Impact of ground motion record selection on higher-mode responses in RC wall buildings: comparing uniform hazard and conditional spectra approaches

Abstract

Reinforced concrete (RC) wall buildings are multi-storey structures comprising several modes of response that contribute to both the displacement-based and force-based demands acting on them during seismic shaking. This research investigates how code-specified methods for selecting ground motion records affect this response, with particular attention to higher-mode contributions. The study contrasts two target spectra: the uniform hazard spectrum (UHS) and the conditional spectrum (CS), both now permitted in the upcoming Eurocode 8 revision. It examines the influence of their choice for several shear wall buildings designed using Eurocode 8 (EC8). By assessing record selection strategies, including UHS alone and different CS with conditional periods beyond the primary mode, T₁, impacts on engineering demand parameters (EDPs) such as peak-storey drift, wall shear demand, and overturning moment are quantified. The findings indicate that the UHS approach generally leads a more conservative higher structural demand estimate across all EDPs, which is not always very cost-effective. Not surprisingly, using the more advanced CS approach tends to yield much lower demands, but is found to be heavily dependent on the conditioning criteria used. While drift demands can be well-represented when conditioning to the first mode period, CS(Sa(T₁)), EDPs such as shear forces and bending moments can be severely underestimated. More critically, when other conditional selection strategies are used (e.g., CS(Sa(T₂)), CS(Sa(T₃)), etc.), these EDPs increase notably, which could lead to uncon servative designs. This study emphasises the importance of understanding the trade-offs between the presumed increase in accuracy when using the CS versus the UHS for RC wall buildings. It is also an important clarification for the next generation of Eurocode 8 that engineers must be aware of.