What is the main concept or principles of limit state design in civil engineering?

The main concepts of limit state design are the following:

1. All the separate conditions that make a structure unfit for use are considered.

2. The design is based on the actual behaviour of materials and performance of structures and members in service.

3. Ideally, design should be based on statistical methods with a small probability of the structure reaching a limit state.

Concept (1) means that a structure should not overturn under the loads and its members and joints should be strong enough to withstand the forces to which they are subjected, and that excessive deflection or unacceptable vibration though may not cause collapse of structure or part of it, should not make the structure unfit for use.

In the second concept concept (2), the strengths are calculated using plastic theory and post-buckling behaviour. The effect of imperfections on design strength is also included.

Concept (3) implies recognition of the fact that loads and material strengths vary. Approximations are used in design and imperfections in fabrication and erection affect the strength in service. All these facts are estimated in statistical terms. The use of probability basis for design is yet to be universally adopted. Instead a factor of safety called partial factor of safety is introduced to take account of all the uncertainties.