Assessment of seismic design provisions for floor diaphragms and higher-mode responses using data from instrumented buildings in California

This article assesses the seismic design provisions for floor diaphragms per ASCE/SEI 7-22 Section 12.10.1 and Section 12.10.3, with emphasis on how these provisions account for the higher-mode effects on peak horizontal floor accelerations of earthquake-resistant buildings. A framework is developed for processing and analyzing strong-motion acceleration data from buildings monitored by the California Strong Motion Instrumentation Program (CSMIP), which is used to assess the floor diaphragm design acceleration coefficients in ASCE/SEI 7-22. These coefficients, which estimate peak floor accelerations at the design-level earthquake, are modified in this study to consider the intensity of measured ground motions and the level of earthquake-induced inelastic building response. First, the spectral response factors, defined as the ratio between the spectral response acceleration at a given floor and that at the ground, are evaluated at the second- and third-mode periods and compared with those at the first-mode period to assess the higher-mode effects. Then, two metrics are defined to compare the modified acceleration coefficients with the measured peak floor accelerations. The results confirm the need to consider the characteristic dynamics of the buildings to estimate the design forces for floor diaphragms. This consideration is included in ASCE/SEI 7-22 Section 12.10.3 but not rationally considered in Section 12.10.1. The modified acceleration coefficients based on ASCE/SEI 7-22 Section 12.10.1 can considerably underestimate the peak floor accelerations, whereas those based on Section 12.10.3 can capture the increased peak floor accelerations caused by the higher-mode earthquake-induced inertial forces. Thus, the authors recommend using ASCE/SEI 7-22 Section 12.10.3 over Section 12.10.1 to estimate the design forces for floor diaphragms; however, certain assumptions considered in Section 12.10.3 require further revision.