Study on the influence of near-fault vertical ground motion on the dynamic response of separation and collision of bridge

This study examines the contact nonlinear behavior of multi-span girder bridges in near-fault regions through dynamic analysis, evaluating the impact of seismic parameters such as excitation amplitude, period, and apparent wave velocity on structural separation thresholds and collision responses. A contact nonlinear dynamic model, based on beam-spring-rod theory, was developed. Parametric simulations were conducted to elucidate the evolution of separation and collision energy transfer at the pier-girder interface, quantitatively assessing the separation risk index and potential collision damage zones. Results indicate that in non-uniform continuous girder bridges, the initial separation is influenced by the span ratio. When the span ratio (middle span/side span) exceeds 1, separation is more likely to initiate in the middle span. The frequency of separation and peak collision force at the middle span contact point are significantly higher than those in the side span, with spatially non-uniform seismic excitation amplifying this risk. When the excitation period approaches the structure's fundamental vertical natural vibration period, the bearing's axial pressure amplitude increases non-linearly. During the non-separation phase, the axial force grows linearly with increased excitation amplitude. Throughout the separation and collision phases, the rate of bearing pressure growth diminishes. The main girder's span ratio notably impacts the contact dynamics, with a higher ratio correlating with reduced likelihood of side-span separation and decreased susceptibility of the side-span's dynamic response to vertical seismic forces.