In many parts of the world. the repetition of medium-strong intensity earthquake ground motions at brief intervals of time has been observed. The new design philosophies for buildings in seismic areas are based on multi-level design approaches, which take into account more than a single damageability limit state. According to these approaches. a sequence of seismic actions may produce important consequences on the structural safety. In this paper, the effects of repeated earthquake ground motions on the response of single-degree-of-freedom systems (SDOF) with non-linear behaviour are analysed. A comparison is performed with the effect of a single seismic event on the originally non-damaged system for different hysteretic models in terms of pseudo-acceleration response spectra, behaviour factor q and damage parameters. The elastic-perfect plastic system is the most vulnerable one under repeated earthquake ground motions and is characterized by a strong reduction of the q-factor. A moment resisting steel frame is analysed as well, showing a reduction of the q-factor under repeated earthquake ground motions even larger than that of an equivalent SDOF system. Copyright (C) 2002 John Wiley Sons, Ltd.
