Representation of bidirectional ground motions for design spectra in building codes
Earthquake Spectra
By: Jonathan P. Stewart, Norman A. Abrahamson, Gail M. Atkinson, Jack W. Beker, David M. Boore, Yousef Bozorgnia, Kenneth W. Campbell, Craig D. Comartin, I.M. Idriss, Marshall Lew, Michael Mehrain, Jack P. Moehle, Farzad Naeim, and Thomas A. Sabol
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Abstract
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.
Suggested Citation
Stewart, J.P., Abrahamson, N., Atkinson, G.M., Beker, J.W., Boore, D.M., Bozorgnia, Y., Campbell, K.W., Comartin, C.D., Idriss, I., Lew, M., Mehrain, M., Moehle, J.P., Naeim, F., Sabol, T., 2011, Representation of bidirectional ground motions for design spectra in building codes: Earthquake Spectra, v. 27, no. 3, p. 927-937, https://doi.org/10.1193/1.3608001.
| 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 |
| Publication Date | August 01, 2011 |
| 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 |