Dynamic response and behavior improvement of square concrete-filled steel tubes under repeated lateral impacts

In bridge anti-collision systems, industrial protective structures, traffic accident-prone zones, and engineering structures exposed to floating-ice or debris-flow impacts, concrete-filled steel tube (CFST) columns are frequently subjected to repeated lateral impacts caused by vehicles, falling objects, or mechanical equipment during service. In this study, drop-weight tests were conducted on nine square concrete-filled steel tube (SCFST) specimens to investigate the effects of impact energy (4.2–16.6 kJ), steel ratio (7.8%-12.4%), and impact times on failure modes, impact plateau force, and mid-span displacement evolution. Experimental results indicate that residual mid-span displacement accumulates approximately linearly with increasing impact times. Under the first impact at an energy level of 4.2 kJ, increasing the steel ratio from 7.8% to 10.3% and 12.4% enhances the impact plateau force by about 53.7% and 77.8%, respectively, while reducing the residual mid-span displacement by approximately 49.5% and 67.3%, respectively. Finite element models were subsequently developed to compare the dynamic responses of SCFST members with inner steel tubes (IS) and without inner steel tubes (NS) under varying hollow ratios (11.1%-30.8%), impact locations, and energy levels. The results show that, for IS members, increasing the hollow ratio from 11.1% to 19.7% and 30.8% increases the first-impact plateau force by 7.3% and 15.5%, respectively, whereas the corresponding change for NS members is negligible; mid-span impact remains the most critical loading condition. A simplified calculation method for the dynamic flexural capacity enhancement factor R_d under repeated impacts is proposed, showing satisfactory agreement with experimental and numerical results. Furthermore, based on 260 numerical simulation cases, ridge regression models were established to predict the dynamic flexural capacity enhancement factor R_d of both IS and NS specimens, showing good agreement with the FE results within the studied parameter range.