Department of Civil Engineering, University of Engineering and Technology Peshawar
Advance Design of Reinforced Concrete Structures CE-5115 By: Prof Dr. Qaisar Ali Civil Engineering Department UET Peshawar drqaisarali@nwfpuet.edu.pk mumarbsc@yahoo.com Prof. Dr. Qaisar Ali
CE 5115 Advance Design of Reinforced Concrete Structures
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Department of Civil Engineering, University of Engineering and Technology Peshawar
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 Length
6
Concrete Structural Systems
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Course Content Lecture No.
Topic
7
Analysis and Design of Slab Systems: One Way Slabs, One Way Joist Systems
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Analysis and Design of Two-way Slab System without Beams (Flat Plate and Flat Slabs), Two Way Joist Slabs & Two-way Slabs with Beams
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Idealized Structural Modeling of RC Structures
10
Gravity Load Analysis of RC Structures
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Case Studies on Gravity Load Analysis of RC Structures
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Course Content Lecture No.
Topic
12
Earthquake Resistant Design of RC Structures
13
Design of Beam-Column Connections in Monolithic RC Structures
14
Slenderness Effects in RC Structures
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Design of Foundations
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Special Topics: Shear Walls, Shear Friction, Corbels, Ledge Beams, Strut and Tie Models: Deep Beams
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Grading Policy Mid Term
= 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.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Lecture-01
Introduction
Prof. Dr. Qaisar Ali
CE 5115 Advance Design of Reinforced Concrete Structures
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Topics Addressed Historical
Development
of
Cement
and
Reinforced Concrete Building Codes and the ACI Code Objectives of Design Design Process
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Topics Addressed Limit States and the Design of Reinforced Concrete Basic Design Relationship: Structural Safety Probabilistic Calculation of Safety Factors Design Procedure Specified in the ACI Code
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Topics Addressed Load Combinations used in the ACI code Strength Reduction Factors used in the ACI code Design Loads for Buildings and other Structures Customary
Dimensions
and
Construction
Tolerances
Prof. Dr. Qaisar Ali
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Department of Civil Engineering, University of Engineering and Technology Peshawar
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.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
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.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
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.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Building Codes and the ACI Code 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).
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Building Codes and the ACI Code General Building Codes in USA
The National Building Code (NBC),
Published
by
the
Building
Officials
and
Code
Administrators
International is used primarily in the northeastern states.
The Standard Building Code (SBC),
Published by the Southern Building Code Congress International is used primarily in the southeastern states.
The Uniform Building Code (UBC),
Published by the International Conference of Building Officials, is used mainly in the central and western United States.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Building Codes and the ACI Code 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 2009 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.
In future only IBC will exist.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Building Codes and the ACI Code 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.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Building Codes and the ACI Code 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.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Building Codes and the ACI Code 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.
Prof. Dr. Qaisar Ali
Many others.
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Building Codes and the ACI Code The ACI 318 Code
The
American
Concrete
Requirements for
Institute
Structural
“Building
Code
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.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Building Codes and the ACI Code The ACI 318 Code
7 parts, 22 chapters and 6 Appendices.
Brief visit of the code
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Building Codes and the ACI 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.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Building Codes and the ACI Code 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.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Building Codes and the ACI Code 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.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
The Design & Design Team 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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Department of Civil Engineering, University of Engineering and Technology Peshawar
Objectives of Design Four Major Objectives of Design 1.
2.
Appropriateness: This include,
Functionality, to suit the requirements.
Aesthetics, to suit the environment.
Economy
The overall cost of the structure should not exceed the client’s budget.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Objectives of Design Four Major Objectives of Design 3.
4.
Structural Adequacy (safety)
Strength.
Serviceability.
Maintainability
Prof. Dr. Qaisar Ali
The structure should be simple so that it is maintained easily.
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Department of Civil Engineering, University of Engineering and Technology Peshawar
The Design Process Three Major Phases of Design 1.
The client’s needs and priorities.
2.
Development of project concept.
3.
Design of Individual systems.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
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
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Limit State and the Design of 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
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Limit State and the Design of Reinforced Concrete
The Major UL States are
Loss of equilibrium
Rupture
Formation of plastic mechanism
Instability
Progressive collapse
Fatigue
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Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Limit State and the Design of 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.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Limit State and the Design of Reinforced Concrete
The SL States are
Excessive deflections
Excessive crack widths
Undesirable vibrations
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Limit State and the Design of 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.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Limit State and the Design of 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.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
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
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Basic Design Relationship:
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
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Basic Design Relationship:
Limit State Design approach Capacity is reduced and demand is increased based on
scientific rationale. In LSD approach, we have
Mn ≥ Mu (α Ms )
Vn ≥ Vu (α Vs )
Pn ≥ Pu (α Ps )
Tn ≥ Tu (α Ts )
= strength reduction factor
α = load amplification factor
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
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.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Structural Safety Variability in Resistance
Effects of simplifying assumptions The fig shows Comparison of
measured
(Mtest)
computed
(Mn)
and failure
moments for 112 similar RC beams
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
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.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
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)
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Structural Safety Conclusion
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.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
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 vs. R”
S > R is failure i.e., load effects greater than resistance & S < R is Safety.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Probabilistic Calculation of Safety 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
Safety margin vs. frequency (success or failure)
figure.
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Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Probabilistic Calculation of Safety 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 Safety margin vs. frequency (success or failure)
given as Y = 0 + βσY
Where, σY = Standard Deviation; β = 1,2,3 …
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Probabilistic Calculation of Safety 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.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Probabilistic Calculation of Safety Factors Calculation of Pf:
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
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Probabilistic Calculation of Safety Factors Calculation of Pf:
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.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Probabilistic Calculation of Safety Factors
Selection of Pf 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.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Probabilistic Calculation of Safety Factors
Selection of Pf and β
In strength design method, we know that:
γMs = ΦMn, therefore,
Safety factor Mn/ Ms = γ/ Φ
For ΦMn = 1.2MD + 1.6ML
Let ML = MD ; then ΦMn = 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
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Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Design Procedures Specified in the ACI Code
The Design Philosophy 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.
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Design Procedures Specified in the ACI Code
The Design Philosophy of the ACI Code This process is called strength design in 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.
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Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Load Combinations LOAD COMBINATIONS in Section 9.2 of ACI 318-02 code
U = 1.4(D + F)
U = 1.2(D + F + T) + 1.6(L + H) + 0.5(Lr or S or R)
U = 1.2D + 1.6(Lr or S or R) +(1.0L or 0.8W)
U = 1.2D + 1.6W + 1.0L + 0.5(Lr or S or R)
U = 1.2D + 1.0E + 1.0L + 0.2S
U = 0.9D + 1.6W + 1.6H
U = 0.9D + 1.0E + 1.6H
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Load Combinations Load Types
Dead (D)
Wind (W)
Live (L)
Seismic (E)
Roof live (Lr)
Soil (H)
Snow (S)
Fluid (F)
Rain (R)
Temperature, creep, shrinkage (T)
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Strength Reduction Factors Strength Reduction Factors in the ACI 318-02 Code, Section 9.3
Prof. Dr. Qaisar Ali
Tension-controlled
0.90
Compression-controlled (spiral)
0.70
Compression-controlled (other)
0.65
Shear and torsion
0.75
Bearing
0.65
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Design Loads for Buildings and Other Structures
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).
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.
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Design Loads for Buildings and Other Structures
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
Prof. Dr. Qaisar Ali
Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Design Loads for Buildings and Other Structures
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 unshored portion of the structure under construction, unless analysis indicates adequate strength to support such additional loads
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Fall 2011
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Department of Civil Engineering, University of Engineering and Technology Peshawar
Customary Dimensions and Construction Tolerance Difference in Working and As-Built Drawings’ Dimensions
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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Department of Civil Engineering, University of Engineering and Technology Peshawar
References Reinforced Concrete - Mechanics and Design (4th Ed.) by James MacGregor. ACI 318-02 PCA 2002
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Department of Civil Engineering, University of Engineering and Technology Peshawar
The End
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