Expected Fatigue Damage of Seismic Structures

Whenever a structure is subjected to strong-motion earthquake excitation, certain parts of the structure may undergo plastic deformations which are indications of structural damage. It has also been known that the damage due to repeated applications of plastic deformation is cumulative and could cause low-cycle fatigue failures. Nevertheless, fatigue problem is not being considered in the present design codes in earthquake engineering. Because of the low number of repeated plastic deformations involved, as well as the rare occurrences of strong earthquakes, attention is given to only low-cycle fatigue in this study. The linear cumulative damage criterion is used along with the counting process derived by Middleton. A method of computing the statistics of the cumulative low-cycle-fatigue damage of structures to earthquake excitation is presented herein. As examples, the expected cumulative damage of a structural member to three types of earthquake excitation is computed numerically at various magnitudes.