Nonlinear analysis of a steel frame

Abstract

Nonlinear seismic analysis is used in structural engineering profession to design steel frames for moderate to strong earthquakes. Following the Northridge earthquake of 1994 in Southern California, widespread damage to many steel frames was discovered. As a result of multi-million dollar efforts by the professionals, many post-Northridge steel beam–column connection designs were proposed. They are essentially partially-restrained (PR) connections, and their presence adds yet another major source of nonlinearity that must be considered appropriately in nonlinear seismic response analysis. In this paper, a case study of analytical investigation of nonlinear seismic performance of a 9-story steel frame is presented. We consider major sources of nonlinearity in the system including the nonlinear behavior of the connections. Using the four-parameter Richard model, a mathematical model is proposed first to represent moment–relative rotation ( M – θ ) curves for a proprietary steel connection. The model can generate ( M – θ ) curves for other beam–column assemblies. Time-domain, nonlinear seismic response of a steel moment-resisting frame (SMRF) in the presence of this type of connection and pre-Northridge bolted-web, welded-flange (denoted here as BWWF) connections are evaluated. The results are then compared with fully-restrained (FR) steel connections typically used in the design. The results analytically confirmed the beneficial effects of connections observed in full-scale testing.