http://en.wikipedia.org/wiki/Limit_state_design

A limit state is a condition of a structure beyond which it no longer fulfills the relevant design criteria.[1] The condition may refer to a degree of loading or other actions on the structure, while the criteria refer to structural integrity, fitness for use, durability or other design requirements. A structure designed by LSD is proportioned to sustain all actions likely to occur during its design life, and to remain fit for use, with an appropriate level of reliability for each limit state. Building codes based on LSD implicitly define the appropriate levels of reliability by their prescriptions.