SHAKEOUT 2008: Tall Steel Moment Frame Building Response

Abstract

In 2008, there was a significant campaign undertaken in southern California to increase public awareness and readiness for the next large earthquake along the San Andreas fault that culminated in a large-scale earthquake response exercise. The USGS ShakeOut scenario was a key element to understanding the likely effects of such an event. In support of this effort, a study was conducted to assess the response of tall steel structures to a M7.8 scenario earthquake on the southern San Andreas Fault. Presented here are results for two structures. The first is a model of an 18-story steel moment frame building that experienced significant damage (fracture of moment-frame connections) during the 1994 Northridge earthquake. The second model is of a very similar building, but with a structural system redesigned according to a more modern code (UBC 97). Structural responses are generated using three-dimensional, non-linear, deteriorating finite element models, which are subjected to ground motions generated by the scenario earthquake at 784 points spaced at approximately 4 km throughout the San Fernando Valley, the San Gabriel Valley and the Los Angeles Basin. The kinematic source model includes large-scale features of the slip distribution, determined through community participation in two workshops and short lengthscale random variations. The rupture initiates at Bombay Beach and ruptures to the northwest before ending at Lake Hughes, with a total length of just over 300 km and a peak slip of 12 m at depth. The resulting seismic waves are propagated using the SCEC community velocity model for southern California, resulting in ground velocities as large as 2 m/s and ground displacements as large as 1.5 m in the region considered in this study. The ground motions at the sites selected for this study are low-passed filtered with a corner period at 2 seconds. Results indicate a high probability of collapse or damage for the pre-1994 building in areas of southern California where many high-rise buildings are located. Performance of the redesigned buildings is substantially improved, but responses in urban areas are still large enough to indicate a high-probability of damage. The simulation results are also used to correlate the probability of building collapse with damage to the structural system.

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