Difference between limit state and ultimate limit state
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UltimateUltimate Load (Strength) method : This method is primarily based on “strength concept ”. The working loads are increased by multiplied them with the load factor to give the ultimate load . The load factor gives structure , a proper and adequate safety against failure .
This method was accepted by IS CODE 1964 as an alternative to working stress method.
Working Stress Method:
Working Stress Method of design is based on the linear theory (elastic theory).The stresses in the materials, which are developed due to working loads are restricted giving a safe design.
The assumptions in working stress method of design are
The steel and concrete behaves as linear elastic material
The design is carried out by the factor of safety of “3” for concrete cube strength and a factor of safety “1.8” for yield strength of steel
The stresses in steel and concrete are related by a factor known as “modular ratio”
M= 280/ 3 sigma cbc
Sigma cbc = allowable stress in concrete
As per IS- 456-2000
Limit state method :
Limit state refers to the acceptable limit for safety and serviceability of structure before the failure of structure.
This method is considered as as ideal method for design as it includes, the structural safety of structure against collapse as well as serviceability. The design of members is initially done by considering the limit state of collapse and the members is checked for serviceability requirement after the design .
As per IS 456-2000 , the structure should be designed by limit State method
This method was accepted by IS CODE 1964 as an alternative to working stress method.
Working Stress Method:
Working Stress Method of design is based on the linear theory (elastic theory).The stresses in the materials, which are developed due to working loads are restricted giving a safe design.
The assumptions in working stress method of design are
The steel and concrete behaves as linear elastic material
The design is carried out by the factor of safety of “3” for concrete cube strength and a factor of safety “1.8” for yield strength of steel
The stresses in steel and concrete are related by a factor known as “modular ratio”
M= 280/ 3 sigma cbc
Sigma cbc = allowable stress in concrete
As per IS- 456-2000
Limit state method :
Limit state refers to the acceptable limit for safety and serviceability of structure before the failure of structure.
This method is considered as as ideal method for design as it includes, the structural safety of structure against collapse as well as serviceability. The design of members is initially done by considering the limit state of collapse and the members is checked for serviceability requirement after the design .
As per IS 456-2000 , the structure should be designed by limit State method
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