Introduction to RC

7/25/2019 Introduction to RC

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INDIAN INSTITUTE OF TECHNOLOGY

KHARAGPUR 721302 INDIA

NATIONAL MISSION ON EDUCATION THROUGH ICT

(MHRD, GOVT OF INDIA)

INTRODUCTION TO RC DESIGN

DR. SUSHANTA CHAKRABORTY


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CONCRETE - Building Material for over 150 years

Advantage

Durability, specially resistance against water

Easy to cast in various size and shape

Excellent compressive bearing Economicto use

Disadvantage

Remarkably weak in tension (1/10thof compressivestrength)


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Basic Considerations in RC Design of Structures

Strength & Integrity Ability to resist stresses due to environmental loading,

fire etc.

Stability

Against overturning, sliding and buckling

Serviceability

Adequate stiffness to counteract deflection, vibrationresponse, crack width etc.

Economics

Aesthetics


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Goal of RC Design

To ensure ductile failure, instead of the sudden brittle failure

of plain concrete

concrete is embedded with steel bars (cuts across principal

tensile planes, i.e. across the potential tensile cracks), as if

stitchingthe (to be) separated concrete parts


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Role of Structural Analysis in Design

The geometric entities and material properties of a structure is

defined. It is desired to find out the stresses, deformations and

allowable load onto that member. The solutions are usuallyunique.

Explore the various combinations of geometric and materialproperties which may fulfill the purpose in hand, e.g.to carry a

defined loading. There may be multiple or even infinite

possibilities.

Analysis problem

Design problem


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Importance of Specifications and Standards

Actual construction practice needs supports from real

experimentalor observational evidences, past records of

performances of similar structures.

Various Codes of Practices across the countries try to lay down

guidelines, for design and construct structures, supported by

empirical laws, collective observational recordsof experts.

These codes are updated periodically with respect to new

findings.


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Importance of Specifications and Standards

The main functions of these codes are to bring valuable and

sophisticated information in the form of simple formulae and

charts, readable by nominally trained practicing engineers.

Apart from complying with any such code, the engineer must

ensure adequate technical supervision to ensure quality of

material used and appropriate construction techniqueswith

proper sequences.

The students must be introduced properly with the existence of

such professional ethics right at this moment


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Code of Practise for Design

IS 456: 2000 Plain and Reinforced Concrete- Code of Practice

IS 875: 1987 Loading (other than earthquake)

IS 1893: 2002 Criteria for Earthquake Resistant Design of Structures

IS 13920: 1993 Ductile Detailing of Reinforced Concrete Structures Subjected to

Seismic Forces

SP16: 1980 Design Aids for Reinforced Concrete to IS 456: 1978

SP34: 1987 Handbook on Concrete Reinforcement and Detailing


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Actual Stress Strain Behaviour of Concrete


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Design Stress Strain Behaviour of Concrete in

Flexural Compression


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Discussion of Design Stress Strain Curve

c= partial safety factor for concrete=1.5

The graph consists of an initial parabolic portion up to a

strain of 0.002and a straight line up to the an ultimate strain

of 0.0035

Under uniform compression (not from flexure) the ultimate

strain is limited to 0.002only.

For a combination of axial compression and flexure, the

ultimate strain is limited to a value between 0.002and 0.0035

depending upon the location of neutral axis.


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Actual Stress Strain Behaviour of Reinforcing Steel


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Characteristic and Design Stress Strain Behaviour

of Reinforcing Steel

002.087.0 E

yf

y

s= partial safety factor for steel=1.15


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Discussion of Design Stress Strain Curve

s= partial safety factor for steel=1.15

For cold-worked type of steels (FE415 or Fe500) there is no

specific yield point.

Full design yield strength is assumed to be 0.87fycorrespond

to the proof strain of 0.002

The design yield strain is to be taken as

002.087.0 E

yf

y


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Difference in Implementing Stress-strain Relation Of

The Concrete And Steel

The partial safety factor sfor steel is applicable in inelastic

region only because,

the Modulus of elasticity is independent of yield strength.

Whereas,

The partial safety factor cfor concrete is applicable in all the

stresses because,

the stress strain curve behaviour depends on the

characteristic strength of concrete.


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DIFFERENT METHODS OF RC DESIGN

WORKING STRESS METHOD (WSM)

ULTIMATE LOAD METHOD (ULM)

LIMIT STATE METHOD (LSM)


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Working Stress Method (WSM)

Linear elasticbehaviour is assumed.

Permissible stresses are kept well below the material strength.

Factor of Safety

= (Strength of the material)/(Permissible stresses)

Factor of safety remains same for all kind of loading.

The method is unableto depict the uncertaintiesassociated

with different types of loading.

WSM is based on service load alone


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Ultimate Load Method (ULM)

Non-linear elasticbehaviour may be assumed.

Permissible stresses are kept well below the material strength.

Load factor

= (Ultimate Load)/(Working Load)

Factor of safety is different for different kind of loading.

The method failsto fulfill serviceability condition.

ULM is based on ultimate load alone


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LIMIT STATE METHOD (LSM)

Limit State Method (LSM) is to provide safety against ultimate

load andserviceability at working load.

The methodology actually is based on the probability of

failure in statistical terms and expressed as probability

density function of failure.

Variations in loading, material properties, geometric entities

having varying degrees of uncertainties are incorporated usingMultiple Safety Factor format.

Partial factor of

safety for Material

Partial factor of

safety for load


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Probability Density Function of Failure

ProbabilityD

ensity

Load or Resistance

R = Resistance

S = Load Effect


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Design Load & Design Strength


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THE END

Introduction to RC

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