Design philosophies for steel structures

Three philosophies have been used for steel design.

1. Working Stress Method (WSM / elastic design): stresses under working loads are kept below permissible stresses ($\approx 0.6f_y$ in tension). Behaviour is assumed to be linearly elastic, and one single factor of safety is used. Adopted in IS 800:1984.
2. Ultimate load / plastic design: the structure is analysed up to the collapse load (formation of plastic hinges and a mechanism). Working loads are multiplied by a load factor (e.g. 1.7 for DL+LL) and the section is designed for the collapse moment $M_p$. Economical for continuous beams and portal frames.
3. Limit State Method (LSM): the structure is designed so that it does not reach any limit state of strength or serviceability, using separate partial safety factors on loads and on material strength. Adopted in IS 800:2007, the present code.

LSM is preferred because it is more rational, takes the probabilistic nature of loads and strengths into account, and gives more uniform safety.