Estimate of V/H spectral acceleration ratios for firm soil sites in Eastern Canada

Abstract

This study aims to provide an estimate of vertical-to-horizontal (V/H) pseudo-spectral acceleration (PSA) ratios in the Eastern Canada seismic zone for firm soils (360 < Vs30 < 760 m/s) referred to as Site Class C in the National Building Code of Canada (NBC). According to previous studies, the 2/3 V/H empirical ratio prescribed in NBC is deemed overestimated for far-field areas and underestimated for near-field areas. In this study, the V/H PSA ratios were computed for 248 records from 67 historic earthquakes in the Eastern Canada region with a magnitude Mw ≥ 3.0 and an epicentral distance (Repi) < 150 km. Given the lack of available records for Site Class C in this region, sets of records from other site classes, mostly Site Class A (Hard rock), were selected and converted to the corresponding records on Site Class C. To this end, the equivalent linear method, using the Pressure-Dependent Modified Kondner Zelasko (MKZ) model of analysis in the frequency domain was selected using the software DEEPSOIL. Computed V/H PSA ratios were then calibrated with those obtained from available Ground Motion Prediction Equations (GMPEs) compatible with Site Class C of the studied region. The computed mean V/H PSA ratios were found to exceed the common value of 2/3 recommended in most codes, especially for short periods up to 1.3 sec, and new V/H ratios were proposed as a function of the fundamental period of the building. Finally, a profile of vertical acceleration design spectra (ADSver) was proposed for Site Class C in Montreal and compared with those obtained by ASCE/SEI 7–16 and ASCE 41-17 provisions.