Behavior insights of steel U-shaped dampers under in-plane loading: Theoretical, experimental, and numerical investigations

Steel U-shaped dampers (UDs) exhibit full hysteresis loops and excellent deformation capability, which have been widely used to dissipate earthquake-induced energy in seismic design. To evaluate the mechanical behavior of steel UDs under horizontal in-plane loading, previous studies have proposed theoretical models for predicting the yield force, yield displacement, and maximum bearing force. However, the existing models generally underestimate the maximum bearing force because they neglect the cyclic hardening effect of steel. In addition, limited attention has been devoted to predicting the ultimate deformation capacity of steel UDs. These limitations underscore the need for accurate theoretical formulations to predict the in-plane behavior of steel UDs. This paper proposes improved theoretical design equations for predicting the load-bearing capacity and deformation capability of steel UDs, considering the cyclic hardening effect of steel. Quasi-static cyclic tests were conducted to investigate the mechanical behavior of steel UDs under various loading conditions. The experimental results demonstrate that steel UDs with different materials or dimensions exhibit stable and full hysteresis loops together with excellent fatigue resistance under various loading conditions. However, once the stable rolling–bending stage was exceeded, the load-bearing capacity increased rapidly, ultimately resulting in ductile failure of steel UDs. The maximum bearing force and displacement of steel UDs during the stable rolling–bending stage are affected by the cyclic hardening effect. Furthermore, finite element models with varying materials and dimensions were developed to perform a parametric analysis. Compared with the experimental and numerical results, the proposed theoretical model provides reliable predictions of the load-bearing capacity and deformation capability of steel UDs. These findings validate the proposed equations, advancing the seismic design of steel UDs in engineering practice.