Many existing deep reinforced concrete coupling beams that have low span-to-depth ratios are not desirable for seismic design due to their potential brittle failure with limited ductility and deformability. Previous experimental and numerical studies have demonstrated that the laterally restrained steel plate retrofitting method could effectively enhance the seismic performance of deep coupling beams. This article aims to develop a nonlinear finite element model based on the OpenSees software to accurately predict the behavior of coupled shear walls with or without laterally restrained steel plate coupling beams. The numerical results reveal that the seismic performance of the laterally restrained steel plate retrofitted coupled shear walls can be significantly enhanced. Furthermore, a genetic algorithm is adopted to determine the optimal positions of laterally restrained steel plate coupling beams. It is found that desirable seismic performance can be achieved by retrofitting coupling beams in the middle and higher floors of a building with coupled shear walls. However, only retrofitting coupling beams on the lower floors should be avoided because this could result in the serious degradation of the lateral load-carrying capacity and stiffness of the coupled shear walls.

Introduction

Reinforced concrete (RC) coupled shear walls are typically used in lateral load resisting systems in the design of high-rise buildings. Coupling beams, which connect individual wall piers, are a critical component that provides lateral strength, stiffness, and deformation capacity to an entire building. In the past decades, the design of many concrete buildings in moderate seismicity regions, like the South China region including Hong Kong, did not take earthquake actions into account. Following the new seismic design codes, many existing coupling beams, especially those with a low span-to-depth ratio (less than 1.5), are presumed to have inadequate earthquake resistance which could fail in brittle shear modes and are not acceptable in seismic assessments of coupled shear wall systems (Kwan and Zhao, 2002; Paulay, 1971). The local failure of coupling beams could lead to global failure of coupled shear walls (Smith et al., 1991). To improve the seismic resistance of existing buildings, coupling beams that are deficient in shear or lack deformability will need to be retrofitted. Harries et al. (1996) investigated the different ways to fix a thin steel plate to one side of coupling beams with a span-to-depth ratio of 3. Their study revealed that a hybrid method of bolting and epoxy bonding to attach steel plates both in the span of the beams and at the ends is more desirable. Su and Zhu (2005) conducted experimental and numerical studies on coupling beams with a span-to-depth ratio of 2.5 reinforced with bolted steel plates without adhesive bonding. In their study, minor buckling of the steel plates was observed but the effects of local buckling on the behavior of the composite coupling beams were not investigated. Cheng and Su (2011) proposed the use of bolted steel plate with lateral restraints (without stiffeners) to retrofit deep RC coupling beams with a span-to-depth ratio of 1.1. Steel angles were used to suppress plate buckling mode and enhance stable shear yielding mechanism of steel plate. Their test results revealed that this method, namely laterally restrained steel plate (LRSP) method, could substantially enhance the postpeak behavior, deformability, and energy dissipation ability of deep RC coupling beams. However, the shear buckling of the steel plates in the early stages usually results in reduced strength, stiffness, and energy dissipation capacity and shows significant pinching. To mitigate these problems, Cheng et al. (2016) added stiffeners to LRSP to further suppress the premature plate buckling and expand the yield zone for better energy dissipation. Cheng et al. (2017) used nonlinear finite element analysis to investigate the effect of stiffener arrangements on the performance of the LRSP retrofitted coupling beams. Their numerical studies showed that when the stiffeners diagonally pass through the center of the steel plate, there is better control of the concrete cracks and a more ductile failure mode is achieved.