Comparative Evaluation of Vertical Seismic Effect Methods in ASCE 7 Across the Contiguous United States

Section 12.4.2 of the ASCE/SEI 7‐22 provisions offers two alternative methods for estimating the seismic load effects due to vertical ground motions: one based on horizontal spectral acceleration at a short horizontal period and the other uses the vertical spectral acceleration at a vertical structural period. This study examines similarities and discrepancies between the two methods for the contiguous U.S., covering Seismic Design Categories (SDCs) A through E, and vertical structural periods ranging from 0.01 to 10 s. Overall, 331,231 sites in the U.S. are analyzed in this study. A lognormal probabilistic model is developed to maintain approximate mathematical tractability of the distribution of the ratio of vertical seismic load effects computed by the two methods across the considered vertical period range for each SDC. Results indicate that, for sites located in the Western U.S. (WUS), and for vertical structural periods between 0.05 and 0.2 s, the approach based on vertical spectral acceleration generally results in higher vertical seismic load effects than the one based on short‐period horizontal spectral ordinates. For sites located in the Central and Eastern U.S. (CEUS), the short‐period horizontal spectral acceleration approach consistently yields comparable or higher estimates of the vertical seismic load effects than the one based on the vertical spectral accelerations.