Fatigue Damage in Seismic Structures

Low-cycle fatigue damage factors are computed for single-degree-of-freedom systems subjected to accelerograms of the El Centro earthquake (NS component) of May 18,1940 and the Taft earthquake (S69°E component) of July 21, 1952. Two interrelated hysteresis loops, namely, load-maximum strain curve and deflection-maximum strain curve, are proposed for columns with an average wide-flange section. These hysteresis loops are used for finding the time history of maximum strains in the shear type frame under earthquake loads. The history of plastic strains are then used to compute the corresponding cumulative damage factor of the structure. Variables studies include frequency (in the elastic range) and the yield load of the structure. Numerical results show that (a) it is possible for a structure to fail in low-cycle fatigue under seismic loads, and (b) this type of structural failure cannot be predicted just by studying the maximum structural displacements of the structure.