The central concepts of limit state design are as follows:
- All the separate conditions that make the structure unfit for use are taken into account. These are the separate limit states. The structure should not overturn under applied loads and its members and joints should be strong enough to carry the forces to which they are subjected. In addition, other conditions such as excessive deflection of beams or unacceptable vibration should not make the structure unfit for use.
- Design should be based on statistical methods with a small probability of the structure reaching a limit state. Loads and material strengths vary, approximations are used in design and imperfections in fabrication and erection affect the strength in service. Al these factors can only be realistically assessed in statistical terms. However, it is not yet possible to adopt a complete probability basis for design, and the method adopted is to ensure safety by using suitable factors. Partial factors of safety are introduced to take into account of all the uncertainties in loads, materials strengths, etc.