Effects of Ground-motion Duration on the Response of a 9-Story Steel Frame Building

Subduction zone earthquake events present a significant hazard in Northern California and the Pacific Northwest. Compared to shallow crustal earthquake motions, subduction zone earthquake motions present longer duration and lower frequency content. As a consequence, the response of structures subjected to these two types of ground motions can be considerably different, especially for mid-rise and high-rise buildings. However, recent and current design codes were calibrated considering only the crustal motions, and the increased vulnerability of existing structures to subduction zone earthquakes is largely unknown. In the present paper, the performance of a steel moment resisting frame building structure is studied under subduction zone earthquake motions and shallow crustal earthquake motions. The 9-story building is part of a set of buildings designed and analyzed under the SAC steel building project. The building that was designed according to pre-Northridge codes and standards is modeled in the Open System for Earthquake Engineering Simulation (OpenSees) using a centerline two dimensional model. Columns are modeled using a nonlinear force-based fiber-section beam-column elements. Beams are modeled using finite-length plastic hinge elements considering a recently proposed bilinear model that accounts for strength and stiffness deterioration. P-Delta effects are accounted for in the analysis by using a leaning column. The performance of the structure under different seismic scenarios is evaluated using nonlinear dynamic response analyses. The results obtained showed significant differences in the seismic demands caused by the two different types of earthquake motions. The increased loss of strength and stiffness and increased residual displacements obtained for the subduction zone earthquake motions illustrate the fact that considering only shallow crustal earthquake motions in design and assessment can lead to under-prediction of the expected damage.