The usual seismic design of buildings, based on current codes of practice, does not account for the occurrence of seismic sequences. Structures are designed to resist one single earthquake and then it is supposed that, before the following event, there is enough time to retrofit the damaged construction. This paper investigates the effects of seismic swarms on single storey Concentric X-Braced steel Frames, a very common construction system for industrial buildings. The selected seismic sequences have been chosen so that the first event is considered as the mainshock, and following events have a similar or greater PGA (Peak Ground Motion). The sequences can represent a swarm or a sequence of earthquakes spaced over time, occurring before the structures are retrofitted. The investigation of the effects on steel frames has been conducted in a simplified way through the analysis of Single Degree Of Freedom (SDOF) systems with a behaviour calibrated on the response of Multi-Degree Of Freedom (MDOF) real Concentrically Braced Frames (CBFs). One of the main focuses of this work is on the additional ductility to be used in the seismic design to account for the effects of seismic sequences. For this purpose, the 95th percentile of the required ductility ratio has been calculated in the typical fundamental frequency range of single storey concentric braced steel frames, as to estimate the increase of ductility or strength to be adopted in design. As a result, a reduction of behaviour factor up to 37% has been found.
