Limit state design is a concept used by structural engineers. Engineers use limit states to define and check a structure's performance. A limit state is a set of performance criteria (e.g. vibration levels, deflection) that must be met when the structure is loaded a particular way. The most commom limit states are the serviceability limit state and the ultimate limit state.
In the serviceability limit state, designers must ensure that when everyday loads (machinery and people in buildings, cars on bridges) are imposed, the activities for which the structure was designed can still proceed. For instance, if the floors of a building deflect too much, objects such as machinery or partitions that sit on that floor may break. As well, if a bridge vibrates too much, people may not use it.
In the ultimate limit state, designers must ensure that the structure will not collapse when loaded by the most severe loads it can be expected to encounter. Partial safety factors are used to ensure that these loads can be carried safely. Limit state design and partial safety factors have replaced the older concept of working stress design.
