About the author  ⁄ Michael T. Valley, P.E, S.E.

Michael Valley is a Principal with Magnusson Klemencic Associates. (mvalley@mka.com)

How Collapse Potential is Affected by the Method of Considering Accidental Torsion

Structural Engineers have long observed that torsional building response is an indicator of earthquake collapse risk. The Building Code’s explicit treatment of torsion dates back at least to the 1961 Uniform Building Code (UBC), which introduced the requirement of adding 5% eccentricity to any inherent torsion when distributing lateral earthquake forces to the vertical seismic force-resisting elements. Although today’s code includes additional penalties for torsionally irregular structures, the treatment of “accidental torsion” remains much the same. This often-maligned but critically important provision prohibits the design of cruciform-type structures without any torsional strength. It also offers increased collapse protection by indirectly accounting for the non-uniform degradation of the vertical seismic force-resisting elements that occur in the true non-linear response of structures.

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Salesforce Tower.

Salesforce Tower.

New Benchmarks in High-Rise Seismic Safety

When completed in 2017, Salesforce Tower will be the tallest building in San Francisco at a height of 1,070 feet (901 feet to the top occupied floor). This super-tall building advances the state-of-the-art of high-rise seismic design through implementation of a number of first-ever design and analysis methods that push limits and set new industry benchmarks. The structural innovations required to create this record-setting, city-defining tower address enhanced performance objectives, foundation challenges, and interactions with adjacent buildings… issues applicable both to this building and future tall buildings in areas of high seismicity.

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STRUCTURE magazine