The Relationship of Ground Motion Hazard to the Design of Tall Buildings

MARSHALL LEW, MARSHALL LEW MACTEC ENGINEERING AND CONSULTING, INC.

ABSTRACT:

Many tall buildings in high seismic regions of the United States are being designed using performance-based earthquake engineering (PBEE) principles under alternative design provisions allowed in the current building code. Still-evolving PBEE design criteria for tall buildings generally require that these buildings be designed for collapse prevention due to earthquake ground motion hazard (with a long recurrence interval, on the order of 2,475 years corresponding to a two percent probability of being exceeded in 50 years). Alternative design criteria for tall buildings generally specify some deviations from prescriptive building code provisions, thus allowing for greater building heights, use of high strength materials, and less structural redundancy among other things. However, because of the deviations from prescriptive provisions, nonlinear response history analysis is required, i.e., earthquake time histories are required. When using probabilistic seismic hazard analysis, the ground motions developed for very rare earthquakes are generally dominated by uncertainties. There is great difficulty in identifying or developing realistic strong ground motion time histories for these rare events. In addition, trying to account for source-to-site effects (such as basin, near-source, and directivity) becomes increasingly complex. Developing representative ground motion time histories may involve scaling or modification of actual time histories, in either the time or frequency domains, to match the target response spectrum. A proposed change to ASCE 7 will redefine the methodology by which the ground motion hazard is determined and will likely have the effect of increasing design ground motions in some regions.

 

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