Improving Seismic Capacity of New Concrete Ductile Core High-Rises: Costs and Benefits of Managing Seismic Risk in the Legal Arena

Mark N White

Abstract


In San Francisco and other urban centers, developers are increasingly choosing concrete ductile core designs for their new high-rise projects (most are more than 160 feet in height and many are upwards of 40 stories).  Many of these new designs incorporate only a single system for resisting lateral seismic loading.  This design approach has not yet been tested by a significant earthquake in the western United States.  For instance, the August 24, 2014 Napa earthquake (M 6.0) generated peak ground acceleration on the order of three percent gravity (or less) at ground zero for San Francisco’s relatively new concrete ductile core high-rises, nowhere near the level of ground acceleration expected in DBE or MCE earthquakes with epicenters inside 15 miles of ground zero.  ASCE 7 provides guidance to the developer when threshold seismic performance targets are selected, specifying appropriate “Risk Categories†and “Importance Factorsâ€.  When the expected occupancy use is residential, most owners and structural designers use Risk Category II, with an Importance Factor of 1.00.  In contrast, when the structure is expected to be used as an office building, many owners and structural designers use Risk Category III, with an Importance Factor of 1.25.  Using Risk Category III and an Importance Factor of 1.25 for new high-rise residential projects employing the concrete ductile core design approach should increase the seismic resiliency and simultaneously reduce the owner’s legal risk profile because of expected improved seismic performance in DBE and MCE events. Taking into account the extra costs of construction, adhering to Risk Category III design criteria should yield substantial long-term benefits for owners of these high-rises and help to manage their seismic risk in the legal arena.

Keywords


Legal Risk; Seismic Risk; High-Rises; Structural Design; Seismic Performance

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References


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