Dynamic soil-structure interaction and energy dissipation mechanisms in the touchdown impact of collapsing high-rise buildings

During high-rise building demolition, collapsed structural components release substantial kinetic energy upon touchdown impact. This touchdown impact induces intense vibrations that severely threaten surrounding infrastructure, constituting the most prominent hazardous effect in demolition engineering. To reveal the mechanism of soil-structure interaction (SSI) during the touchdown impact of a collapsed substructure, we carried out touchdown-impact tests on a 1:3 verification frame model under different drop heights. These tests evaluated the dynamic response characteristics of both the substructure and the soil. We established a corresponding numerical model in LS-DYNA to quantify the energy conversion, energy dissipation, and SSI mechanisms during the impact process. Based on the energy method and the Winkler foundation model, we derived a simplified impact-load estimation model for the collapsed substructure. The results indicate that with increasing drop height, the damage to the frame structure progresses from local cracking of beam-column joints to column-base crushing, beam fracture, and connection failure. As the drop height increases, the dynamic response mode of the soil shifts. It transitions from compression accompanied by partial elastic unloading to short-duration, high-amplitude, compression-dominated indentation. During the touchdown impact, the soil dissipated approximately 70%-74% of the input energy, whereas the frame structure dissipated 17%-20%. Energy was dynamically transferred between the structure and soil through the contact interface. The simplified impact-load estimation model provides reasonable estimates when the first soil layer dominates the load bearing and the participation of deeper soil layers in load bearing is limited. Under these specific conditions, the relative errors remain within 6%. These findings provide a theoretical basis for the safety evaluation and impact-resistant protective design of adjacent infrastructure.