Advance Design of Reinforced Concrete Structures CE-5115

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Department of Civil Engineering, University of Engineering and Technology Peshawar

Prof. Dr. Qaisar Ali CE 5115 Advance Design of Reinforced Concrete Structures

Advance Design of

Reinforced Concrete Structures

CE-5115

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By: Prof Dr. Qaisar Ali

Civil Engineering Department

UET [email protected]

www.drqaisarali.com


Prof. Dr. Qaisar Ali CE 5115 Advance Design of Reinforced Concrete Structures 2

Course Content

Lecture No.

Topic

1 Introduction

2 Materials

3 Design of RC Members for Flexure and Axial Loads

4 Design of RC Members for Shear and Torsion

5 Serviceability Requirements, Development and Splices of Reinforcement

6 Analysis and Design of RC Slabs

7 Idealized Structural Modeling of RC Structures

8 Gravity Load Analysis & Design of RC Structures

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Course Content

Lecture No.

Topic

9 Seismic Analysis & Design of RC Structures (Part-I)

10 Seismic Analysis & Design of RC Structures (Part-II)

11 Design of Beam-Column Connections in Monolithic RCStructures

12 Slenderness Effects in RC Structures

13 Analysis & Design of RC Shallow Footings

14 Special Topics: Strut and Tie Models, Brackets and

Corbels, Deep Beams etc.



� Midterm = 30 %

� Final Term = 60 %

� Assignment = 05 %

� Term Project = 05 %

� Attendance = 75 % is must to pass the course

� Final term exam also includes the course taught before mid term

exam.

Grading Policy

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� All lectures and related material will be available on

the website:

www.drqaisarali.com

Lectures Availability



Lecture-01

Introduction

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By: Prof. Dr. Qaisar Ali

Civil Engineering Department

UET Peshawar

[email protected]

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Topics Addressed

� Historical Development of Cement and Reinforced Concrete

� Building Codes and the ACI Code

� The Design and Design Team

� Concrete Structural Systems

� Limit States and the Design of Reinforced Concrete

� Basic Design Relationship



Topics Addressed

� Structural Safety

� Probabilistic Calculation of Safety Factors

� Design Procedure Specified in the ACI Code

� Design Loads for Buildings and other Structures

� Load Combinations used in the ACI code

� Strength Reduction Factors used in the ACI code

� Customary Dimensions and Construction Tolerances

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Historical Development of Cement and

Reinforced Concrete

� Cement

� In 1824 Joseph Aspdin mixed limestone and clay and heated them

in a kiln to produce cement.

� The commercial production started around 1880.

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Historical Development of Cement and

Reinforced Concrete

� Reinforced Concrete

� Joseph Monier, owner of a French nursery garden began

experimenting (in around 1850) on reinforced concrete tubs with

iron for planting trees.

� The first RC building in the US was a house built in 1875 by W. E.

Ward, a mechanical engineer.

� Working Stress Design Method, developed by Coignet in around

1894 was universally used till 1950.

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Building Codes and the ACI Code

� General Building Codes

� Cover all aspects of building design and construction from

architecture to structural to mechanical and electrical---. UBC, IBC

and Euro-code are general building codes.

� Seismic Codes

� Cover only seismic provisions of buildings such as SEAOC and

NEHRP of USA, BCP-SP 07 of Pakistan.




� Material Specific Codes

� Cover design and construction of structures using a specific

material or type of structure such as ACI, AISC, AASHTO etc.

� Others such as ASCE

� Cover minimum design load requirement, Minimum Design Loads

for Buildings and other Structures (ASCE7-02).

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� General Building Codes in USA

� The National Building Code (NBC),

� Published by the Building Officials and Code Administrators

International was used primarily in the northeastern states.

� The Standard Building Code (SBC),

� Published by the Southern Building Code Congress International was

used primarily in the southeastern states.

� The Uniform Building Code (UBC),

� Published by the International Conference of Building Officials, was

used mainly in the central and western United States.




� General Building Codes in USA

� The International Building Code IBC,

� Published by International Code Council ICC for the first time in 2000,

revised every three years.

� The IBC has been developed to form a consensus single code for USA.

� Currently IBC 2012 is available.

� UBC 97 is the last UBC code and is still existing but will not be updated.

Similarly NBC, SBC will also be not updated.

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� Seismic Codes in USA

� NEHRP (National Earthquake Hazards Reduction Program)

Recommended Provisions for the Development of Seismic

Regulations for New Buildings developed by FEMA (Federal

Emergency Management Agency).

� The NBC, SBC and IBC have adopted NEHRP for seismic design.

� SEAOC “Blue Book Structural Engineers Association of California

(SEAOC), has its seismic provisions based on the Recommended

Lateral Force Requirements and Commentary (the SEAOC “Blue

Book”) published by the Seismology Committee of SEAOC.

� The UBC has adopted SEAOC for seismic design.




� Building Code of Pakistan

� Building Code of Pakistan, Seismic Provision BCP SP-07 has

adopted the seismic provisions of UBC 97 for seismic design of

buildings.

� IBC 2000 could not be adopted because some basic input data

required by IBC for seismic design does not exist in Pakistan.

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� The ACI MCP

� ACI MCP (American Concrete Institute Manual of Concrete

Practice) contains 150 ACI committee reports; revised every three

years.

� ACI 318: Building Code Requirements for Structural Concrete.

� ACI 315: The ACI Detailing Manual.

� ACI 349: Code Requirement for Nuclear Safety Related Concrete

Structures.

� Many others.




� The ACI 318 Code

� The American Concrete Institute “Building Code Requirements

for Structural Concrete (ACI 318),” referred to as the ACI code,

provides minimum requirements for structural concrete design or

construction.

� The term “structural concrete” is used to refer to all plain or

reinforced concrete used for structural purposes.

� Prestressed concrete is included under the definition of reinforced

concrete.

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� The ACI 318 Code

� 7 parts, 22 chapters and 6 Appendices.

� Brief visit of the code




� Legal Status of The ACI 318 Code

� The ACI 318 code has no legal status unless adopted by a state or

local jurisdiction.

� It is also recognized that when the ACI code is made part of a

legally adopted general building code, that general building code

may modify some provisions of ACI 318 to reflect local conditions

and requirements.

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� The Compatibility Issue in BCP SP-2007

� Building Code of Pakistan, Seismic Provision BCP SP-07 has

adopted the seismic provisions of UBC 97 for seismic design of

buildings.

� As the UBC 97 has reproduced ACI 318-95 in Chapter 19 on

concrete, the load combinations and strength reduction factors of

ACI 318-02 and later codes are not compatible with UBC 97 and

hence BCP SP-07. Therefore ACI 318-02 and later codes cannot

be used directly for design of a system analyzed according to the

seismic provisions of UBC 97.




� The Compatibility Issue in BCP SP-2007

� To resolve this issue, BCP SP-2007 recommends using ACI 318-05

code for design except that load combinations and strength

reduction factors are to be used as per UBC 97.

� The IBC adopts the latest ACI code by reference whenever it is

revised and hence are fully compatible.

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� General:

� The design covers all aspects of structure, not only the structural

design.

� The structural engineer is a member of a team whose members

work together to design a building, bridge, or other structure.

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The Design and Design Team



� Liaison between Engineer and Architect

� Close cooperation with the architect in the early stages of a

project is essential in developing a structure that not only meets

the functional and aesthetic requirements but exploits to the

fullest the special advantages of reinforced concrete.

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� Four Major Objectives of Design

1. Appropriateness: This include,

� Functionality, to suit the requirements.

� Aesthetics, to suit the environment.

2. Economy

� The overall cost of the structure should not exceed the client’s

budget.




� Four Major Objectives of Design

3. Structural Adequacy (safety)

� Strength.

� Serviceability.

4. Maintainability

� The structure should be simple so that it is maintained easily.


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� Three Major Phases of Design

1. The client’s needs and priorities.

2. Development of project concept.

3. Design of Individual systems.




Concrete Structural Systems

� Selection Criterion

� Depending on structural spans, loading conditions, purpose of

building, availability of formwork, skilled labor and material etc., a

number of different structural systems such as flat plate, flat slab,

one-way or two way joist system etc. are possible.

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� Flat Plate

� A flat plate is a slab floor system in

which the slab of uniform thickness is

supported directly on columns.

� Flat plates are economical for:

� Short and medium spans

� Economical range: 15 ft – 25 ft, and

� Moderate live loads

� Punching shear is a typical problem in

flat plates.




� Flat Slab

� Beamless systems with drop panels or column capitals or both

are termed as flat slab systems.

� Short and medium spans, economical range 20 ft. – 30 ft.

� Drop Panel: Thick part of slab in the vicinity of columns

� Column Capital: Column head of increased size

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� Beam-Supported Slab

� In beam supported slab, the perimeter beams are usually concrete

cast monolithically with the slab, although they may also be

structural steel, often encased in concrete for composite action and

for improved fire resistance.

� Suitable for long spans and for parking structures.

� Specially for intermediate and heavy loads for span up to about 30 ft.




� Beam-and-Girder Floors

� A beam and girder floor consists of a series of parallel beams

supported at their extremities by girders, which in turn frame into

concrete columns placed at more or less regular intervals over entire

floor area.

� Adapted to any loads and spans. Normal range in column spacing isfrom 16 to 32 ft.


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� Banded-slab System

� For light loads, a floor system has been

developed in which the beams are

omitted in one direction, the one-way

slab being carried directly by column line

beams that are very broad and shallow.

� These beams, supported directly by the

columns, become little more than a

thickened portion of the slab. This type

of construction is known as banded slab

construction.



Rib


� One-Way Joist

� A one-way joist construction consists of a monolithic combination of

regularly spaced ribs and a top slab (T beam or Joist) arranged to

span in one direction.

� Long spans, economic range: 30 ft. – 50 ft.

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Joist


� Two-Way Joist

� A two-way joist system, or waffle slab, comprises evenly spaced

concrete joists spanning in both directions and a reinforced concrete

slab cast integrally with the joists.

� Like one-way joist system, a two way system will be called as two-

way joist system if clear spacing between ribs (dome width) does not

exceed 30 inches.




� Composite Construction with Steel Beams

� In this system, columns, beams, and girders consist of structural

steel whereas the floors are reinforced concrete slabs.

� The spacing of beams is usually 6 to 8 ft.

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� Steel Deck Reinforced Concrete Slab

� In this structural system, the steel deck serves as stay in place form

and, with suitable detailing the slab becomes composite with the

steel deck, serving as the main tensile flexural steel.

� The spacing of beams is usually 12 ft.




� Post-Tension Slab

� In the post-tensioned slab systems, hollow conduits are provided in

slab through which the tendons are placed. Tendons are tensioned

after the concrete has gained its strength. Columns and beams are

regular reinforced concrete members.

� Longer spans can be achieved due to pre-stress, which can be usedto counteract deflections.

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Limit State and the Design of

Reinforced Concrete

� Limit State

� When a structure or structural element becomes unfit for its

intended use, it is said to have reached a limit state.

� The three limit states

1. Ultimate Limit States

2. Serviceability Limit States

3. Special Limit States




Reinforced Concrete

� The Ultimate Limit States

� These involve a structural collapse of part or all of the structure.

� Such a limit state should have a very low probability of

occurrence, since it may lead to loss of life and major financial

losses

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Reinforced Concrete

� The Major UL States are

� Loss of equilibrium

� Rupture

� Formation of plastic mechanism

� Instability

� Progressive collapse

� Fatigue




Reinforced Concrete

� Serviceability Limit States

� These involve disruption of the functional use of the structure, but

not collapse.

� Since there is less danger of loss of life, a higher probability of

occurrences can generally be tolerated than in the case of an

ultimate limit state.

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Reinforced Concrete

� The SL States are

� Excessive deflections

� Excessive crack widths

� Undesirable vibrations




Reinforced Concrete

� Special Limit States

� This class of limit state involves damage or failure due to

abnormal conditions or abnormal loadings.

� The SpL States include

� Damage or collapse in extreme earthquakes.

� Structural effects of fire, explosions, or vehicular collisions.

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Reinforced Concrete

� Limit State Design of RC Buildings

� RC buildings are designed for ULS

� Subsequently checked for SLS

� Under special condition also checked for SpLS

� Note: SLS and not ULS may be governing LS for structures such as

water retaining structures and other structures where deflection and

crack control are important.



Basic Design Relationship

� The Capacity and Demand

� Capacity must be ≥ Demand (in same units)

� Demand: An imposed action on structure

� Capacity: The overall resistance of structure

� Load Effects: Bending, torsion, shear, axial forces, deflection,

vibration

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� The Capacity and Demand

� Capacity < Demand is failure

� Capacity > Demand is success with FOS

� Capacity = Demand is success without FOS

� Working Stress Design approach

� Capacity is reduced by half

� Demand is kept the same




� Limit State Design approach

� Capacity is reduced and demand is increased based on scientific

rationale. In LSD approach, we have

� φ Mn ≥ Mu (α Ms )

� φ Vn ≥ Vu (α Vs )

� φ Pn ≥ Pu (α Ps )

� φ Tn ≥ Tu (α Ts )

� φ = strength reduction factor

� α = load amplification factor

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Structural Safety

� Variability in Resistance

� The actual strengths (resistances) of beams, column, or other

structural members will almost always differ from the values

calculated by the designer (nominal strength). The main reasons

for this are as follows:

� variability of the strength of the concrete and reinforcement,

� differences between the as-built dimensions and those shown on the

structural drawings,

� effects of simplifying assumptions made in deriving the equations for

member resistance.



Structural Safety

� Variability in Resistance

� Effects of simplifying

assumptions

� The fig shows Comparison of

measured (Mtest) and

computed (Mn) failure

moments for 112 similar RC

beams

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Structural Safety

� Variability in Loads

� All loadings are variables, especially live loads and environmental

loads due to snow, wind, or earthquakes.

� In addition to actual variations in the loads themselves, the

assumptions and approximations made in carrying out structural

analysis lead to differences between the actual forces and

moments and those computed by the designer.



Structural Safety

� Variability in Loads

� Fig shows variation of Live loads

in a family of 151sft offices.

� The average (for 50 % buildings)

sustained live load was around

13 psf in this sample.

� 1% of measured loads exceeded

44 psf.

� Building code specify 50 psf for

such buildings (ASCE 7-02)

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Structural Safety

� Consequences of variability of load and resistance

� Due to the variability of resistances and load effects, there is

definite chance that a weaker-than-average structure will be

subjected to a higher- than-average load.

� In extreme cases, failure may occur.

� The load factors and resistance factors are selected to reduce the

probability of failure to a very small level.



Probabilistic Calculation of Safety

Factors

� Resistance vs. Load Effects� R = The distribution of a population of

resistance of a group of similar

structure.

� S = Distribution of the maximum load

effects, S, expected to occur on those

structure during their life times

� The 45° line in this figure corresponds

to a load effect equal to the resistance

(S = R).

� S > R is failure i.e., load effects greater

than resistance & S < R is Safety.

“S vs. R”

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Factors

� Resistance vs. Load Effects� Safety margin can be represented as Y

= R – S

� Graph shows plot between safety

margin (Y) and frequency of

occurrence (success or failure)

� If Y is greater than 0, then safety

margin exists and failure is avoided.

� Failure will occur if Y is negative,

represented by the shaded area in

figure.

Safety margin vs. frequency (success or failure)




Factors

� The Probability of Failure� The probability of failure, Pf, is the

chance that a particular combination of

R and S will give a negative value of Y.

� In normal distribution curve, Pf is equal

to the ratio of the shaded area to the

total area under the curve in figure.

� From the figure, mean value of Y is

given as Y = 0 + βσY

� Where, σY = Standard Deviation;β = 1,2,3

…

Safety margin vs. frequency (success or failure)

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Factors

� The Safety Index� Now larger the distance βσY, the lesser

will be the negative part and more will

be the positive part in the curve, which

means less chance of failure and more

safety. The factor β is called the safety

index.

� More positive part on the curve means

increasing R. But increase in

resistance will require compromise on

economy.




Factors

� Calculation of P f:

� The probability of failure (Pf) which is Probability that (Y = R – S)

< 0, can be calculated by converting the normal distribution

(which is function of Y) to standard normal distribution (which is

a function of Z ) and then using standard normal distribution

tables to find the area under the curve

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Factors

� Calculation of P f:

� For β = 3.5, the probability of failure P (Z) = 0.0001 = 0.01 % =

1/9091. (from standard statistics tables)

� It means that roughly 1 in every 10,000 structural members

designed on the basis that β = 3.5 may fail due to excessive load

or under strength sometime during its life time.




Factors

� Selection of P f and β

� The appropriate values of Pf and hence of β are chosen by

bearing in mind the consequences of failure.

� Based on current design practice, β is taken between 3 and 3.5

for ductile failure with average consequences of failure and

between 3.5 and 4 for sudden failure or failures having serious

consequences.

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Factors

� Selection of P f and β

� In strength design method, weknow that:

� γMs = ΦMn, therefore,

� Safety factor Mn/ Ms = γ/ Φ

� For ΦMn = 1.2MD + 1.6ML

� Let ML = MD ; thenΦMn = 2.8MD

� Therefore, Safety factor (Mn/MD)= 2.8/Φ

� For Φ = 0.65→ Mn/ MD = 4.3 ≈ β

� For Φ = 0.90→ Mn/ MD = 3.11 ≈ β



Design Procedure of the ACI Code

� 9.1.1- structures and structural members shall be designed to

have design strengths at all sections at least equal to the

required strength calculated for the factored loads and forces in

such combinations as are stipulated in this code.

� 9.1.2- members also shall meet all other requirements of this

code to ensure adequate performance at service load levels.

� This process is called strength design in the ACI code.

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Design Procedure of the ACI Code

� In the AISC Specifications for steel design, the same design

process is known as LRFD (Load and Resistance Factor

Design).

� Strength design and LRFD are methods of limit-state design,

except that primary attention is always placed on the ultimate

limit states, with the serviceability limit states being checked

after the original design is completed.



Design Loads in the ACI Code

� ACI 318-02, Section 8.2-LOADING:

� 8.2.2: Service loads shall be in accordance with the

general building code of which this code forms a part,

with such live load reductions as are permitted in the

general building code.

� Section R8.2 : The provisions in the code are for live, wind,

and earthquake loads such as those recommended in

“Minimum Design Loads for Buildings and Other

Structures,” (ASCE 7).

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� ACI 318-02, Section 8.2-LOADING:

� If the service loads specified by the general building code (of

which ACI 318 forms a part) differ from those of ASCE 7, the

general building code governs. However, if the nature of the

loads contained in a general building code differs considerably

from ASCE 7 loads, some provisions of this code may need

modification to reflect the difference




� ASCE Recommendations on Loads:

� ASCE 7-02 sections 1 to 10 are related to design loads for

buildings and other structures.

� The sections are named as: general, load combinations, dead,

live, soil, wind, snow, rain, earthquake and ice loads.

� Brief visit of ASCE 7-02, Section 1 to 10

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� Loads on Structure During Construction

� During the construction of concrete buildings, the weight of the

fresh concrete is supported by formwork, which frequently

rests on floors lower down in the structure.

� ACI section 6.2.2 states the following:

� No construction loads exceeding the combination of superimposed

dead load plus specified live load (un-factored) shall be supported

on any un-shored portion of the structure under construction, unless

analysis indicates adequate strength to support such additional

loads



Load Combinations of ACI and UBC

Section 9.2 of ACI 318-02 code

� 1.4D

� 1.2D + 1.6L

� 1.2D + 1.0E + 1.0L

� 0.9D + 1.0E

� Load Types

Dead = D, Live = L, Seismic = E

Section 1928.1.2.3 of UBC code

� 1.4D

� 1.4D + 1.7L

� 1.2D + 1.5E + 0.5L

� 0.9D + 1.5E

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Strength Reduction Factors Of ACI and

UBC

Strength Reduction Factors

ACI 318-02 Code, Section

9.3

φ

UBC 1909.3.2

φ

Tension-controlled 0.90 0.90

Shear and torsion 0.75 0.85

Compression-controlled (spiral)

0.70 0.75

Compression-controlled (other)

0.65 0.70

Bearing 0.65 0.70



Customary Dimensions and

Construction Tolerance

� The actual as-built dimensions will differ slightly from those

shown on the drawings, due to construction inaccuracies.

� ACI Committee 117 has published a comprehensive list of

tolerance for concrete construction and materials.

� As an example, tolerances for footings are +2 inches and – ½

inch on plan dimensions and – 5 percent of the specified

thickness.

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References

� Reinforced Concrete - Mechanics and Design (4th Ed.) by

James MacGregor.

� ACI 318-02

� PCA 2002



The End

Advance Design of Reinforced Concrete Structures CE-5115

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