Seismic Analysis of MR Steel Frames based on Refined Hysteretic Models of Connections

Evaluation of seismic performance of moment-resisting (MR) steel frames is usually carried out assuming the elastic-perfectly-plastic type of hysteresis model for plastic zones (in particular for beam-to-column connections). This type of model is associated with a conventional available ductility, giving an indication of the maximum deformation beyond which strength degradation is likely to occur. This methodology leads to conservative results and the elastic-perfectly-plastic type of hysteresis model is certainly adequate in obtaining reliable information on deformation when the above ductility limits are not exceeded. However, the safety of the frame at 'collapse' remains unknown. The analyses presented in this paper show that safety margins against global collapse of the structure can be predicted only by considering more realistic hysteresis behaviour, i.e. using mathematical models able to take account of strength degradation and pinching phenomena. Moreover, it is shown that the design of conventional steel building systems according to the European seismic code may lead to over-resistant structures, due to the limitation on inter-storey drift angles for non-structural damage control under frequent earthquakes. It is also noticed that this result is inevitable, owing to the indications provided by Eurocode 8 in terms of both prescribed inter-storey drift limits and assumed base shear-force demand under frequent earthquakes.