Damage evaluation of steel frame subjected to falling debris impact at midspan

Falling debris impact can cause severe structural damage or even trigger progressive collapse. This study investigates the impact response and post-impact performance of steel frames with welded unreinforced flange–bolted web (WUF-B) connections subjected to falling debris. A finite element (FE) model, calibrated against experimental data, is employed to simulate the dynamic response of steel frames. Damage to this steel frame is characterized by using two indices based on residual load bearing capacity and residual energy dissipation capacity. A series of numerical simulations across a wide range of impact mass–velocity (m–v) combinations is conducted and the results reveal that the beam-column connection is the most vulnerable component under midspan impact. It is also found that only considering residual load bearing capacity may underestimate damage severity. Incorporating residual energy dissipation capacity provides a more reliable damage assessment. Critical m–v diagrams are developed to classify damage into four levels, i.e., minor (D=0–0.2), medium (D=0.2–0.5), severe (D=0.5–1), and collapse (D=1), based on the proposed indices (D). Finally, empirical formulae are established to predict the parameters of m–v diagram as functions of frame parameters, enabling efficient damage evaluation of steel frames with varying configurations, which can be used for vulnerability assessment of steel frames subjected to falling debris impact.