Working Stress Method: The stresses in the member is obtained from the working loads and compared with permissible stresses. Working Stress method will lead to large FOS and over-sized sections, thus the sections will be uneconomical. WSM is still being used in special structures such as water tanks, bridges, chimneys in India.
Ultimate Load Method: In this method the ultimate strength of the material is considered which gives much slender sections for columns and beams compared to WSM method.
Limit State Method: This method is based on the concept of safety and serviceability. Limit state is a condition just before collapse. A structure designed by limit state should give proper strength and serviceability throughout its life. In limit state method, the limit state of collapse deals with the safety of structure and limit state of serviceability deals with the durability of structure.
Limit state of collapse- The structure is designed for limit state of collapse: flexure, limit state of collapse: shear, limit state of collapse: torsion, limit state of collapse: compression. The structure shall be designed to resist all these forces. The resistance offered by the structure shall not be less than designed value and that designed value shall be based on load combinations (IS 456:2000).
Limit state of serviceability- Limit state of serviceability deals with control on deflection, cracking, vibration and corrosion. Different load combinations have been given in table 18 (IS 456:2000) for limit state of collapse and limit state of serviceability.