This is the most rational method which takes into account the ultimate strength of the structure and also the serviceability requirements. It is a judicious combination of workings stress and ultimate load method of design. The acceptable limit of safety and serviceability requirements before failure occurs is called a limit state. This method is based on the concept of safety at ultimate loads (ultimate load method) and serviceability at working loads (working stress method). The two important limit states to be considered in design are
- Limit state of collapse.
- Limit state of serviceability.
- Limit State of Collapse
This limit state is also called as strength limit state as it corresponds to the maximum load carrying capacity i.e. the safety requirements of the structure. The limit state of collapse is assessed from collapse of the whole or part of the structure. As per this limit state, the resistance to bending, shear, torsion and axial loads at every section shall not be less than that produced by the most unfavourable combination of loads on that structure. The following limit states of collapse are considered in design:
- Limit state of collapse in flexure (bending).
- Limit state of collapse in compression.
- Limit state of collapse in shear.
- Limit state of collapse in torsion.
Limit State of Serviceability
A structure is of no use if it is not serviceable. Thus this limit state is introduced to prevent excessive deflection and cracking. It ensures the satisfactory performance of the structure at working loads. It is estimated on the basis of elastic theory or working stress method because deformation is of significance under working load and not at collapse. Limit state of serviceability of following limit stales:
- Limit state of deflection
- Limit state of cracking.
The structure should be designed which considering all the appropriate limit state of safety and serviceability. The structure should be designed on the basis of most critical limit state and then checked for all other limit states.