introduction to rc
TRANSCRIPT
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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