Cumulative Fatigue Damage in Seismic Structures

The viability of cumulative fatigue damage as a design criterion for single degree-of-freedom steel seismic structures is investigated. Two ensembles of earthquake acceleration records are generated and used as excitation input in simulations involving structures having varied strength factors and periods of vibration. Strain and stress histories are obtained for the extreme fibers of critical sections of the structures. These are evaluated for cumulative fatigue damage by a model which incorporates the rain flow cycle counting method, a factor to account for the mean stress of each strain reversal and an empirical adjustment to the strain-fatigue life function to account for the sequence effect of the strain histories. Fatigue damage is found to be significant in structures having periods of vibration at the lower end of the medium period range of the design earthquake spectra. The damage may be estimated from a hysteretic energy parameter pertaining to the critical section of the frame.