Safety of Tall Pre-Northridge Steel Frame Buildings and Implications on Cordoning and Recovery Deierlein, G. G.; Yen, W. Y.; Hulsey, A. M.; Galvis, F.; Baker, J. W.; Molina Hutt, C.
The downtown business districts of San Francisco, Los Angeles and other cities in the western United States have significant inventories of older (pre-2000) tall steel buildings that have elevated risk, compared to new code-conforming buildings, of damage and collapse from strong earthquakes. Many of these buildings have welded connections of the type that experienced sudden brittle fractures during the 1994 Northridge earthquake, and they were designed under building codes with less stringent requirements on seismic drifts, minimum column strengths, and seismic base shears. Compounding these deficiencies are new seismic hazard models which indicate that long-period spectral accelerations and durations of earthquake ground motions are larger than considered in older design codes. This study applies newly developed models to assess the response of older steel buildings from the onset of damage up to collapse. The analysis results are used to examine issues associated with post-earthquake inspection and safety, which have implications on building reoccupancy and recovery. The study considers the implication of structural damage on recovery of both the tall building itself, as well as implications of safety cordons around the tall building on the re-occupancy and recovery of neighboring buildings. A method is proposed for quantifying the post-earthquake safety of damaged tall buildings to inform criteria for establishing safety cordons. The building-specific performance-based assessment method of FEMA P58 is extended and applied to quantify the post-earthquake recovery time for neighboring buildings. In addition to providing more comprehensive estimates of community downtime, this framework can include metrics for whether certain buildings are likely to induce inordinately long downtimes in surrounding buildings or how a cordoning strategy will affect a community. This type of downtime assessment can inform a range of policy and planning decisions, such as requiring seismic retrofit of vulnerable tall buildings, evaluating emergency response strategies, and developing postearthquake recovery plans.
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