department of civil engineering, university of engineering and technology peshawar structure codes...

Department of Civil Engineering, University of Engineering and Technology Peshawar

Structure Codes and the Design Basis of RC Structures

By: Prof Dr. Qaisar Ali

Civil Engineering Department

UET Peshawardrqaisarali.com

[email protected]


Topics Addressed

Building Codes and the ACI Code

Objectives of Design

Design Process

Limit States and the Design of Reinforced Concrete

Basic Design Relationship

Structural Safety


Topics Addressed

Design Procedure Specified in the ACI Code

Design Loads for Buildings and other Structures

Customary Dimensions and Construction Tolerances

Admixtures

Factors Affecting Strength of Concrete

High Strength Concrete


Topics Addressed

Durability of Concrete

Concrete Subjected to High Temperatures

Reinforcing Steel

Chapter 3 : Materials

Chapter 4 : Durability of Concrete

Chapter 5 : Concrete Quality, Mixing and Placing



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).



General Building Codes in USA

The National Building Code (NBC),

The Standard Building Code (SBC),

The Uniform Building Code (UBC),



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.

In future only IBC will exist.



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.



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.



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.



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.



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.



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.


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.


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


Variability in Resistance

Effects of simplifying

assumptions

The fig shows Comparison of

measured (Mtest) and

computed (Mn) failure

moments for 112 similar RC

beams

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)

Structural Safety


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.


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.


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.

Design Procedures Specified 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).

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.

A

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




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


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.


Admixtures

A material (usually in liquid form) other than cement, water

and aggregates, that is used as an ingredient of concrete

and is added to the batch immediately before or during

mixing to change properties of fresh or hardened concrete.


Admixtures

Uses

Admixtures are used to:

achieve certain properties in concrete more effectively than by other means.

maintain the quality of concrete during the stages of mixing, transporting, placing, and curing in adverse weather conditions.

reduce the cost of concrete construction.


Admixtures

Types As per ACI Committee 212, admixtures have been

classified into following groups:

Air-entraining Admixtures: causes the development of a

system of microscopic air bubbles in concrete, mortar, or

cement paste during mixing. Air-entrained concrete should

be used wherever water saturated concrete may be

exposed to freezing and thawing. Air entrainment also

improves the workability of concrete.


Admixtures

Types

Accelerating Admixtures: causes an increase in the rate of

hydration of the hydraulic cement and thus shortens the

time of setting, increases the rate of strength development,

or both.

Water Reducing and Set-Controlling Admixtures: Reduce

the water requirements of a concrete mixture for a given

slump, modify the time of setting, or both.


Admixtures

Types

Admixtures for Flowing Concrete: Flowing Concrete is

concrete that is characterized as having a slump greater

than 190 mm (7-1/2 in.) while maintaining a cohesive

nature.

Miscellaneous

Freeze Resistant, Pigments, Bonding, Grouting etc. (Refer

ACI 212 for details and more types of miscellaneous

admixtures)


Factors Affecting Concrete Strength

In addition to mixing, conveying, placing and compaction,

the strength of concrete primarily depends on:

Water Cement Ratio: Decrease in water cement ratio increases the

strength.

Aggregate Cement Ratio: Decrease in aggregate cement ratio

increases the strength up to a value of around 2.0. Further decrease may

cause decrease in strength.

Properties of Concrete


Factors Affecting Concrete Strength

Aggregate: The concrete strength is affected by the aggregate strength,

its surface texture, its grading and maximum size of the aggregate.

Curing: Prolonged moist curing leads to the highest concrete strength



Rate of Strength Gain

ACI Committee 209 [3-21] has proposed the following

equation to represent the rate of strength gain for concrete

made from Type 1 cement and moist-cured at 70°F.

f ’c(t) = f ’

c(28) {t/(4 + 0.85t)}

Where f ’c(t) = is the compressive strength at age t in days.



Rate of Strength Gain and Cement Types Figure shows the effect of type of cement on strength gain of concrete

(moist cured; w/c = 0.49).

I = NormalII = ModifiedIII = High early strengthIV = Low heatV = Sulfate resisting




Concretes with strengths in excess of 6000 psi are referred to

as high strength concrete.

The resulting concrete has a low void ratio.

Only the amount of water needed to hydrate the cement in the

mix is provided.



UET Lab Results for Producing High Strength Concrete Mix design results for 6000 and 8000 psi concrete.

Admixture used: Sikament 520BA

Table-A

Trial Test Proportion No. of cylinders

Date of preparation

Date of Testing

Slump (in)

Avg. Strength

(psi)

6000 psi (1:1:2)w/c (0.36)

6 25/6/2010 22/7/2010 2.5 6100

8000 psi (1:0.8:1.5)w/c (0.31)

6 28/6/2010 25/7/2010 2 8000


Durability of Concrete

Three most common durability problems in concrete are: Corrosion of steel in concrete.

Breakdown of the structure of concrete due to freezing and thawing.

Breakdown of the structure of concrete due to chemical action.


Concrete Subjected to High Temperatures

Compressive Strength of Concrete at High Temperatures


ASTM A 615, Specification for

Deformed and Plain Carbon-Steel

Bars for Concrete Reinforcement.

ASTM A 706, Specification for Low-

Alloy Steel Deformed and Plain Bars

for Concrete Reinforcement.

ASTM A 996, Specification for Rail-

Steel and Axle-Steel Deformed Bars

for Concrete Reinforcement.

Deformed Bar Reinforcement



Variation in Yield Strength

Distribution of mill test yield strength for grade 60 steel.



Strength of Reinforcing Steel at High temperatures

Deformed reinforcement subjected to high temperatures in

fires tends to lose its strength.


ACI Chapter 3: Materials

Tests of Materials

A complete record of tests of materials and of concrete shall be

retained by the inspector for 2 years after completion of the project,

and made available for inspection during the progress of the work.

Water

Water used in mixing concrete shall be clean and free from injurious

amounts of oils, acids, alkalis, salts, organic materials, or other

substances deleterious to concrete or reinforcement.


ACI Chapter 3: Materials

Steel Reinforcement

Reinforcement shall be deformed reinforcement, except

that plain reinforcement shall be permitted for spirals or pre-

stressing steel; and reinforcement consisting of structural

steel, steel pipe, or steel tubing shall be permitted as

specified in this code.


ACI Chapter 4: Durability of Concrete

Sulfate exposures

Concrete to be exposed to sulfate-containing solutions or

soils shall conform to requirements of Table 4.3.1 or shall

be concrete made with a cement that provides sulfate

resistance and that has a maximum water-cementitious

materials ratio and minimum compressive strength from

Table 4.3.1.


ACI Chapter 5: Concrete Quality, Mixing and Placing

Average Strength of Concrete Produced in the Field

It is emphasized in this chapter that the average strength of

concrete produced in the filed should always exceed the

specified value of fc′ used in the structural design calculations.

This is based on probabilistic concepts, and is intended to

ensure that adequate concrete strength will be developed in the

structure.


Average Strength of Concrete Produced in the Field Variation in Strength

Variations in the properties or proportions of constituents of

concrete, as well as variations in transporting, placing, and

compaction of the concrete, lead to variations in the strength of

the finished concrete. In addition, discrepancies in the tests will

lead to apparent differences in strength.



Average Strength of Concrete Produced in the Field Variation in Strength

Figure shows the distribution of strengths in a sample of 176 concrete

cylinder tests for the concrete having nominal strength of 3000 psi

Strength less than nominal = 9

Strength more than nominal = 167



ACI recommendations for achieving specified strength in the field

The ACI code recommends that selection of concrete

proportions for achieving a specified concrete strength in the

field shall be based on the required average compressive

strength of concrete f cr′ and not on the specified strength. ACI table 5.2.2.



Selection of Concrete Proportions

Proportions of materials for concrete shall be established to

provide:

Workability and consistency to permit concrete to be worked

readily into forms and around reinforcement under conditions

of placement to be employed, without segregation or

excessive bleeding;

Resistance to special exposures as required by Chapter 4 of

ACI;

Conformance with strength test requirements of ACI 5.6.



Selection of Concrete Proportions

Recommendations for selecting proportions for concrete

are given in detail in “Standard Practice for Selecting

Proportions for Normal, Heavyweight, and Mass Concrete”

(ACI 211.1).



Sampling Frequency for Strength Tests

ACI R5.6.2.1: As a measure of quality control, the code

recommends following criteria for collecting samples of

concrete cylinders from a given class of concrete:

Once each day a given class is placed, nor less than

Once for each 150 yd3 of each class placed each day, nor less than

Once for each 5000 ft2 of slab or wall surface area placed each day.

In calculating surface area, only one side of the slab or wall should be considered.



Strength Test

A strength test shall be the average of the strengths of two

cylinders made from the same sample of concrete and

tested at 28 days or at test age designated for

determination of fc′.



Criterion for Satisfactory Concrete Strength ACI 5.6.3.3: Strength level of an individual class of concrete

shall be considered satisfactory if both of the following requirements are met:

(a) Every arithmetic average of any three consecutive strength

tests equals or exceeds fc′;

(b) No individual strength test (average of two cylinders) falls

below fc′ by more than 500 psi when fc′ is 5000 psi or less; or by

more than 0.10fc′ when fc′ is more than 5000 psi.



The steps taken to increase the average level of

test results: It will depend on the particular circumstances, but could include

one or more of the following:

An increase in cementitious materials content

Changes in mixture proportions

Reductions in or better control of levels of slump supplied

Closer control of air content

An improvement in the quality of the testing, including strict compliance with standard test procedures



Investigation of Low Strength Test Results

If any strength test of laboratory-cured cylinders falls below

specified value of fc′ by more than the values given in

5.6.3.3(b), steps shall be taken to assure that load-carrying

capacity of the structure is not jeopardized.




If the likelihood of low-strength concrete is confirmed and

calculations indicate that load-carrying capacity is

significantly reduced, tests of cores drilled from the area in

question in accordance with “Method of Obtaining and

Testing Drilled Cores and Sawed Beams of Concrete”

(ASTM C 42) shall be permitted.

In such cases, three cores shall be taken for each strength

test that falls below the values given in 5.6.3.3(b).




According to ACI 5.6.5.4, concrete in an area represented

by core tests shall be considered structurally adequate if

the average of three cores is equal to at least 85 percent of

fc′ and if no single core is less than 75 percent of fc′.




If criteria of ACI 5.6.5.4 are not met and if the structural

adequacy remains in doubt, the responsible authority shall

be permitted to order a strength evaluation in accordance

with Chapter 20 for the questionable portion of the

structure, or take other appropriate action.



Regular refresher courses for field staff at least twice a year

Code implementation in full letter and spirit

Provision for structural design cost in PC-1

Field execution check lists to be developed

Certification courses for contractors, fabricators and masons

Dissemination of FPM and the like material

Retrofitting of vulnerable structures

Strong link with Universities.

Some Humble Suggestions


The End

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department of civil engineering, university of engineering and technology peshawar structure codes...

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