Effect Of Pier-beam Separation On Pier Failure Under Near-Fault Earthquake

With the increase of seismic monitoring data and the improvement of monitoring level,more and more phenomena are found that the amplitude of vertical excitation acceleration of near site epicenter is higher than the standard value,and the bottom of bridge pier is destroyed under compression.For the beam bridge connected by rubber bearings,the high amplitude vertical ground motion may lead to pier and beam separation collision due to the lack of tensile connection of bearings.The separation and collision process not only produces huge vertical impact force,but also greatly changes the horizontal dynamic response of the bridge structure due to the release of the longitudinal constraints,which leads to a new change in the failure mode of the bridge pier.In this paper,a double-span continuous girder bridge with plate-type rubber bearings is taken as the analysis model,and the dynamic response theory of the separable bridge is deduced by theoretical analysis and numerical calculation method.The influence of pier beam separation conditions and pier beam separation collision on pier failure law is studied.The main research work of this paper includes:(1)By using transient wave function expansion method and indirect mode superposition method,the vertical and longitudinal dynamic response formulas of separable two-span continuous girder Bridges are derived,and the mechanical model and theory of pier beam separation and collision process are established.(2)Under bidirectional harmonic excitation,the relationship between excitation frequency,amplitude and pier beam separation is discussed.The vertical impact force of pier beam and the longitudinal deviation of pier beam are analyzed and the calculation methods of their limit conditions are given.(3)The actual near-site vibration records were selected for fast Fourier transform,which was expanded into simple harmonic superposition of each frequency.The vertical contact force of the pier beam and the ultimate condition of the longitudinal offset of the pier top were calculated,and the failure law of the pier in the process of pier beam separation and collision was proposed: The increase of vertical collision force will change the failure mode at the bottom of the pier,while the increase of the longitudinal deviation of the pier top will lead to the coexistence of multiple failure modes at the bottom of the pier.(4)The separation and collision process of bridge pier and beam under different bridge structure and material parameters and the failure mode of bridge pier at this time are systematically studied.Based on the failure of pier bottom under the most unfavorable condition,the influence of structural parameters on the failure mode of pier is determined.Reasonable bridge structural design may reduce the influence of pier and beam separation caused by vertical earthquake on the failure mode of pier.(5)The finite element dynamic calculation model of the bridge structure is established to calculate the dynamic response of the bridge in the process of pier beam separation collision under the simultaneous vertical and longitudinal excitation.The applicable range of the above-mentioned vertical-vertical decoupling theory is given.