Assessment Method for Inelastic Higher Mode Effects in Outrigger Braced Tall Building Under Seismic Loads

Abstract

Outrigger system is a more popular structural system in tall buildings and higher mode participation in such buildings is an important aspect in design. During severe earthquake impacts, buildings undergo inelastic deformations, and this can lead to lengthening of modal periods of the building. Since higher mode effects have a significant impact on the overall behavior in tall buildings, the impact due to lengthening of modal periods needs to be addressed in the design. In this study, a simple assessment method is presented to quantify the effect of higher mode response on the behavior of outrigger braced buildings under seismic loads. The method comprises of simplifying the building to vertical cantilever with lateral degrees of freedom equal to number of stories and rotational springs with equivalent spring stiffness at corresponding outrigger locations. A pin-based structure was assumed to limit the first mode response to represent the transition from elastic to inelastic status of the building. The proposed method is capable of estimating the inelastic lengthened periods for higher modes and their responses in outrigger braced tall buildings. The developed method was applied to a case study outrigger braced tall building and reasonable results are predicted.