Cyclic loading may progressively degrade the global structural response of thin-walled steel members before final failure becomes apparent. This study examines that process in full-scale steel girders with sinusoidally corrugated webs using a stiffness-based approach derived from load-displacement loops. The stiffness parameter Ki is used together with the Ninit criterion, defined by a persistent decrease relative to the reference stiffness K0, to identify the onset of measurable degradation in global terms. To complement this diagnostic framework, the observed stiffness histories are also expressed in a normalized two-stage form using ηK = Ki/K0 and n = N/Nfail. The method is applied to an experimental programme including a reference girder, girders subjected to cyclic loading until failure, and girders tested statically after prior fatigue loading. The results show that the global stiffness remains nearly constant during most of fatigue life and then decreases rapidly in the final stage preceding failure. The identified Ninit consistently precedes Nfail, indicating an intermediate stage between apparently stable response and final failure and reflecting the stage at which local degradation begins to produce measurable consequences for the global response. The normalized representation captures the main features of the degradation histories and provides a compact comparative and interpretative description of degradation development. The identified degradation state also supports interpretation of the subsequent ultimate response, including reduced load-carrying capacity and possible changes in the governing failure mechanism.