The most important factor to consider in residual strength prediction of a cracked built-up structure is to decide whether the structure is skin or stiffener critical.  Normally, a short crack length is likely to be a skin critical case and a long crack length a stiffener critical case.  However, there is no clear cut demarcation between the two cases.  Factors such as percentage stiffening, spacing of stringers, lands in the structure, and other structural details will influence the type of failure.  Hence, a good technique is to determine the residual strength of a given structure based on both skin critical and stiffener critical cases.  The minimum fracture stress of the two will then represent the residual strength of the structure and should be considered to be the governing case.