Methods of Structure Design

Different countries have their own structural design codes based on theoretical and laboratory certified methods. Two methods which are predominantly used in India are working state and limit state method. IS (Indian standard) code standards are currently using limit state method from early 2000s, before that it was following working state method.

Working state method calculates permissible stress and using theoretical formulas answers the allowable load on it. Factor of safety is divided by yield stress to calculate the permissible stress. Factor of safety depends on material type and the type of stress will be developed on it. Allowable load can be computed by multiplying cross section area with permissible stress. So it is pretty much easy to work out and logical to some extent.

Limit state method is broadly classified into two benchmarks/criteria : Limit state of collapse and limit state of serviceability. Structures are designed in accordance with both the specifications. Limit state of collapse mostly talks about strength of the material and maximum load on it, where limit state of serviceability talks about cracks and deflections in the structure. It considers various failure ways to formulate these limits.

Basic assumptions which are common in both the methods:

1. Materials are homogeneous and isotropic
2. The behavior of structure is linear (strain vary linearly throughout cross section)
3. Plane section remains plane after the bending also

Major differences:

1. Linear stress strain relationship for concrete in WSM, In LSM it is non linear (more realistic)
2. WSM can be considered as a deterministic approach where LSM is probabilistic
3. There is no concept of factor of safety for characteristic load in WSM
4. Permissible stress is calculated using yield stress in WSM, Material strength after yield stress was ignored. In limit state design, strength is calculated dividing characteristic strength with partial safety factor, Characteristic strength is calculated experimentally
5. Characteristics of different loads (wind, dead load etc.) was not considered in WSM

Section results from LSM are more economical than WSM