Impact of urban layout and ground motion directionality on building responses and damages in San Francisco during the Loma Prieta earthquake

Current regional seismic risk assessment methods do not take into account either ground motion directionality or building orientation. A recent study examined the combined effect of ground motion directionality and building orientation for a testbed of imaginary buildings with the same height, structural material, lateral resisting system, and underlain soil properties. That study concluded that future urban seismic risk analyses should consider the combined effects of ground motion directionality and building orientation, and indicated that further research was warranted to conduct this type of assessment in real-world conditions. This investigation specifically examines the combined effects of ground motion directionality and urban layout on the seismic responses and damages of 150 tall buildings in San Francisco’s Financial District during the 1989 Loma Prieta earthquake, using an advanced probabilistic regional seismic risk framework previously developed by the authors. Building responses and damages were computed for three scenarios: (1) buildings in their current orientations, (2) buildings rotated so that one principal axis is aligned with the RotD100 orientation during the Loma Prieta earthquake in downtown San Francisco, and (3) buildings rotated so that both building principal axes are 45° from the RotD100 orientation. The results indicate that peak building responses and damages can vary by up to 30% and 100%, respectively, depending on changes in the urban grid layout. However, responses and damages at individual building stories can change by much more than 30% and 100%, respectively. Larger responses and damages are expected to occur when the city has an orthogonal grid layout and the orientation of RotD100 aligns with the city’s street grid. These findings, which highlight the combined impact of urban layout and ground motion directionality on building responses and damages, have important implications for city officials, planners, insurers, reinsurers, and other stakeholders involved in regional seismic risk assessments.