Abstract
Current building code requirements for seismic design are primarily intended to minimize life-safety risks due to structural damage under extreme earthquakes. While tall buildings designed under current standards are expected to achieve the life-safety goal, this study estimates that they may require up to 7.5 months of repair to return to functionality after a design-level earthquake (roughly equivalent to ground motion shaking with a 10% probability of exceedance in 50 years), and over 1 year after a risk-targeted maximum considered earthquake (roughly equivalent to ground motion shaking with a 2%–4% chance of exceedance in 50 years). These long downtimes, which correspond to median predictions, far exceed recovery goals for major employers and other recovery-critical uses and can have disproportionately harmful effects on businesses and residents. To address such extensive downtime risks, we evaluate the impact of recovery-based design guidelines for reducing recovery times through (1) more stringent drift limits under expected ground motions and (2) measures to mitigate externalities that impede recovery. The results suggest that by combining these strategies, expected recovery times following a design-level earthquake can be reduced to roughly 1 month, and to 2 months following a risk-targeted maximum considered earthquake. These findings are illustrated for an archetype 42-story reinforced concrete shear wall residential building and a 40-story steel buckling-restrained braced frame office building in San Francisco, CA.
Highlights
Enforced building code requirements for earthquakes are primarily intended to provide acceptable safety in extreme earthquakes
The maximum maximum considered earthquake (MCER) story drift ratios, in both reinforced concrete shear wall (RCSW) residential and buckling-restrained braced frame (BRBF) office buildings, are significantly less than the 3% limit currently prescribed by PEER TBI guidelines (PEER, 2017) for performance-based seismic designs
Current building requirements for tall buildings limit the maximum story drift ratios to 3% under MCER ground motions, which is roughly equivalent to story drift ratios of 2% under design earthquake (DE) ground motions
Summary
Enforced building code requirements for earthquakes are primarily intended to provide acceptable safety in extreme earthquakes. Non-prescriptive design procedures, such as the guidelines for performance-based seismic design of tall buildings, employ nonlinear analysis to evaluate expected building response to large earthquakes and, thereby, provide greater assurance that buildings meet the performance targets intended in current building codes to minimize life-safety risks under extreme earthquakes. Two recent studies of a hypothetical reinforced concrete shear wall (RCSW) residential building, which represents many of the recently constructed tall buildings in San Francisco, indicate that these structures may incur damage requiring repairs costing about 15% of building replacement cost under a design earthquake (Tipler, 2014) and 5% under a magnitude-7 Hayward fault earthquake (Almufti et al, 2018) These values of economic losses are not uncommon in seismic performance assessments of modern buildings. Tall building construction slowed down in the 1990s, but has resurged since with almost 25% of the inventory constructed since 2000
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