Fatigue Failure of Nonlinear Multistory Seismic Structures

Two four-story structures are excited by the 1940 El Centro Earthquake NS component. In one structure, all floors are strained beyond the elastic limit, whereas inelastic deformation is prevented in the first floor of the other structure. An elasto-plastic load-deflection hysteresis loop and a trilinear load-deflection relationship are assumed for evaluating the time history of the maximum strain at the critical section of each floor. The cumulative low-cycle fatigue damage factors are calculated for different fundamental frequencies of the structures. The variation of the maximum deflection of a floor with reference to the floor below is also studied and is shown to be different from the corresponding variation of low-cycle fatigue damage factors. Though the low-cycle fatigue damage in the first floor is more than that in the other floors, the safety of the frame as a whole is not improved by stiffening the first floor. In general, the stiffer structures are damaged to a greater extent than the flexible ones.