Select a double span box girder bridge as the analysis model, the vibration control differential equation of ultra-high pier is established, solved by the Runge-Kutta method with variable step length, combined with B-R motion criterion, the dynamic response of ultra-high pier induced by vertical seismic excitation is calculated, and the instability mechanism is studied. Theoretical analysis shows that for high piers, ignoring the bidirectional coupling effect will underestimate the dynamic response of piers. The vertical seismic excitation results in the increase of the axial pressure of bridge pier, which will increase the horizontal deformation of pier. The separation of the main beam and bridge pier will change the extreme deformation value of the bridge pier and increase the risk of pier instability. It has important guiding significance for the theoretical analysis and engineering practice of dynamic instability of ultra-high pier.

By considering the near-field vertical seismic spectrum and calculating the change in vertical contact force between the main beam and the pier, the possible vertical separation contact condition of a bridge is deduced. By calculating the extreme value of the pier–beam vertical contact force and the longitudinal deformation of the pier under the structural separation, the influence of the separation on the failure of the pier is determined. Separation increases the risk of pier failure under compression, bending, and shear, and different separation times lead to different longitudinal responses from the pier, and the first failure mode is different. Therefore, it is of great significance to reasonably design bridges near faults.

In this study, a double span continuous girder bridge is used to derive the dynamic response of the structure. By calculating the variation of vertical contact force of pier and beam, the vertical separation condition of pier and beam is put forward. By comparing the changes of the vertical contact force of pier and beam and the extreme value of longitudinal deformation of pier under the two conditions of separation or not, the influence of the dynamic response change caused by separation on pier failure is analyzed. According to the calculation, the separation of pier and beam is related to the excitation frequency and amplitude, and the separation will increase the risk of pier failure under compression, bending and shear. And the failure mode of bridge pier at the initial moment is separated. Therefore, it is of great significance to reasonably design bridges near faults and balances all kinds of possible pier damage.