For any construction purpose.First, we need to make a plan and estimation.For the design of structural members, there are several methods used such as working stress method and limit state method.
Working stress method is a traditional method while the limit state method is modern.As we know that, there is no material which is pure and completely homogeneous in this world.
So in order to overcome his fact, we need to make some assumptions in limit state method in our design procedure.
Limit state method assumptions
- The first Assumptions in Limit State Method is, Plane sections normal to the axis remain plane even after bending.It means the strain distribution across the depth of the cross-section is linear as shown below
- At Limiting state, the maximum strain in concrete, which occurs at outermost compression fibre is 0.0035
- The stress-strain curve for concrete is having parabolic shape up to 0.002 strain and the constant up to limit state of 0.0035.However, IS Code does not prevent using other shapes like rectangle and trapezoidal
- The tensile strength of concrete is ignored
- The stress in reinforcement is derived from the representative stress-strain curve for the type of steel used.For design purpose a partial safety factor of 1.15 is used Hence maximum stress in steel is limited to fy/1.15= 0.87 * fy
- Th maximum strain in steel at failure shall not be less than fy/1.15Es + 0.002 fy is the characteristic strength of steel. Es is the modulus of elasticity of steel
These are the Assumptions in Limit State Method. Thank you
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