Aspects of seismic response margins and overstrength of massive low-rise RC shear walls

Abstract

The nonlinear seismic behavior of massive low-rise shear walls under various earthquake motions is studied to contrast design assumptions with actual structural behavior. Various perspectives are herein adopted to understand the evolution of structural changes and their connection to possible margins. We specifically focus on behavior up to the reinforcement yield. A dedicated analytical model is used to calculate the structural response. The change in the apparent vibration frequency due to stiffness degradation is assessed using a time-frequency analysis, separately for the wall responding to the yield limit and to the ultimate limit. A considerable decrease in the apparent vibration frequency is noted also in the first case. When approaching the problem through load categorization, we see that such a categorization changes as the structure utilizes its ductile capacity. Next, assuming an elastoplastic idealization, we compare the code-based response spectrum with response spectra derived for scaled acceleration records, which would provoke a structural response to the yield limit. For such excitation, spectral ordinates higher than code-based values have been found. Seismic overstrength, defined in terms of yield reduction factors, has been calculated for various natural periods of the system. The strengthened yield reduction factors have been found to be 0.85–2.43 for the wall when responding to its yield limit and 1.16–5.11 when responding to its ultimate limit. Utilization of the overstrength over the idealized yield strength strongly depends on the character of the excitation and the natural period of the structure and can mean slight or considerable margins.