Experimental Study And Theoretical Analysis Of Earthquake-induced Pounding Effects On Bridges

Earthquake-induced pounding has been identified as a critical factor affecting structural behavior.Poundings may lead to local damage.Apart from that,the generated impact loads will alter the seismic response of the whole structure.Poundings may occur in both the longitudinal direction(i.e.,between adjacent girders or between girders and abutments)and transverse direction(i.e.,at shear keys)of bridges.However,previous investigations mainly focused on the effect of longitudinal poundings.Only a few studies on poundings in the transverse direction,especially for high-speed rail(HSR)bridges,were performed,and very limited research has been conducted on the load transmission in bridge piers due to earthquake-induced impacts.To fill the knowledge gap,the following works were done:(1)To investigate the pounding effect on bridges due to earthquakes with different horizontal excitation angles(0°,30°,45° and 90°),a smallscale(1/100)model of a typical viaduct was tested using shake table.The poundings between the bridge girder and abutments(i.e.,along the longitudinal direction)and between the girder and shear keys of the abutments(i.e.,in the transverse direction)were considered.The result shows that with different excitation angles,the pounding effects on the bridge are significantly different,and the intensity of longitudinal poundings is much greater than that of transverse poundings.Although pounding the longitudinal direction would lead to a larger girder acceleration,it simultaneously limits the development of girder displacement and bending of the pier.(2)To investigate the effect of transverse poundings on the nonisolated HSR bridge with 32 m standard span,a large-scale(1/6)model was tested by shake table.The pounding reduction of rubber bumpers was also investigated.The result indicates that although the transverse pounding can effectively limit the development of pier-girder relative displacement,large impact loads will be transferred to the piers and therefore lead to an increase in the bending moment at the pier support(over 50%).This finding is significantly different from that of longitudinal pounding effect on bridges.Installing rubber bumpers can reduce the intensity of poundings and thus reduce the impact load transferred to the girder and piers,i.e.,it will decrease the girder acceleration and the bending moment at the pier support.(3)Laboratory tests were also conducted to evaluate the effect of transverse pounding on the HSR bridge isolated by friction pendulum bearings(FPBs).It is found that in the case of no pounding,FPBs are more effective in isolating the seismic force transferring between the upper-and lower-structures of the bridge compared to spherical steel bearings(SSBs).Pounding will significantly decrease the isolation effectiveness of FPBs.In the case of pounding,the maximum girder acceleration of the isolated bridge is even larger than that of the non-isolated one.(4)To further investigate the effect of pounding on the seismic behavior of the HSR bridge,FFT and WT of seismic responses were performed to analyze the responses in the frequency domain.In the case of no pounding,the frequency content of the responses of the non-isolated bridge is determined by the structural fundamental frequency as well as the predominant frequency of the ground motion.For the isolated bridge,it is mainly influenced by the structural fundamental frequency.However,in the case of pounding,the pounding-induced impulse effect and its influence on the dynamic properties of the structure will increase the contribution of high-frequency response.It is also found that installing rubber bumpers can effectively reduce the high-frequency response.(5)To study the mechanical behavior of HSR bridge piers subjected to the earthquake-induced poundings at the pier top,horizontal impact tests were conducted on 1/20-scaled piers.The parameters including impact velocity and mass,impact material,pier height,and impact direction were investigated.The result indicates that a typical pounding includes three phases,i.e.,the pounding phase I,the separation phase,and the pounding phase II.Among them,the pounding phase I has a shorter duration and a larger pounding force,while a longer duration and a smaller contact force are found in phase II.During phase I,the impact load is mainly equilibrium by inertial force in the pier,but in phase II it is jointly equilibrium by inertial force and restoring force of the pier.The pier-top displacement is mainly affected by the impact impulse,while the maximum acceleration is mainly determined by the amplitude of pounding force.In addition,the result of quasi-static tests shows that the capacity of the piers following the impact tests would be significantly reduced.(6)Considering the strain-rate effect of materials,a finite element model of HSR piers due to earthquake-induced horizontal impacts was established using ANSYS/LS-DYNA,and the internal forces developed in the piers were analyzed.Due to the inertia effect,the shear force and bending moment under the impact loads are significantly different from those due to quasi-static loads.Obvious oscillations are found in the bending moment along the pier,and a large negative bending moment can be found at the pier top when subjected to impact loads.In addition,based on the verified simulation method,a finite element model of a multi-span HSR bridge is established,and the influence of the CRTS-II track system on the seismic behavior of the bridge is discussed with considering the transverse pounding.(7)A simplified method for calculating the response of bridge piers due to the horizontal impact load at the pier top was proposed.The method simplifies the piers to a single-degree-of-freedom system,and the impact load is equivalent to a rectangular impulse without changing the impact impulse and duration.The maximum response of the pier can be conveniently obtained by the impact spectrum.Compared with the experimental and numerical results,the error of the maximum pier-top displacement and bending moment at the pier support calculated by the simplified method is within 20%,which verifies the accuracy of the proposed method.On this basis,a calculation method for determining the strength demand of shear keys for HSR bridges is proposed,and the design process is given.The method can minimize the risk of girder collapse on the premise of ensuring that shear keys are damaged prior to bridge piers.The calculation method can provide a reference for designing shear keys in HSR bridges.