The residual strength requirement of a fail-safe structure is to assure damage tolerance following a partial failure of the structure.  Damage tolerance is maintained through detection of this failure prior to total loss of the structure and sufficient residual strength capability for operating safely within the partial failure prior to inspection.  The fail-safe structure is typically a built-up structure with multiple load paths or crack arrest features in its design.  In the event of failure of a structural member, its load must be transferred to and withstood by the remainder of the structure, which also contains crack damage, without causing the loss of whole structure.  The residual strength of the built-up structure must be determined under such critical circumstances so that the fail-safe design requirements are met.

The analysis of residual strength capability for built-up structure requires the estimation of the critical stress level at which the partial failure occurs, as well as an understanding of the capability of the total structure to withstand this partial failure at and subsequent to the time of failure.  The required load associated with the time subsequent to failure is based on the inspection category and, the partially-failed structure must be able to maintain this load until the time of inspection.