Seismic fragility of cold-formed steel structures considering the performance degradation of hold-downs

Abstract

Hold-downs are the key components to ensure the force transmission of a cold-formed steel (CFS) structure, and their mechanical properties have a significant impact on the ability of the structure to resist earthquakes and wind loads. A collapse limit considering the influence of the performance degradation of hold-downs was analyzed first. Then, taking ground motion duration into consideration, equivalent brace models considering and without considering the nonlinear behavior of hold-downs were respectively established, to compare the influence of the performance degradation of hold-downs on the seismic fragility of CFS structures. The results show that (1) Since the anchoring effect of hold-downs on the boundary studs of CFS shear walls is easily weakened due to the looseness of anchor bolts, resulting in a premature collapse of the structure, the rotation deformation of the walls caused by the yielding of hold-downs should be considered when defining the story drift limit corresponding to the CP (collapse prevention) state; (2) compared with the models with hold-downs being assumed to be ideally hinged, the seismic capability, especially the collapse resistance capacity of the models considering the performance degradation of hold-downs, is significantly reduced; (3) long-duration ground motions will increase the impact of the performance degradation of hold-downs on the seismic capacity of CFS structures; during seismic design of CFS structures, the influence of long-duration ground motions should be fully considered, the requirements on the ductility, stiffness, strength and other properties of the structures should be higher than that of the structures under short-duration ground motions.