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Vertical Ground Motion: Characteristics, Relationship with Horizontal Components, and Building Code Implications

by Yousef Bozorgnia, Kenneth Campbell and Mansour Niazi

Bozorgnia, Yousef, Kenneth Campbell and Mansour Niazi (1999). Vertical Ground Motion: Characteristics, Relationship with Horizontal Components, and Building Code Implications. SMIP99 Seminar on Utilization of Strong-Motion Data, p. 23 - 50.

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Abstract

In this study, the characteristics of peak vertical ground acceleration and vertical response spectra are examined and the differences between the vertical and horizontal components are investigated. This was accomplished with a comprehensive database of 2,823 free-field components (three per recording) of uncorrected peak ground acceleration from 48 worldwide earthquakes and 1,308 free-field components of corrected peak ground acceleration and response spectral acceleration from 33 worldwide earthquakes, all recorded within 60 km of the causative fault from earthquakes ranging from 4.7 to 7.7 in magnitude. Peak and spectral acceleration attenuation models were developed for both the vertical and horizontal components as a function of magnitude, source-to-site distance, type of faulting, and local soil conditions. An analysis of residuals indicated that the vertical-to-horizontal (V/H) spectral ratios predicted by these attenuation relationships show no significant bias with respect to observed VH and the modeled parameters. The study clearly demonstrates the strong dependence of V/H on oscillator period, source-to-site distance, and local soil conditions. V/H shows a weaker and more limited dependence on magnitude and type of faulting. The largest short-period V/H ratios are observed to occur on Holocene Soil at short periods and short distances where they can reach values in excess of 1.5 at 0.1-sec period. The largest long-period V/H ratios are observed to occur on Hard Rock where they can reach values as high as 0.7. Generally V/H is 0.5 or less at the longer periods (0.3 to 2.0 sec). We conclude that the standard engineering practice of assigning VH a value of two-thirds is unconservative at short periods, especially for unconsolidated soil, but conservative at long periods, and should be modified. We propose a simplified model for estimating a design vertical response spectrum for engineering purposes from a simplified model of V/H that better fits the observed trends in V/H. The procedure seems to have merit and will be refined in a future study.