The effect of joint ductility on the seismic fragility of a regular moment resisting steel frame designed to EC8 provisions

In order to evaluate the seismic reliability of moment resisting frames at performance levels associated with highly nonlinear structural responses, structural modelling of the rotational capacity of the joints needs to address potential failure modes. This paper begins by reviewing European experimental studies on the cyclic behaviour of steel joints. A linear regression equation, which relates the joint plastic rotation capacity to the beam depth, is proposed based on an analysis of the test data. Thus, the effect of joint ductility and failure on structural response is quantified, within a probabilistic context, through seismic fragility analysis of a mid-rise steel frame designed to Eurocode 8. The variability in structural demand is estimated at increasing ground shaking intensity levels, and fragility curves, conditional on a given ground motion record, are derived for two different performance levels. The inherent randomness in ground motion is taken into account by using an ensemble of recorded accelerograms. The effect of joint rotation capacity is noticeable in mean fragility curves corresponding to high seismic demand and response levels.