Evaluation of Pre-Northridge Low-Rise Steel Buildings. II: Reliability

The redundancy of building structures under earthquake ground motion attracted more attention after the 1994 Northridge and 1995 Kobe earthquakes. Very little, however, is known on redundancy under stochastic dynamic loads primarily due to the analytical difficulty associated with such problems. This paper proposes a method for evaluation of structural reliability and redundancy of steel buildings under earthquakes in terms of maximum column drift ratio (MCDR). The seismic hazard is expressed in terms of annual exceedance probability of biaxial spectral displacement (BSD). A power law relationship between MCDR and BSD is then established from regression analyses of building response under SAC (SEAOC, ATC, CUREe) ground motions for a wide range of intensity. The annual exceedance probability of MCDR is then determined from the BSD hazard curve for a given MCDR threshold multiplied by a correction 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 calculating the required design force for structures of different degrees of redundancy such that a uniform reliability requirement is satisfied.