Dynamic behaviour analysis of planar subframes with CFST composite connections under falling impacts

Concrete-filled steel tubular (CFST) structures may experience partial or large-scale collapse under accidental impact loads during service life. However, limited studies have been reported on the anti-falling impact capacity of CFST composite planar frames, especially ultimate capacity under multiple consecutive impacts. This study numerically analyzed the dynamic response, damage mode and energy-absorbing mechanisms of CFST planar subframes with bolt-welded hybrid (BW) and fully welded (FW) connections under single and multiple falling impacts, considering mid-span and beam-end impact positions. Results indicate that planar subframes with FW connection exhibit slightly better anti-impact capacity than BW connection subframes under single impact. For the two connection form subframes, connection cracks developed vertically under mid-span impact, while diagonal buckling of web was observed under beam-end impact. Additionally, the maximum displacement rose by 10.9 % as impact area increased from 0.01 m² to 0.09 m². As the impact times increased, the planar subframes exhibited overall tensile bending damage under mid-span impact and shear damage mode under beam-end impact after the third impact. The peak values of impact force curves were decreased by 8.2 %–12.6 % from the first to the third impact. During the first impact, 60–70 % of the load-bearing capacity was provided by flexural effect, with the catenary effect increasing as impact times increased. The energy absorption of steel beams accounted for about 70 % under mid-span impact, whereas the energy absorption percentage of ring plates and steel tubes was significantly increased under beam-end impact. Finally, the simplified formulas for impact resistance and energy absorption were proposed based on the inertial force, flexural and catenary effects under different impact conditions. The calculation results were compared with the results of existing tests and FE analysis to verify the accuracy of proposed formulas.