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Unformatted text preview: Journal of Earthquake Engineering, Vol. 8, No. 2 (2004) 175207 c Imperial College Press THE VERTICAL-TO-HORIZONTAL RESPONSE SPECTRAL RATIO AND TENTATIVE PROCEDURES FOR DEVELOPING SIMPLIFIED V/H AND VERTICAL DESIGN SPECTRA YOUSEF BOZORGNIA Applied Technology & Science ( ATS ) , 5 Third Street, Suite 622 San Francisco, CA 94103, USA yousef@A-T-S.com KENNETH W. CAMPBELL ABS Consulting and EQECAT, Inc., 1030 NW 161st Place Beaverton, OR 97006, USA KCampbell@absconsulting.com Received 18 April 2003 Revised 17 July 2003 Accepted 8 August 2003 This paper presents the results of a study on the engineering characteristics of vertical ground motion. A ground-motion model for the vertical-to-horizontal ratio (V/H) of peak ground acceleration and 5%-damped acceleration response spectra was derived by using a database of 443 near-source accelerograms recorded from 36 worldwide earth- quakes of M W 4.7-7.7. The recordings were all located within 60 km of the seismogenic rupture zone of worldwide shallow crustal earthquakes in active tectonic regions. We found no bias in the V/H estimates from independent analyses of vertical and horizontal response spectra. We also found V/H to be a strong function of natural period, local site conditions, and source-to-site distance; and a relatively weaker function of magnitude, faulting mechanism, and sediment depth. V/H exhibits its greatest differences at long periods on firm rock (NEHRP site category BC), where it has relatively low amplitudes, and at short periods on firm soil (NEHRP site category D), where it has amplitudes that approach 1.8 at large magnitudes and short distances. We propose a tentative simplified model for estimating V/H spectra and two relatively simple procedures for developing a preliminary vertical design spectrum that could be used for practical engineering appli- cations where site-specific spectra (e.g. from a seismic hazard analysis) are not available. Extensive vetting from the seismological and engineering communities will be needed be- fore such simplified spectra are officially accepted and adopted in codified procedures in the United States. Keywords : Vertical ground motion; vertical design spectrum; vertical component; attenuation. 1. Introduction The characteristics of vertical ground motion have been examined by a number of investigators. Niazi and Bozorgnia [1989, 1990, 1991, 1992] analysed several hun- dred horizontal and vertical response spectra from several earthquakes recorded by the SMART-1 strong-motion array in Taiwan to study the effect of magnitude 175 176 Y. Bozorgnia & K. W. Campbell and distance on the vertical-to-horizontal ratio (V/H) of strong-motion response spectra. In two follow-up studies, Bozorgnia and Niazi  examined horizontal and vertical response spectra on soil and rock sites from the 1989 Loma Prieta, California, earthquake, and Bozorgnia et al. [1995, 1996] analysed horizontal and vertical response spectra recorded on soil sites during the 1994 Northridge, Califor-...
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