibre Composite Trusses xxii
Symbols
k = stiffness
σ = stress
ε = strain
δ = deflection
φ = ultimate limit state capacity factor
ν = Poisson’s ratio
σ11.c.ult = ultimate compressive stress in the fibre direction
ε11.c.ult = ultimate compressive strain in the fibre direction
ε11.t.ult = ultimate tensile strain in the fibre direction
σ11.t.ult = ultimate tensile stress in the fibre direction
ι12 = shear stress
σ22.c.ult = ultimate compressive stress perpendicular to the fibre direction
ε22.c.ult = ultimate compressive strain perpendicular to the fibre direction
σ22.t.ult = ultimate tensile stress perpendicular to the fibre direction
ε22.t.ult = ultimate tensile strain perpendicular to the fibre direction
εfirst noise = strain at which first noise occurs
σu = ultimate limit state stress
A = area
Acore = area of core
Afaces = area of faces
Ag = gross area
b = cross sectional width
d = cross sectional depth
E = elastic modulus
Ixx = second moment of area about the X-X axis
E11.c = compressive elastic modulus in the fibre direction
E11.t = tensile elastic modulus in the fibre direction
E11.t.core = tensile elastic modulus of core in the fibre direction
E11.t.eq = equivalent tensile elastic modulus in the fibre direction
E11.t.faces = tensile elastic modulus of faces (nominally 20 GPa for E-glass)
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Development and Structural Investigation of Monocoque Fibre Composite Trusses xxiii
E22.c = compressive elastic modulus perpendicular to the fibre direction
E22.t = tensile elastic modulus perpendicular to the fibre direction
EAF1 = elastic modulus of face material in area 1
EAF2 = elastic modulus of face material in area 2
EBP = elastic modulus of “BASS Pacific” laminate
Eeq = equivalent elastic modulus based on transformed cross section
Ef = final elastic modulus
Ei = initial elastic modulus
EL = elastic modulus in the member’s longitudinal direction
EPFR = elastic modulus of PFR
G12 = shear modulus
h = depth of truss (between panel points)
I = second moment of area
Imin = second moment of area about the minor principal axis
Ixx.AF1 = second moment of area of face area 1 about the X-X axis
Ixx.AF2 = second moment of area of face area 2 about the X-X axis
Ixx.BP
= second moment of area of “BASS Pacific” laminate about the X-X
axis
Ixx.PFR = second moment of area of PFR about the X-X axis
Iyy = second moment of area about the Y-Y axis
L = length
Lav = average debond length
Nc = nominal member capacity in compression
Ns = nominal section capacity of compression member
P = point load
PE = Euler critical buckling load
Sav = average crack spacing
T = tension force in truss member
t = thickness
w = uniformly distributed load
δ = deflection
ε = strain
ε11.c.ult = ultimate compressive strain in the fibre direction
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Development and Structural Investigation of Monocoque Fibre Composite Trusses xxiv
ε11.t.ult = ultimate tensile strain in the fibre direction
ε22.c.ult = ultimate compressive strain perpendicular to the fibre direction
ε22.t.ult = ultimate tensile strain perpendicular to the fibre direction
εfirst noise = strain at which first noise occurs
φ = ultimate limit state capacity factor
ι12 = shear stress
ν = Poisson’s ratio
σ = stress
σ11.c.ult = ultimate compressive stress in the fibre direction
σ11.t.ult = ultimate tensile stress in the fibre direction
σ22.c.ult = ultimate compressive stress perpendicular to the fibre direction
σ22.t.ult = ultimate tensile stress perpendicular to the fibre direction
σu = ultimate limit state stress
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Development and Structural Investigation of Monocoque Fibre Composite Trusses 1 - 1
Chapter 1 - Introduction
Fibre composite materials are gaining recognition in civil engineering applications as
a viable alternative to traditional materials. Their migration from customary
automotive, marine, aerospace and military industries into civil engineering has
continued to gain momentum over the last three decades as new civil engineering
applications develop. The use of fibre composite materials in civil engineering has
now evolved from non-structural applications, such as handrails and cladding, into
primary structural applications such as building frames, bridge decks and concrete
reinforcement.
In recent years, researchers around the world have been seeking to develop new and
innovative structural forms that can successfully exploit the benefits offered by these
materials (Karbhari & Zhao, 2000; Gowripalan, 1999; van Erp, 1999d; Hooks et. al.,
1997 and Creative Pultrusions, 2002a). One structural form that appears to have
significant potential is the truss. The framed nature of trusses can be used to
minimise the shortfalls of fibre composite materials and maximise their benefits. To
date, much of the development of fibre reinforced polymer (FRP) trusses has
focussed on connection of continuous profile members using mechanical joints or
secondary bonding (Creative Pultrusions, 2002b; NASA, 2002; Goldsworthy 1995;
Morsi & Larralde, 1994a and Strongwell, 2002). Mechanical joints can introduce
performance compromises into fibre composite trusses while the use of continuous
profile members tends to reduce the freedom of the designer to use materials
efficiently and effectively.
This dissertation presents an investigation into a novel fibre composite truss
proposed by The University of Southern Queensland Fibre Composite Design and
Development. The truss uses monocoque construction to eliminate the use of
mechanical fasteners or secondary bonding to connect members, whilst allowing the
designer freedom to locate material where it will perform most efficiently and have
the greatest effect. It will be shown that structures of this type can be developed with
predictable structural behaviour and a level of strength and stiffness suitable for civil
engineering applications.
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Development and Structural Investigation of Monocoque Fibre Composite Trusses 1 - 2
1.1 Background
Composite materials combine and maintain two or more distinct phases to produce a
material that has properties far superior than either of the base materials. Fibre
composites are two-phase materials in which one phase reinforces the other. High
strength fibres are used as the primary means of carrying load and a polymer resin
binds the fibres into a cohesive structural unit. The combination of fibres and resin
produces a bulk material with strength and stiffness governed primarily by the fibres
and chemical resistance provided by the resin.
Evidence has been found to suggest that fibre composite materials have been used in
construction for thousands of years. Straw has been used to reinforce bricks for over
3000 years and this method is still used today. Chinese bridge builders used timber as
early as 1600 BC and Greek builders were apparently the first to reinforce masonry
with metal around 1000 BC. By comparison, the development of modern synthetic
fibre composites is relatively new, beginning in the early 20th century.
In the first half of the 1900’s the development of synthetic resins which could cure at
room temperature, combined with the serendipitous discovery of a method to
manufacture fine glass fibres, led to an increased use of synthetic fibre composites.
The unique properties offered by glass fibre reinforced composites made them
particularly desirable for marine, aerospace, military and automotive applications.
Material characteristics such as low weight, tailorable durability, good fatigue
resistance and manufacturing versatility were some of the characteristics initially
recognised as potential advantages over existing materials. Over the last sixty years
fibre composites have enjoyed widespread use in these industries and have evolved
significantly. Developments include production of advanced reinforcing fibres, such
as carbon and aramid, which offer improved strength and elastic modulus and better
impact performance over glass, and the evolution of modern polyester, vinylester and
epoxy resin formulations which offer improved performance over the original high
temperature curing phenolic resin in areas such as mechanical properties, chemical
resistance and bonding.
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Development and Structural Investigation of Monocoque Fibre Composite Trusses 1 - 3
As these materials find new applications their significance to engineering fields with
which they were previously unfamiliar is becoming more pronounced. In the last
twenty years fibre composite materials have established themselves as a viable and
competitive option for rehabilitation and retrofit of existing civil structures. By
externally bonding fibres, pre-fabricated strips and jackets to deteriorated or obsolete
structures, strength and stiffness can be re-introduced or improved to increase service
life. Fibre composites are also used to replace steel as a reinforcing and stressing
material in concrete for some specialised applications. To a lesser extent new civil
structures have been created almost entirely from fibre composite materials by
joining standard structural sections or modular components to produce complete
structures.
Civil and structural engineering is seeing an increased push for the use of these
materials in mainstream structures. This push is being driven by both composites and
civil engineering groups. Civil engineers are driven primarily by the desire to realise
the potential performance benefits offered by these materials. Potential benefits
include high strength, low weight and environmental durability. For the civil /
structural engineer high strength and light weight may translate into reduced
construction times and costs through the use of products which can be brought to
near-complete state in an off-site factory and then easily transported to site for rapid
deployment. The advantages of a material with superior durability include potential
reductions in maintenance over the life of a structure and hence lower cost for the
owner.
The composites industry, on the other hand, is driven by a desire to participate in
what is arguably the world’s largest industry. The global civil engineering and
construction market turnover has been estimated at US$800 billion per annum
(Marsh, 2000). With the global composites market in the year 2000 being valued at
only around US$8 billion (Marsh, 2000), even a 1% stake in the construction market
would double its current size.
Regardless of the reasons, the international pressure to use composites in mainstream
civil engineering has been growing over the past decade. During this time a large
number of experimental and commercial structures have been constructed around the
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Development and Structural Investigation of Monocoque Fibre Composite Trusses 1 - 4
world to demonstrate the potential of composites in major civil engineering
structures. Figure 1.1 provides examples of these structures.
While these applications help to demonstrate that fibre composite materials are
structurally capable, there are issues which are slowing the ingress of fibre composite
materials into the civil engineering industry. One such issue is that there is currently
a significant cost premium associated with their use.
b) Road Bridge - AUS (Source: FCDD, 2002)
a) Lattice Towers - USA
(Source: Strongwell, 2002)
c) Dome roof structures – Libya
(Source: Hollaway, 2002)
Figure 1.1 – Applications of fibre composites in civil engineering structures
Other issues exist such as difficulties in realising potential benefits, general lack of
civil engineers’ familiarity with the material and relatively little standardisation in
the composites industry. For composites to truly offer a viable alternative to
traditional construction materials in the civil engineering marketplace, it is essential
that these issues are addressed. It is proposed that this situation could be improved by
demonstrating that potential benefits offered by composites can be achieved with
hallaThis figure is not available online. Please consult the hardcopy thesis available from the QUT Library
hallaThis figure is not available online. Please consult the hardcopy thesis available from the QUT Library
hallaThis figure is not available online. Please consult the hardcopy thesis available from the QUT Library
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Development and Structural Investigation of Monocoque Fibre Composite Trusses 1 - 5
familiar civil engineering forms that are well suited to fibre composite materials and
whose structural behaviour is consistent with safe civil engineering structures and
predictable with existing structural engineering analysis and design methods.
One structural form that would appear to offer significant potential for composites in
civil engineering is the truss. Trusses have been accepted as efficient structural
elements for centuries and offer a number of advantages over solid web members.
Typically they use much less material than solid web members and their framed
nature allows material to be located where it has the greatest effect. Given the
significantly higher costs of fibre composite laminates in comparison to traditional
structural materials such as steel and concrete, this low material usage may address
some of the cost disparities between traditional and composite structures. In addition
to this, the loading within truss elements is largely axial. It is thought that this would
enable the easy tailoring of reinforcement along the load paths, resulting in a high
level of efficiency in material usage.
The concept of a fibre composites truss is not entirely new. Several examples of this
type of structure have been constructed around the world using a variety of joint
configurations (see Figure 1.2). However, these trusses are very much a reflection of
traditional technology developed for timber or metal members. As a result they fall
short of fully exploiting the potential offered by composites.
a) FRP truss bridge with pultruded
members (Source: Berenberg, 1997)
b) Bolted connection of pultruded
members (Source: Berenberg, 1997)
Figure 1.2 – Examples of fibre composite trusses
hallaThis figure is not available online. Please consult the hardcopy thesis available from the QUT Library
hallaThis figure is not available online. Please consult the hardcopy thesis available from the QUT Library
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Development and Structural Investigation of Monocoque Fibre Composite Trusses 1 - 6
c) Inserts used to connect FRP truss
members (Source: Hollaway, 2002)
d) Serrated FRP truss joint
(Source: Goldsworthy 1995)
Figure 1.2 – Examples of fibre composite trusses
In 1998 the USQ proposed a new type of fibre composite truss. Unlike previous
composite trusses, the new concept was based on monocoque design and avoided the
need for secondary joints. Initial investigations into this truss were extremely
promising and the concept was identified as one with significant development
potential. However, there is a need to fully understand the structural behaviour of this
new monocoque fibre composite (MFC) truss before it can be used confidently.
1.2 Aims
The primary aim of this thesis is to develop and improve the fundamental
understanding of the structural behaviour of monocoque fibre composite trusses and
to advance the civil engineering community’s knowledge in the use of fibre
composite materials in civil engineering structures.
In fulfilling the broad aim above, the study will:
develop the MFC truss concept into a working structure
investigate the structural behaviour of the principal truss elements (tension
and compression members and joints)
hallaThis figure is not available online. Please consult the hardcopy thesis available from the QUT Library
hallaThis figure is not available online. Please consult the hardcopy thesis available from the QUT Library
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Development and Structural Investigation of Monocoque Fibre Composite Trusses 1 - 7
demonstrate that the structural behaviour of principal truss elements can be
predicted using well established structural engineering analysis and design
methods
design a prototype truss in Pratt, Howe and Warren configurations using
understanding gained from truss element and joint investigations
evaluate the structural performance of the three prototype trusses through
physical testing and computer analysis
characterise the behaviour of the MFC trusses in terms of stiffness, strength,
failure mode, predictability and warning of failure
conclude on the ability of the MFC trusses to provide a safe, predictable and
adequate structural solution
1.3 Scope
In order to maintain focus on the primary aims of this investigation, adhere to
budgetary constraints of this research project and work within limitations of available
fabrication and testing equipment, the following restrictions were imposed on the
scope of this project:
Truss tests were undertaken for a single load / support system with
defined restraint conditions and loading points
A single prototype of each full-scale MFC truss design was fabricated
and tested (in this thesis the term “full-scale” refers to trusses that are
the same size as the finite element model)
Structural behaviour investigations were limited to short-term,
pseudo-static loading at ambient temperature in a non-aggressive
environment
Long-term durability was not considered
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Development and Structural Investigation of Monocoque Fibre Composite Trusses 1 - 8
1.4 Thesis structure
Chapter 2 – Fibre Composites In Civil Engineering: presents a more in-depth
discussion of the background to this project. It examines the history of composites
and the potential benefits that make them attractive to civil engineers. Three basic
areas of fibre composite use in civil engineering type applications are discussed,
followed by an examination of the issues that affect the use of fibre composite
materials in civil engineering.
Chapter 3 – Monocoque Fibre Composites Trusses: introduces the MFC truss
concept and the methodology used to develop the concept into a working structure.
The Chapter begins with a brief discussion of the history of trusses and some of the
characteristics that saw them become the structure of choice for bridge construction
during mid 1800’s. Focus is then shifted to existing FRP trusses, a selection of which
are critically examined to identify potential shortfalls of the current approaches. The
philosophy behind the MFC truss is then presented and compared with existing FRP
truss construction, in particular how the MFC truss has the potential to overcome
some of the shortfalls of existing FRP truss technology. Chapter 3 then examines and
evaluates a number of materials and structural form options for the MFC truss,
finally presenting the adopted MFC truss configuration that will form the basis of the
study.
Chapter 4 - Static Structural Behaviour of MFC Truss Tension Elements:
presents the results of an investigation into the behaviour of axial MFC truss tension
elements constructed using the prototype cross-section developed in Chapter 3. MFC
truss tension members with a variety of core materials are investigated
experimentally and analytically to determine their load response, failure mode,
predictability and warning of failure. An incremental method based on established
structural engineering principles is developed to predict the tension member
behaviour. These predictions are compared to results obtained from testing to
determine their accuracy.
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Development and Structural Investigation of Monocoque Fibre Composite Trusses 1 - 9
Chapter 5 – Static Structural Behaviour of MFC Truss Compression Elements:
investigates the structural behaviour of typical MFC truss compression members.
Three classes of compression member are investigated, namely; stocky members
with a compact cross-section, stocky member with a non-compact cross section and
slender members. Established closed form solutions as well as finite element analysis
are used to predict their structural behaviour. These predictions are then compared to
results obtained through testing to determine the accuracy of the predictions.
Chapter 6 - Monocoque Fibre Composite Truss Joints: shifts the focus of the
investigation to the MFC truss joints. In this Chapter the work is primarily concerned
with the ability of the joint to provide adequate connection to members. Joint
strength is studied experimentally and analytically and the effect of the rigid nature
of the joint on connected members and truss deflection is investigated. Finally, the
suitability of a proposed simplified approach to joint analysis is discussed.
Chapter 7 - Static Structural Behaviour of MFC Trusses: applies the findings of
Chapters 4, 5 and 6 in the examination of a series of prototype truss designs. These
designs are analysed using finite element analysis techniques and predicted
behaviour is evaluated through fabrication and testing of several sample trusses.
Observations including strength, failure mode, stiffness and provision of warning of
imminent failure are made.
Chapter 8 – Conclusions and Recommendations: draws together key findings on
the structural behaviour of MFC trusses and methods for predicting such behaviour.
Finally, several recommendations for future work are identified and discussed.
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Development and Structural Investigation of Monocoque Fibre Composite Trusses 2 - 1
Chapter 2 - Fibre Composites in Civil Engineering
Synthetic fibre composite materials have existed for over a century and have been
used widely in military, aerospace, sporting and automotive applications. In the last
three decades they have been increasingly considered for civil applications mainly in
retrofit of existing structures, reinforcement for concrete and to a lesser extent
complete fibre composite structures. Their use in civil engineering applications has
been the focus of worldwide research by both the composites industry and the civil
engineering industry. This research has highlighted a number of potential benefits as
well as some concerns with the use of this material in civil engineering applications.
To date the bona fide use of fibre composite materials as load bearing structural
elements in civil applications remains rare and it is unclear why this is the case. This
chapter examines key fibre composites issues in relation to civil engineering, draws
conclusions on their viability as civil engineering materials and recommends a way
forward.
2.1 Introduction
Modern fibre composites originated in the late 19th century when the first man-made
polymer, phenol-formaldehyde, was reinforced with linen fibre to make Bakelite. In
1936, DuPont patented the first room temperature curing resin, unsaturated polyester.
The first epoxy resin system was produced in 1938 and Ciba introduced the widely
recognised Araldite epoxy resin system in 1942. At the same time reinforcing fibres
were undergoing rapid development and in 1941 Owens-Corning began production
of the world’s first woven glass fabric.
The defence and marine industries were amongst the first to exploit some of the
potential advantages of reinforced polymer composites such as relatively high
strength-to-weight ratio, good durability and fatigue performance and radiowave
transparency. The automotive industry was able to capitalise on characteristics such
as efficient production techniques leading to inexpensive tooling, rapid turn-around
production and high quality surface finishes. The Cold War prompted significant
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Development and Structural Investigation of Monocoque Fibre Composite Trusses 2 - 2
effort by the military in the development of fibre composite materials. This led to
improvements in composites processing technology and mechanical properties of
laminates. The reader is directed to Gibson (1994), Herakovich (1997), Swanson
(1997), Kaw (1997), Vinson & Chou (1995), Mallick (1997), Johnson (1994), Lubin
(1982), Ayers (2002) and Owens Corning (2002) for more detailed information.
The use of fibre composite materials in civil structures can be traced back to the
1960’s when glass reinforced plastic rods were used to reinforce concrete. In the
1970’s civil FRP structures included roofs, pedestrian bridges, pipes, in-ground tanks
and phone boxes (see Yoosefinejad and Hogg, 1997; Liao et al, 1998; Holloway,
2002 and Sharjah Airport Corporation, 2002). However, the end of the Cold War in
the late 1980’s contributed to a glut of fibre composite resources and the composites
industry began a concerted effort to migrate fibre composites technology to
infrastructure applications.
The 1990’s saw significant development in the application of fibre composite
technology to civil infrastructure led by rehabilitation and retrofit projects.
Developments were also made in the use of fibre composite materials as concrete
reinforcement and to a lesser extent civil structures comprised primarily of fibre
composite materials. These developments will be discussed in more detail in the next
section.
In the last few years the use of fibre composites in civil engineering applications has
made some progress. However, these materials have not enjoyed the level of
widespread acceptance predicted by its proponents over the last decade. This slow
ingress appears to be due to a number of issues, which are discussed later in this
chapter.
2.2 Fibre composite materials in construction and civil engineering
Non-structural fibre composites have enjoyed widespread use in the construction
industry for many years in non-critical applications such as baths and vanities,
cladding, decoration and finishing. However, the use of structural fibre composites in
critical load-bearing applications remains rare mainly consisting of rehabilitation and
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Development and Structural Investigation of Monocoque Fibre Composite Trusses 2 - 3
retrofit of existing infrastructure, reinforcement of new concrete structures and new
civil structures constructed predominantly of fibre composite materials. These are
discussed next.
2.2.1 Rehabilitation and retrofit
Rehabilitation and retrofit currently represents the largest structural use of fibre
composite materials in civil engineering applications. The widespread deterioration
of infrastructure in Canada, the USA and Europe is well documented (Head, 1994;
Karbhari, 1997, 1998, 2000; Rizkalla, 1999 and Green, 2000). The estimated cost to
rehabilitate and retrofit existing infrastructure worldwide is around CAD$900B
(ISIS, 1998). In Australia it is estimated that $500M per annum is required to repair
and upgrade concrete structures (Oehlers, 2000). In addition to this a large amount of
infrastructure is reaching the end of its design life as revisions in structural codes and
loading codes combined with increased traffic demands are raising load limits on
existing infrastructure. Earthquakes in Loma Prieta (1989), Northridge (1994) and
Kobe (1995) have demonstrated the vulnerability of many of the existing concrete
structures to the effects of earthquake (Karbhari, 1998; Rizkalla, 1999). In many
cases demolition and rebuilding of these structures is difficult to justify in
economical terms so engineers seek inexpensive and effective methods to strengthen
them.
Traditional rehabilitation and retrofit methods use concrete or external steel sheets to
re-introduce or improve structural properties such as strength and ductility. The
ability of concrete to form complex shapes and its suitability to submerged
installation has seen it used for encapsulation of elements such as bridge piers
(Carse, 1997). However, concrete’s relatively low stiffness, high density, frequent
requirement of complex formwork and difficulties in achieving sufficient bonding to
the substrate and sufficient compaction to properly protect reinforcing steel are seen
as drawbacks in general retrofit applications. Steel, on the other hand, can be bonded
or bolted to deteriorated concrete structures to provide strength and stiffness
improvements with relatively little additional weight. However, steel plates can be
difficult to use on complex shapes and protective coatings required by steel plates
can be compromised during installation. Both the concrete and steel systems tend to
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Development and Structural Investigation of Monocoque Fibre Composite Trusses 2 - 4
inherently provide additional stiffness to the structure which can attract additional
load. This can be a disadvantage in some cases, particularly if foundation capacity is
limited.
Fibre composites are often used as a surface layer that either protects and/or
improves the response of the encapsulated element. In these cases the materials are
usually bonded externally to the structure in the form of tows (fibre bundles), fabrics,
plates, strips or jackets. The strength of circular or near circular concrete members
can be improved through confinement provided by tangentially oriented reinforcing
fibres without the introduction of significant stiffness. Strength or stiffness
improvement in bending members, such as beams and slabs, can be achieved by
bonding laminated strips to the underside of the member.
The advantages offered by composites in these forms include their ability to bond
well to many substrate materials and to follow complex shapes. Composites also
offer a potential benefit over isotropic retrofit materials, such as steel, by allowing
enhancement of strength without increasing stiffness and vice versa. This can be an
advantage for strength enhancement of bridge piers where increasing stiffness could
attract unwanted extra load.
An important consideration in the decision to use traditional or FRP rehabilitation
methods is cost. FRP rehabilitation methods are available, but they are currently
carried out by specialist subcontractors and tend to be expensive. However, Carse
(2003) provides an example of an application in which FRP rehabilitation was
competitive on cost and provided a more desirable solution in terms of aesthetics and
protection compared to traditional methods. In this case bridge piers were
rehabilitated with concrete below the waterline and FRP above the waterline.
2.2.2 Concrete structures reinforced with fibre composites
Concrete reinforced with FRP materials has been under investigation for decades.
Unstressed FRP reinforcement has been developed in a number of forms including
ribbed FRP rod similar in appearance to deformed steel reinforcing bar, undeformed
E-glass and carbon fibre bar bound with polyester, vinylester or epoxy resin, E-glass
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Development and Structural Investigation of Monocoque Fibre Composite Trusses 2 - 5
mesh made from flat FRP bars and prefabricated reinforcing cages using flat bars and
box sections (Shapira et al, 1997; Ko, 1997; Harris, 1998 and Gowripalan, 1999).
Stressed FRP reinforcement is also available, usually consisting of bundles of rods or
strands of fibre reinforced polymer running parallel to the axis of the tendon. These
are used in a similar fashion to conventional steel tendons (El Kady et al, 1999 and
Gowripalan, 2000).
The durability performance of FRP reinforcements is considered by Ko (1997),
Harris (1998) and Gowripalan (1999) to offer a possible solution to the problem of
corrosion of steel reinforcement, a primary factor in reduced durability of concrete
structures. Other reported advantages of FRP rebar include enhanced erection and
handling speeds (Karbhari, 1999) and suitability to applications which are sensitive
to materials which impede radiowave propagation and disturb electromagnetic fields.
However, in many cases corrosion of reinforcing steel can be traced back to
deterioration of concrete resulting from poor design, materials or workmanship.
Well-designed steel-reinforced concrete can produce extremely durable structures
and examples exist of concrete elements found to be in excellent condition despite
continuous tidal zone wetting and drying in a saline marine environment for over 70
years (Carse, 1997). Good design and construction is likely to be a more
economically feasible approach than use of expensive FRP rebar which can be up to
eight times (Tilco, 2002) as expensive as uncoated reinforcing steel and around one
and a half times as expensive as stainless steel reinforcement (Arminox, 2003).
In terms of erection and handling speeds, FRP rebar can sometimes prove more
difficult to work on site than traditional reinforcing steel. An example of this is the
common requirement for reinforcing steel to be bent on site, a characteristic not
possessed by thermoset FRP rebars. In relation to radiowaves and electromagnetic
fields, carbon fibre rebar would offer little benefit over traditional steel
reinforcement as it is not transparent to radio waves or electromagnetic radiation. In
these cases E-glass bars are often used as they are able to combine good radiowave
and electromagnetic transparency with adequate structural performance. However,
GFRP rebar has disadvantages such as relatively low strength, which could require
up to seven times the area of steel to satisfy deemed-to-comply requirements of
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Development and Structural Investigation of Monocoque Fibre Composite Trusses 2 - 6
AS3600 for minimum strength (SA, 2001) and susceptibility to stress rupture which
can require a large amount of reinforcing bar to keep stresses low. In addition to
this, the relatively low stiffness of GFRP rebar can require deeper sections and
greater reinforcing areas to achieve serviceability limits.
Applications that are not sensitive to radiowave or electromagnetic transparency may
use stronger and stiffer carbon fibre rebar. CFRP rebar can offer up to three times the
stiffness and three times the strength of GFRP rebar. However, these bars are usually
more expensive than GFRP rebars and in most cases will never be able to develop
their claimed high strength due to stringent serviceability limits which restrict the
maximum strain developed at serviceability failure. For example, members
supporting masonry commonly adopt a deflection limit of around span/500 (SA,
2001). Based on this deflection limit, a concrete member with a typical span-to-depth
ratio of around 12 will develop approximately 0.1% strain in the rebar. Carbon fibre
typically has an ultimate strain of around 1% and as CFRP rebar is predominantly
unidirectional, the material will be used to approximately 1/10 of its capacity at the
serviceability limit state. The result is inefficient use of FRP material and difficulties
in producing a serviceable concrete member. It is likely that unstressed FRP rebar
will be limited to applications not governed by serviceability and will need to
consider the material cost fibre composites compared to other available traditional
materials. This is discussed further in Section 2.3.1.
On the other hand, stressed FRP rebar can allow more efficient materials usage at
serviceability limit states. The action of pre-stressing can take up some of the excess
strain capacity of FRP materials allowing fuller development of the material’s
characteristic high strength and production of a large area of concrete in compression
resulting in a stiff and strong concrete member. However, tendons are required to
carry sustained load for long periods of time and issues such as stress rupture and
creep must be addressed. Materials such as CFRP and AFRP are generally favoured
over GFRP as they are not susceptible to stress rupture, except at higher stress levels,
and their tendency to creep can be accommodated at the design stage by over
stressing. A significant difference between FRP stressing tendons and steel tendons
is that unidirectional FRP tendons are predominantly brittle and do not cope well
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Development and Structural Investigation of Monocoque Fibre Composite Trusses 2 - 7
with stress concentrations at crack locations, unlike ductile steel. It is therefore
advisable for FRP tendons not to be bonded to the concrete.
The use of stressed FRP reinforcement can provide a more economical use of
material than unstressed FRP and therefore will probably be favoured in applications
requiring FRP reinforced concrete for reasons of cost.
2.2.3 New fibre composite civil structures
A small number of new load bearing civil engineering structures have been made
predominantly from FRP materials over the last three decades. These include
compound curved roofs (Hollaway, 2002 and Sharjah Airport Corporation, 2002)
pedestrian and vehicle bridges and bridge decks (Hazen and Bassett, 1998; Karbhari,
2000; Kollar, 1998 and FHWA, 2002), energy absorbing roadside guardrails (Bank
and Gentry, 2000), building systems (Barbero and GangaRao, 1991), access
platforms for industrial, chemical and offshore (Hale, 1997), electricity transmission
towers, power poles, power pole cross-arms and light poles (Goldsworthy, 1998;
ISIS, 1998 and Weaver, 1999), modular rooftop cooling towers (Barbero and
GangaRao, 1991) and marine structures such as seawalls and fenders (Weaver,
1999). The benefits most often claimed to be offered by fibre composites include
high specific strength and specific stiffness, tailorable durability, good fatigue
performance and the potential to reduce long-term costs. However, in many cases
these claims are difficult to substantiate and are often based on sparse and irrelevant
data. Currently many civil engineers are sceptical of the material’s ability to provide
a viable alternative to traditional materials and bona-fide applications are scarce.
Many of the existing applications are experimental in nature and are aimed at
demonstrating the ability of fibre composite materials to perform in certain
applications. To this end they are often successful in terms of structural performance,
but offer little by way of meaningful financial performance data. In most cases
groups of interested parties combine to design, fabricate and install the structure at a
reduced cost. Examples of this cooperation are evident in projects such as the
Bennett’s Creek crossing on New York State route 248 (Allampali et al, 2000) and
the Tech 21 road bridge (Farhey, 2000).
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Development and Structural Investigation of Monocoque Fibre Composite Trusses 2 - 8
Allampali et al (2000) provides some financial data for the Bennett’s Creek crossing.
These figures indicate a cost of around US$400,000 to produce the temporary bridge,
based on reduced rates and omitting some items such as deck wearing surface and
some engineering costs. Based on average costs to produce a similar pre-stressed
concrete plank bridge in Australia, at around $1000/m2, it is likely that a complete
permanent bridge could have been built for half the cost of the FRP option.
However, the extra cost of an FRP option could be amortised over the expected
service life of a project. This would offer little justification in the case of a temporary
bridge but, as will be discussed later, may be a significant consideration in structures
with a long service life, or a temporary structure which can be re-used. This would
only be an advantage if potential benefits such as high durability could be realized to
keep maintenance costs down and provided that the material and structure were
durable enough to allow multiple re-use.
New civil structures may also benefit from the ability to produce large modular
components allowing rapid deployment of an FRP structure, although the benefit of
this would be most significant where the cost of public inconvenience and traffic
management is high. Other potential benefits such as high specific stiffness and high
specific strength may exist, but at a cost which makes them uncompetitive with
traditional materials. These issues suggest that fibre composites would mainly suit
non-standard civil applications or those which can balance extra cost against some
unique composites property.
2.3 Issues affecting the use of fibre composites in civil engineering applications
To date fibre composite materials have not enjoyed widespread use in civil projects.
To understand the reasons for this it is necessary to examine the key issues affecting
the use of fibre composite materials in structures in a civil engineering context. A
number of issues affecting the use of fibre composite materials in civil engineering
applications have been highlighted by research undertaken over the last decade
(Ballinger, 1990; Meier, 1991; Gangarao, 1993; Morsi and Larralde, 1994; Karbhari,
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Development and Structural Investigation of Monocoque Fibre Composite Trusses 2 - 9
1996; Scalzi et al, 1999; van Erp et al, 2000). The following key issues will be
examined in the context of application to civil engineering:
1. Cost
2. Structural performance
3. Durability
4. Familiarity and education
5. Specification and standardisation
6. Compatibility
7. Temperature and fire performance
2.3.1 Cost
In most civil engineering structures, good design requires provision of a solution
which can satisfy design requirements for the lowest cost. Cost can be considered in
terms of short-term costs, such as design, construction and installation, and long-term
costs such as maintenance, modification, deconstruction and disposal. These can be
further grouped into direct costs, such as materials and production, and indirect costs,
such as interruptions to traffic, depreciation, resale value and impact on the
environment.
Short term costs of fibre composites
Currently, fibre composite materials are expensive when compared to conventional
construction materials on an initial cost basis. This is demonstrated in Tables 2.1 and
2.2 which compare the cost per stiffness and cost per strength of FRP materials with
traditional construction materials. In engineering terms stiffness can be expressed as
LEAk =
Where E is the Modulus of Elasticity of the material (N/mm2), A the area (mm2) and
L the length (mm).
Strictly speaking this equation applies to axial loading only but it can also be used to
compare materials loaded in bending assuming the dimensions of the cross section
are fixed.
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Development and Structural Investigation of Monocoque Fibre Composite Trusses 2 - 10
Determining how much “stiffness” can be bought for a dollar is a realistic way to
compare the stiffness capabilities of different materials. The cost of a material can
be expressed as
( ) ( ) densitylengthareakgAcost ×××= /$
Assuming a length of 1 m, the stiffness in N/mm per dollar is given by
( ) densitykgAEk
×=
/$1000 .
Table 2.1 – Material cost comparison between traditional materials and fibre
composites
Material E
(N/mm2)
Cost
(A$/kg)
Density
(kg/m3)
Stiffness/dollar
(N/mm per A$)
Pultruded glass composites 30,000 7.00 1,800 2,381
Carbon composites 90,000 30.00 1,400 2,142
Standard Construction Steel 200,000 2.50 7,850 10,190
Steel Rebar 200,000 1.20 7,850 21,230
Stainless Steel Rebar 200,000 6.00 7,850 4,246
50 MPa concrete 30,000 0.10 2,500 120,0001
Hard wood timber 16,000 2.50 650 9,846
New polymer concrete 11,000 0.75 1,900 7,719
1. Compression only
A similar table can be assembled for strength. For a 1 m long bar, the load carrying
capacity per dollar can be expressed as:
( ) densitykgANu
××
=/$
106φσ
where φ is an indicative ultimate limit state capacity factor and uσ is the ultimate
limit state stress (N/mm2).
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Development and Structural Investigation of Monocoque Fibre Composite Trusses 2 - 11
Table 2.2 – Indicative strength/dollar ratio for different materials
strength/dollar
(N/A$)
Material uφσ
(N/mm2)
Cost
(A$/kg)
Density
(kg/m3)
tension compression
Pultruded glass
composites
0.6x600(T*)
0.6x480(C)
7.00 1,800 17,143
13,714
Carbon composites 0.7x900(T)
0.6x720(C)
30.00 1,400 15,000
12,000
Standard Construction
Steel
0.9x300 2.50 7,850 13,758 13,758
Steel Rebar 0.9x500 1.20 7,850 47,770 47,770
Stainless Steel Rebar 0.8x500 6.00 7,850 8,492 8,492
50 MPa concrete 0 (T)
0.8x50(C)
0.10 2,500 0
160,000
Hard wood timber 0.6x40(T)
0.6x50(C)
2.50 650 14,770
18,462
New polymer concrete 0.6x10(T)
0.6x50(C)
0.75 1,900 4,210
21,053
(*T is tension, C is compression)
These results clearly show that fibre composites struggle to compete financially with
traditional construction materials both in terms of stiffness and strength. Although
these performance criteria are rarely considered alone, they are often fundamental in
“material-evaluation”.
There are a number of factors contributing to the high cost of composite materials
including; high cost of raw materials and processing, the use of imported materials,
the general acceptance of high prices in markets such as marine and aerospace and
occasional low availability of material (Goldstein, 1996). The production of
materials locally is likely to reduce material cost, however with America and Europe
making up 35% and 27% of the worldwide market respectively (Weaver, 1999),
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Development and Structural Investigation of Monocoque Fibre Composite Trusses 2 - 12
there appears to be little incentive for material manufacturers to provide production
facilities in Australia.
In line with the evolution of other composites uses, such as sporting equipment,
many researchers and composites commentators believe it is likely that production
volume increases resulting from the use of fibre composites in civil engineering
applications will lead to decreased cost of materials (Karbhari, 1998a; Weaver, 1999;
Hastak and Halpin, 2000). However, in the Author’s opinion, this should be viewed
as an optimistic outlook. The majority of fibre composites materials used in Australia
are imported and are therefore subject to a range of international economic variables.
For example, fluctuations in the local price of imported materials would be affected
by overseas production costs, transport and import costs and fluctuations in the
exchange rate between Australia and countries such as Europe, United States and
Japan, which supply us with carbon, aramid, E-glass fibres and many resins.
When this is considered in conjunction with the tendency of suppliers to provide
price reductions for purchase of large quantities of some materials, accurate costing
of an FRP civil project can seem difficult. The uncertainty that exists in our ability to
accurately cost FRP civil project suggests that it could be some time before
anticipated price drops could significantly influence project cost.
Fabrication cost
In addition to relatively high material costs, the short-term cost of FRP materials is
dependant on fabrication. Most fibre composite manufacturing techniques were
originally developed for the aircraft, marine and/or car industries. The civil
engineering industry is vastly different to these as civil and structural engineers tend
to be concerned with the design and construction of rather large-scale structures.
Most of these structures have to meet different design specifications and therefore
very little duplication of design solutions occurs. As a result most civil engineering
projects tend to be ‘one-off’ jobs. This situation is in contrast with the manufacturing
industries, where mass production of one design solution is common. As a result,
design and manufacturing methods which are highly successful in the manufacturing
industry are often not viable in civil engineering.
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Development and Structural Investigation of Monocoque Fibre Composite Trusses 2 - 13
Most civil engineering applications of fibre composites are currently based around
the pultrusion process (similar to extrusion of aluminium). The pultrusion process is
ideal for the continuous production of elements of constant cross-sectional geometry
and moderate complexity. The advantages are relatively low labour cost, minimal
material wastage, consistent quality and high production rates. However, the
pultrusion process has some serious disadvantages such as high initial costs of setting
up for a production run and relatively few producers. Consequently, pultruders offer
a limited range of standard structural sections similar to those available in steel. A
non-standard section would require a significant volume to justify the high setup
cost. In many civil engineering projects, this high volume simply does not exist and
for these situations, a cheaper, more flexible manufacturing approach is required.
One of the most flexible methods of FRP fabrication is hand-layup. This method
allows individual placement of fibres and resin onto surfaces of virtually any
topography. However, this method tends to be inefficient and laborious and is
usually avoided as much as possible where large volumes are required. The most
likely way forward is through use of modular components fabricated using a
combination of manual and automated manufacturing procedures such as embedment
of standard pultruded sections in cast components, filament winding, automated tape
laying and resin transfer moulding. These procedures can be computer controlled to
produce accurate components with lower labour costs. Integration of these methods
could be particularly suitable for civil applications, which often require highly
aligned fibres and fabrication versatility.
Some short-term costs, such as transport and erection, may benefit from production
of large, lightweight, modular components. Lower weight can translate into reduced
transport and cranage costs, while the use of fewer large modular components can
reduce erection time. The implications on indirect short-term costs such as consumer
inconvenience and traffic management can be substantial. Meiers (2000) points out
that although it is difficult to quantify indirect savings, they have a cost that is
present. He believes that savings can be accrued at the systems level due to faster
construction thereby causing less distress and disruption to the community, lower
dead weight requiring smaller and l