Concept of LSM

In RCC design LSM is considered as the most suitable alternative to working stress method structural elements for bending, shear, torsion and axial load. Limit state is a condition of the structure when it stops to function in a way it is expected do the function. It ceases to service. It may not collapse but deflected excessively or cracked vibrated etc. Now all over the world this method is used.

The structure shall be designed to withstand safely all loads that expected to act on the structure, through out the life of the structure. It must be satisfied the serviceability requirements. It will not crack or deflected beyond the limits. This limit is called limit state of serviceability. Beyond that limit failure may occur. When designing most critical limit state is considered and checked for other limit states.

Limit State of Different Elements

        1.      Limit state of collapse in

a)      Flexure

b)     Compression

c)      Shear

d)     Torsion

        2.      Limit state of serviceability against

a)      Deflection

b)     Cracking

c)      Vibration

        3.      Other limit states  

a)      Durability

b)     Fatigue

c)      Fire resistance

The resistance of every section to bending, shear, torsion and axial loads shall not be less than the corresponding values assessed from rupture of one or more critical sections.

 The limit state of serviceability of a structure can be assessed from the amount of deflection, cracks etc. The deflection or cracks shall not exceed the allowable values in the code of practice 

Advantages of LSM

        1)     Working stress method is based on elastic theory, but LSM of analysis, both elastic and plastic                 theory of analysis is used. Concrete is a semi plastic material so elastic theory is not suitable.

        2)     The sizes elements arrived is less then its appearance is better.

        3)     Area of steel providing is less than what it may give according to working stress method.