Seismic response of steel frames under repeated earthquake ground motions

Seismic sequences characterized by the repetition of medium–strong earthquake ground motions occurring at short time one after the other were observed in many parts of the world. The paper investigates the effects produced by these events on steel structures. Single-degree-of-freedom (SDOF) systems with elastic–plastic laws, moment resistant frames with rigid and semi-rigid joints, and a concentrically braced steel frame have been analysed under seismic sequences. Both the SDOF systems and steel frames are characterized by a significant damage accumulation with respect to only one event. According to the approach where, for damage control limit state verifications, the seismic analysis of a non-linear system is performed through an elastic analysis by using the q-factor, a reduction in the q-factor is hence proposed. This reduction in the behaviour factor should be considered in earthquake-prone regions, where the repetition of seismic events may have a high probability of occurrence.