Seismic Design Provisions Based on Uniform Hazard Spectrum

Abstract

In the equivalent static load procedures specified in many seismic codes, including the National Building Code of Canada (NBCC), the elastic base shear is obtained from a design response spectrum, using the first mode period of the structure. The elastic response spectrum is obtained applying appropriate amplification factors to the peak ground motion bounds. In the recent years, methodologies have been developed that allow the direct determination of maximum spectral accelerations for specified values of the period of a single-degree-of-freedom system and for a given probability of exceedance. A plot of such spectral acceleration values is referred to as a uniform hazard spectrum (UHS). Several codes propose to use UHS as the basis for determining the seismic design base shear. However, the base shear obtained from a UHS corresponding to the first mode period needs to be adjusted for higher mode effects. Base shear adjustment factors are derived for several different structural types including moment-resisting frames, braced frames, and flexural walls. The corrected base shear can be distributed across the height according to the NBCC procedures. However, because the NBCC distribution is primarily in the form of the first mode, the resulting overturning moments generally overestimate the true moments, which arise from a combination of various modes. Adjustment factors to be applied to the overturning moments are derived in this chapter for various structural types.