Seismic design and performance analysis of bridge bents retrofitted with multistage buckling-restrained braces

Buckling-restrained braces (BRBs) are often used to retrofit RC bridge bents to increase their strength and stiffness, and dissipate the seismic energy through hysteretic behavior. However, the post-yielding stiffness of BRBs is very low, and the total stiffness of the retrofitted bent drops sharply when the brace yields, which can lead to more serious damage to the primary structure contrary to the design purpose. Accordingly, multistage buckling-restrained braces (MSBRBs), consisting of steel cores with different yield strains, were used for the seismic retrofitting of bridge bents in this study. By controlling the low yield point (LYP) core yield before the bridge bents and high yield point (HYP) core yield after the bents, the expressions for the steel core length related to the geometric parameters of the bridge bents were derived. Furthermore, a preliminary seismic design method based on the structural fuse concept was proposed, which realized that the MSBRB dissipates energy while providing stable stiffness to improve the safety of the entire system. The advantages of the MSBRB in controlling the peak and residual displacement of the RC bent were proved via nonlinear time history analyses. Finally, the effects of the stiffness ratios, yield displacement ratios, and different configurations on the seismic performance of retrofitting bridge bents with MSBRBs were examined. The results showed that seismic performance of reinforced bents can be improved by increasing the stiffness ratio appropriately. The LYP core is primarily related to the energy dissipation capacity of the MSBRBs, and the value of beta L should be less than 0.5. The HYP core is largely responsible for controlling the deformation of bridge bent, and beta H must be greater than 1.0. Compared to other configurations, the diagonal MSBRB system has a greater energy dissipation capacity and higher seismic reduction rate.