The performance of a structure and its components is described using limit state functions that separate desired from undesired states. The physical effects of exceeding a limit state may be reversible or irreversible. If the effects are reversible, the removal of the cause of the exceedance allows a structure to return to the desired state. If the effects are irreversible, a return to the desired state is not possible, and certain consequences, such as damage, may ensue depending on the nature of the limit state. These consequences may themselves be reversible or irreversible. For example, if consequential damage is limited, such as an undesired and localised permanent set, it may be repairable (e.g., by replacing the affected parts). Limit states are examined against different target safety levels, where the target to be attained for any particular type of limit state is a function of the consequences of and ease of recovery from that state.