Impact resistance and residual axial capacity of concrete-filled cold-formed steel built-up columns under two subsequent impacts

Abstract

This study investigates the impact resistance and post-impact axial bearing capacity of novel concrete-filled cold-formed steel (CF-CFS) built-up columns. A total of 12 CF-CFS columns were subjected to two identical impacts using a drop hammer testing machine, followed by axial bearing capacity tests. The experiments recorded the complete impact process, time-history curves of impact force and mid-span deflection, axial load-displacement curves, and final failure modes. The effects of various parameters on the dynamic response and residual ultimate bearing capacity of the specimens were analyzed. Finite element analysis was conducted based on the experimental results to reveal the energy dissipation mechanism of CF-CFS columns and to compare their impact resistance with that of concrete-filled steel tubular (CFST) columns. The results indicate that CF-CFS columns exhibit excellent impact resistance, undergoing bending deformation and retaining residual bearing capacity after two lateral impacts. It was found that the impact force plateau value during the second impact was higher than that of the first impact, and the deflection induced by the second impact was smaller than that of the first impact. In the axial load tests on the impact-damaged columns, specimens with fixed-sliding boundary conditions exhibited the lowest residual ultimate bearing capacity. Due to fastening, high-stress regions in CF-CFS columns were concentrated between the two rows of fasteners near the impact point. When comparing CF-CFS with typical CFST, it was found that under high-impact energy, CF-CFS columns exhibited longer impact duration and greater mid-span deflection compared to CFST columns.