Damage evaluation of steel frame subjected to impact load at midspan

The lower floors of the multi‐storey structure can be impacted by the debris from the upper floor caused by gas and bomb explosion, which may induce damage or failure of the impacted substructure and lead to progressive collapse of large‐scale structure. The behavior of the frame with a steel beam and two adjacent beam‐column joints was numerically analyzed when the beam was impacted at midspan by various combinations of impact mass and velocity. The failure mode of the joints, the impact load and midspan displacement were analyzed. It shows that the beam‐column joint fails prior to the beam under all impact loading scenarios. The residual bearing capacity of the frame was also studied by statically pushing‐down the substructure after impact loading. A damage index was proposed using the residual capacity of the frame after impact loading divided by the static capacity of the joint without impact loading, and four damage levels, i.e., the minor damage, medium damage, severe damage, and collapse, were defined by the damage index. The boundaries of different damage levels of the frame under various impact loads were fitted, and it can be used to evaluate the damage level of the frame subjected to possible impact loads.