Representation of bidirectional ground motions for design spectra in building codes

Earthquake Spectra
By: , and 



The 2009 NEHRP Provisions modified the definition of horizontal ground motion from the geometric mean of spectral accelerations for two components to the peak response of a single lumped mass oscillator regardless of direction. These maximum-direction (MD) ground motions operate under the assumption that the dynamic properties of the structure (e.g., stiffness, strength) are identical in all directions. This assumption may be true for some in-plan symmetric structures, however, the response of most structures is dominated by modes of vibration along specific axes (e.g., longitudinal and transverse axes in a building), and often the dynamic properties (especially stiffness) along those axes are distinct. In order to achieve structural designs consistent with the collapse risk level given in the NEHRP documents, we argue that design spectra should be compatible with expected levels of ground motion along those principal response axes. The use of MD ground motions effectively assumes that the azimuth of maximum ground motion coincides with the directions of principal structural response. Because this is unlikely, design ground motions have lower probability of occurrence than intended, with significant societal costs. We recommend adjustments to make design ground motions compatible with target risk levels.
Publication type Article
Publication Subtype Journal Article
Title Representation of bidirectional ground motions for design spectra in building codes
Series title Earthquake Spectra
DOI 10.1193/1.3608001
Volume 27
Issue 3
Year Published 2011
Language English
Publisher EERI
Publisher location Oakland, CA
Contributing office(s) Earthquake Science Center
Description 11 p.
Larger Work Type Article
Larger Work Subtype Journal Article
Larger Work Title Earthquake Spectra
First page 927
Last page 937
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