lecture 01 introduction

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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 1

Advance Design of Reinforced Concrete

Structures CE-5115

By: Prof Dr. Qaisar Ali

Civil Engineering Department

UET [email protected]

[email protected]


Prof. Dr. Qaisar Ali Fall 2011 2

Course ContentLecture 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

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7 Analysis and Design of Slab Systems: One Way Slabs,One Way Joist Systems

8 Analysis and Design of Two-way Slab System withoutBeams (Flat Plate and Flat Slabs), Two Way JoistSlabs & Two-way Slabs with Beams

9 Idealized Structural Modeling of RC Structures

10 Gravity Load Analysis of RC Structures11 Case Studies on Gravity Load Analysis of RC

Structures




12 Earthquake Resistant Design of RC Structures

13 Design of Beam-Column Connections in MonolithicRC Structures

14 Slenderness Effects in RC Structures

15 Design of Foundations

16 Special Topics:Shear Walls, Shear Friction, Corbels, LedgeBeams, Strut and Tie Models: Deep Beams

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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 midterm exam.


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

Lecture-01

Introduction

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Topics Addressed Historical Development of Cement and

Reinforced Concrete

Building Codes and the ACI Code

Objectives of Design

Design Process



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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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 ConstructionTolerances


Prof. Dr. Qaisar Ali Fall 2011

Historical Development of Cement and Reinforced

Concrete Cement

In 1824 Joseph Aspdin mixed limestone and clay and heatedthem 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 beganexperimenting (in around 1850) on reinforced concrete tubswith iron for planting trees.

The first RC building in the US was a house built in 1875 byW. E. Ward, a mechanical engineer.

Working Stress Design Method, developed by Coignet inaround 1894 was universally used till 1950.

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General Building Codes

Cover all aspects of building design and construction fromarchitecture 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 SEAOCand NEHRP of USA, BCP-SP 07 of Pakistan.

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Material Specific Codes

Cover design and construction of structures using a specificmaterial or type of structure such as ACI, AISC, AASHTOetc.

Others such as ASCE

Cover minimum design load requirement, Minimum DesignLoads 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 AdministratorsInternational is used primarily in the northeastern states.

The Standard Building Code (SBC),

Published by the Southern Building Code Congress International isused primarily in the southeastern states.

The Uniform Building Code (UBC),

Published by the International Conference of Building Officials, is usedmainly in the central and western United States.

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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.

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Seismic Codes in USA

NEHRP (National Earthquake Hazards Reduction Program)

Recommended Provisions for the Development of Seismic Regulations forNew Buildings developed by FEMA (Federal Emergency ManagementAgency).

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 LateralForce Requirements and Commentary (the SEAOC “Blue Book”) publishedby the Seismology Committee of SEAOC.

The UBC has adopted SEAOC for seismic design.

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Building Code of Pakistan

Building Code of Pakistan, Seismic Provision BCP SP-07has adopted the seismic provisions of UBC 97 for seismicdesign of buildings.

IBC 2000 could not be adopted because some basic inputdata required by IBC for seismic design does not exist inPakistan.

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The ACI MCP

ACI MCP (American Concrete Institute Manual of ConcretePractice) contains 150 ACI committee reports; revised everythree years.

ACI 318: Building Code Requirements for Structural Concrete.

ACI 315: The ACI Detailing Manual.

ACI 349: Code Requirement for Nuclear Safety Related ConcreteStructures.

Many others.

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The ACI 318 Code

The American Concrete Institute “Building CodeRequirements for Structural Concrete (ACI 318),”referred to as the ACI code, provides minimum requirementsfor structural concrete design or construction.

The term “structural concrete” is used to refer to all plain orreinforced concrete used for structural purposes.

Prestressed concrete is included under the definition of reinforcedconcrete.

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The ACI 318 Code

7 parts, 22 chapters and 6 Appendices.

Brief visit of the code

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Legal Status of The ACI 318 Code

The ACI 318 code has no legal status unless adopted by astate or local jurisdiction.

It is also recognized that when the ACI code is made part ofa legally adopted general building code, that general buildingcode may modify some provisions of ACI 318 to reflect localconditions 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.

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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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The Design & Design Team

General:

The design covers all aspects of structure, not only thestructural design.

The structural engineer is a member of a team whosemembers work together to design a building, bridge, orother structure.

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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.

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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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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.

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Limit State and the Design of Reinforced Concrete

Limit State

When a structure or structural element becomes unfit for itsintended 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

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The Ultimate Limit States

These involve a structural collapse of part or all of thestructure.

Such a limit state should have a very low probability ofoccurrence, since it may lead to loss of life and majorfinancial losses

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The Major UL States are

Loss of equilibrium

Rupture

Formation of plastic mechanism

Instability

Progressive collapse

Fatigue

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Serviceability Limit States

These involve disruption of the functional use of thestructure, but not collapse.

Since there is less danger of loss of life, a higher probabilityof occurrences can generally be tolerated than in the caseof an ultimate limit state.

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The SL States are

Excessive deflections

Excessive crack widths

Undesirable vibrations

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Special Limit States

This class of limit state involves damage or failure due toabnormal 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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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 aswater retaining structures and other structures where deflection andcrack control are important.

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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

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Limit State Design approach

Capacity is reduced and demand is increased based onscientific 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

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Structural Safety Variability in Resistance

The actual strengths (resistances) of beams, column, orother structural members will almost always differ from thevalues calculated by the designer (nominal strength). Themain 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 thestructural drawings,

effects of simplifying assumptions made in deriving the equations formember resistance.

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Variability in Resistance

Effects of simplifying assumptions

The fig shows Comparison ofmeasured (Mtest) andcomputed (Mn) failuremoments for 112 similar RCbeams

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Structural Safety



Structural Safety Variability in Loads

All loadings are variables, especially live loads andenvironmental loads due to snow, wind, or earthquakes.

In addition to actual variations in the loads themselves, theassumptions and approximations made in carrying outstructural analysis lead to differences between the actualforces and moments and those computed by the designer.

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Variability in Loads

Fig shows variation of Live loadsin a family of 151sft offices.

The average (for 50 % buildings)sustained live load was around13 psf in this sample.

1% of measured loads exceeded44 psf.

Building code specify 50 psf forsuch buildings (ASCE 7-02)

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Structural Safety



Structural Safety

Conclusion

Due to the variability of resistances and load effects, there isdefinite chance that a weaker-than-average structure will besubjected to a higher- than-average load.

In extreme cases, failure may occur.

The load factors and resistance factors are selected toreduce the probability of failure to a very small level.

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Resistance vs. Load Effects

R = The distribution of a population ofresistance of a group of similarstructure.

S = Distribution of the maximum loadeffects, S, expected to occur on thosestructure during their life times

The 45° line in this figure correspondsto a load effect equal to the resistance(S = R).

S > R is failure i.e., load effects greaterthan resistance & S < R is Safety.

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“S vs. R”



Resistance vs. Load Effects

Safety margin can be represented asY = R – S

Graph shows plot between safetymargin (Y) and frequency ofoccurrence (success or failure)

If Y is greater than 0, then safetymargin exists and failure is avoided.

Failure will occur if Y is negative,represented by the shaded area infigure.

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Safety margin vs. frequency (success or failure)

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The Probability of Failure

The probability of failure, Pf, is thechance that a particular combinationof R and S will give a negative valueof Y.

In normal distribution curve, Pf isequal to the ratio of the shaded areato the total area under the curve infigure.

From the figure, mean value of Y isgiven as Y = 0 + βσY

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

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Safety margin vs. frequency (success or failure)




The Safety Index

Now larger the distance βσY, thelesser will be the negative part andmore will be the positive part in thecurve, which means less chance offailure and more safety. The factor β iscalled the safety index.

More positive part on the curve meansincreasing R. But increase inresistance will require compromise oneconomy.

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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 afunction of Z ) and then using standard normal distribution tablesto find the area under the curve

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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 membersdesigned on the basis that β = 3.5 may fail due to excessive loador under strength sometime during its life time.

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Selection of Pf and β

The appropriate values of Pf and hence of β are chosen bybearing in mind the consequences of failure.

Based on current design practice, β is taken between 3 and3.5 for ductile failure with average consequences of failureand between 3.5 and 4 for sudden failure or failures havingserious consequences.

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Selection of Pf 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 ; 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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Design Procedures Specified in the ACI Code

The Design Philosophy of the ACI Code

9.1.1- structures and structural members shall be designed tohave design strengths at all sections at least equal to therequired strength calculated for the factored loads and forcesin such combinations as are stipulated in this code.

9.1.2- members also shall meet all other requirements of thiscode to ensure adequate performance at service load levels.

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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 isknown as LRFD (Load and Resistance Factor Design).

Strength design and LRFD are methods of limit-state design, except thatprimary attention is always placed on the ultimate limit states, with theserviceability limit states being checked after the original design iscompleted.

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Design Procedures Specified in the ACI Code

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LOAD COMBINATIONS in Section 9.2 of ACI 318-02code

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

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Load Combinations



Load Types

Dead (D)

Live (L)

Roof live (Lr)

Snow (S)

Rain (R)

Wind (W)

Seismic (E)

Soil (H)

Fluid (F)

Temperature, creep, shrinkage (T)

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Load Combinations

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Strength Reduction Factors

Strength Reduction Factors in the ACI 318-02

Code, Section 9.3

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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ACI 318-02, Section 8.2-LOADING:

8.2.2: Service loads shall be in accordance with the generalbuilding code of which this code forms a part, with such liveload reductions as are permitted in the general building code.

Section R8.2 :The provisions in the code are for live, wind, andearthquake loads such as those recommended in “Minimum DesignLoads for Buildings and Other Structures,”(ASCE 7).

If the service loads specified by the general building code (of whichACI 318 forms a part) differ from those of ASCE 7, the general buildingcode governs. However, if the nature of the loads contained in ageneral building code differs considerably from ASCE 7 loads, someprovisions of this code may need modification to reflect the difference.

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Design Loads for Buildings and Other Structures

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ASCE Recommendations on Loads:

ASCE 7-02 sections 1 to 10 are related to design loads forbuildings 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 ofthe fresh concrete is supported by formwork, whichfrequently rests on floors lower down in the structure.

ACI section 6.2.2 states the following:

No construction loads exceeding the combination of superimposed deadload plus specified live load (un-factored) shall be supported on any un-shored portion of the structure under construction, unless analysisindicates adequate strength to support such additional loads

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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 oftolerance 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 specifiedthickness.

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References

Reinforced Concrete - Mechanics and Design (4th Ed.) byJames MacGregor.

ACI 318-02

PCA 2002

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The End

lecture 01 introduction

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