Study On Numerical Model And Nonlinear Seismic Response Of Deep-Water Bridge With High Piers Under Fluid-structure Interaction

The infrastructure construction has been gradually focused on the western area of China,which leads to a result that a large amount of long-span deep-water bridges has been built or under construction recent years.These deep-water bridges are used to stride across valleys and reservoirs.Different from eastern area of China,the western area is characterized by frequent earthquakes,rushing water and other poor geographical conditions,these conditions bring challenges to deep-water bridge design.Through past researches,it is recognized that fluid-structure interaction has nonneglectable effect on structure seismic responses,however,there are a small number of researches on deep-water bridge aseismic problem abroad,domestic research on this problem is rather less.Based on the fact,modal tests were conducted on a specimen,which was scaled from a deep-water bridge pier with group pile foundation.Then,the effects of fluid-structure interaction on structural dynamic characteristics were studied from test data.Meanwhile,numerical models of the specimen were built and analyzed.Based on the experimental and numerical results,finite element models of the chosen long-span deep-water bridge were established and seismic responses of the bridge structure under different seismic excitations were calculated and analyzed.After,fluid-structure interaction and pounding effects were considered at the same time and seismic responses of the deep-water bridge models were studied.Finally,seismic responses of the deep-water bridge structure under fluid-structure interaction and nonlinearity conditions were studied.The main research contents are introduced as following.(1)The research status of fluid-structure interaction of deep-water bridges were summarized.Studies on seismic responses of long-span bridges under pounding or nonlinearity condition were acquaintanced.Analysises and attempts were done to prepare for the next research work.(2)Water tank and specimen of deep-water hollow bridge pier were designed.Considering the hollow pier contacting with outer water,inner water and both outer and inner water,meanwhile,considering different added masses designated as 0kg,11.4kg,23.4kg and 31.7kg,and modal tests were carried out on the specimen to study the fluid-structure interaction effects on structural vibration frequencies.Numerical models of the specimen under different combined cases were also been built,and numerical results validated against the experimental results,the adopted numerical method is reliable.(3)Based on the verified numerical method,finite element model of the main bridge and two approach spans with one at each side was established and the model was proved to be reliable.Then,dynamic properties and dynamic responses of the model under different water levels and different earthquakes were analyzed.The results show that fluid-structure interaction can increase vibration periods and dynamic responses,especially the relative displacements between adjacent spans which could lead to beam collision.Further,the pounding effect between adjacent girder at both expansion joints were considered,and seismic responses of the model under different earthquake actions were analyzed.The analysis work includes out of phase vibration under fluid-structure interaction,seismic responses under pounding effect only and seismic responses under the combination effects of pounding and fluid-structure interaction.The results indicate that the fluid-structure interaction can increase the difference between vibration properties of adjacent span,which leads to increased relative displacements.Fluid-structure interaction is the main cause to make the seismic responses enlarged,while the pounding effect is favorable to the main bridge during earthquake.Nevertheless,pounding effect could amplify the deck displacement resulting in increased possibility of span unseating.The existence of inner water would aggravate the effect of fluid-structure interaction.(4)The hollow bridge piers were considered to be yielded during earthquake and bilinear beam element was adopted to model the plastic zone.Numerical model of the long-span deep-water bridge was established to study the seismic responses under fluid-structure interaction and nonlinearity conditions.Changing rules of the seismic responses along the longitudinal and transverse direction of the main bridge as a function of water level were obtained.Meanwhile,seismic responses of approach spans were also studied due to the effect of main bridge.The results indicate that seismic responses of the hollow pier increase with water level generally,especially the shear force at the pier bottom.At a relatively high water level,the pier bottom is more easily to yield,When the yielding happens,the curvature ductility demand of the bottom section is significantly amplified,and the damage index of the bridge pier is distinctly expanded.The existence of the inner water would cause further demand of the curvature ductility,which may lead to aggravated damages to the deep-water piers.Fluid-structure interaction of deep-water bridge pier can also increase the seismic responses of other bridge structures,such as main deck displacement,relative displacement between main deck and approach deck,seismic responses of transition piers and etcetera.