Research Of Seismic Response And Seismic Behavior Of Deep-water Concrete-filled Steel Tubular Bridge Piers

With the economic development and increasing traffic demand,the number of deep-water bridge has increased steadily,including bridges across the sea or across the river.As the piers located in the deep water,under earthquake excitation there will be dynamic interaction existing between the bridge piers and the surrounding water,resulting in hydrodynamic pressure that would affect the dynamic characteristics and seismic response of the bridge piers.Under earthquake excitation,traditional reinforced concrete(RC)piers incline to have brittle failure,which is hard to repair;while concrete filled steel tubular(CFST)piers,due to their high bearing capacity and ductility,have gradually been applied in bridge projects,which is of great significance to improve the safety of bridges.In this paper,seismic response and seismic performance of deepwater composite bridge piers are studied,and the applicability of simplified analyzing method for water-pier interaction is evaluated.The main research work and conclusions are as follows:(1)The influence of hydrodynamic pressure on the seismic response of various types of deep-water piers is analyzed.Focusing on RC piers,CFST piers,and concrete filled double-skin steel tubular(CFDST)piers,modal analyses of different piers under different levels of water-depth were conducted respectively.Through fluid-structure interaction,seismic analyses were also conducted on deep-water piers under different types of earthquake and water depth,and seismic responses were obtained and compared.The results showed that when water was present,frequencies of all the natural vibration modes of the piers were decreased,the decreasing amplitude increased with the water depth,and the higher order modes were affected more.Pier seismic response was increased by the hydrodynamic pressure,which was significant for piers with lower stiffness and less self-weight.Under frequent earthquakes,if the depth-toheight ratio of the solid piers exceeded 0.6 or the water depth-to-height ratio of the hollow piers exceeded 0.4,the influence of hydrodynamic pressure on pier seismic responses would be over 10% and therefore should not be neglected.(2)Elastic-plastic nonlinear dynamic time history analyses were conducted on deep-water composite bridge piers under strong earthquake,and the applicability of additional mass method with Morison equation in the nonlinear dynamic time history analyses were investigated.Analyses were conducted on various composite bridge piers with various slenderness ratio and water-depth using the numerical method with fluid-structure interaction and using the additional mass method with Morison equation respectively,and results were compared.The results showed that: the higher the slenderness ratio,the higher the precision was using the Morison equation method.When the slenderness ratio was greater than 53,the error of using the Morison equation method was less than 10%,which could meet the precision requirements.The change of water depth had little influence on the accuracy of using the Morison equation method.(3)The impact of hydrodynamic pressure on the seismic behavior of solid CFST piers and hollow CFDST piers was analyzed.Based on additional mass method with Morison equation,a semi-analytical finite element model was established,a set of farfield and near-field earthquake ground motions were selected,and Incremental Dynamic Analysis(IDA)analyses were conducted on CFST and CFDST piers with and without considering the presence of water respectively.The research shows that: piers would have a higher tendency of failure when vibrating in the water,and the peak ground acceleration needed for the piers to reach their ultimate displacement was 0.1g~0.2g lower when they were in the water.Under the near field earthquake ground motions,pier displacements were larger,and the influence of hydrodynamic pressure was greater.Compared to solid bridge piers,hollow bridge piers with less self-weight had lower seismic responses under the no-water condition,but since they were more susceptible to the influence of hydrodynamic pressure,their seismic responses under the water condition were on a similar level to the solid piers.