Evaluation of pre-northridge low-rise steel buildings. II: Reliability

The redundancy of building structures under earthquake ground motion attrac ted more attention after the 1994 Northridge and 1995 Kobe earthquakes. Ver y little, however, is known on redundancy under stochastic dynamic loads pr imarily due to the analytical difficulty associated with such problems. Thi s paper proposes a method for evaluation of structural reliability and redu ndancy of steel buildings under earthquakes in terms of maximum column drif t ratio (MCDR). The seismic hazard is expressed in terms of annual exceedan ce probability of biaxial spectral displacement (BSD). A power law relation ship between MCDR and BSD is then established from regression analyses of b uilding response under SAC (SEAOC, ATC, CUREe) ground motions for a wide ra nge of intensity. The annual exceedance probability of MCDR is then determi ned from the BSD hazard curve for a given MCDR threshold multiplied by a co rrection factor for the capacity and modeling uncertainties. The effects of brittle connection failure, biaxial interaction, and torsional motion are examined. A uniform-risk redundancy factor is also developed for calculatin g the required design force for structures of different degrees of redundan cy such that a uniform reliability requirement is satisfied.