Earthquake induced peak floor accelerations in multi-story self-centering concentrically braced frames

Abstract

This study presents theoretical analysis and numerical simulations on the peak floor accelerations (PFAs) of self-centering concentrically braced frames (CBFs), considering it is a pressing need for estimating both structural and nonstructural damages. In the first part, two-degree-of-freedom (2DOF) systems with two flag-shaped springs are employed to elucidate the fundamental characteristics of accelerations. Grounded in structural dynamics, analytical equations are derived to qualitatively estimate peak accelerations of 2DOF systems. In the second part, the multi-story self-centering CBFs with four different story numbers are designed, based on the premise that they attain the same peak interstory drift ratio under the design basis earthquakes. In the nonlinear time history analysis (NLTHA), three suites of earthquake ground motions associated with three seismic hazard levels are employed to excite the structures into different degrees of nonlinearity. The NLTHA results demonstrate that the proposed analytical equations effectively explain the trends of PFAs in the self-centering CBFs. Furthermore, both the 2DOF analysis and seismic analysis of the buildings suggest that the increase of hysteretic parameters of braces could decrease PFAs. Notably, the PFAs are typically higher at the middle of the building than at the lower or upper stories during earthquakes. This phenomenon is elucidated by converting the multi-story frames into the equivalent 2DOF systems, where the equivalent mass ratio increases and equivalent initial stiffness ratios decreases along the structural height.