Effects of vertical earthquake component on columns of reinforced concrete moment frames across different seismic zones

Abstract

This thesis investigates the impact of vertical earthquake ground motion on the seismic performance of reinforced concrete (RC) moment-resisting frames. A 12-story RC moment­ resisting frame, designed in accordance with the NEHRP 2015 seismic provisions for Honolulu, Hawaii, was selected. The building was analyzed using nonlinear time history analysis in OpenSees to capture its seismic response under realistic loading conditions. Earthquake records from four seismically active regions—Kathmandu (Nepal), Anchorage (Alaska), El Centro (California), and Honolulu (Hawaii)—were applied to evaluate the influence of vertical ground motion across different seismic zones. Site-specific amplification and scaling factors were implemented to ensure uniformity in ground motion input. This study compares displacement time histories, moment-displacement relationships, axial force-moment interactions, and stress-strain responses of reinforcement and core concrete under three loading scenarios: horizontal-only, combined horizontal with maximum vertical axial load (P-max), and combined horizontal with minimum vertical axial load (P-min). Results indicate that although vertical ground motion contributes minimally to lateral displacement, it significantly alters axial load variations within columns, thereby affecting flexural strength and strain capacity. Increased axial compression in P-max cases enhances moment capacity but limits deformation, while reduced axial compression in P-min cases decreases moment capacity and allows greater These findings emphasize the importance of incorporating vertical seismic components into structural design and assessment, especially for critical elements in regions subject to strong vertical excitation. This study advances the understanding of vertical earthquake effects on RC moment frames and supports enhanced safety in earthquake-prone areas.