Study On Hydrodynamic Forces On Substructure Of Deep-water Bridge Under Combined Loadings Of Wave-currents And Earthquakes

There are many rivers,lakes and seas in China.In recent years,a large number of deep-water bridges have been built in China's coastal areas and the western mountain reservoir areas.China is located between the two major fault zones,and seismic activity is often found in the site of these bridges.The bridge substructure in deep water is often subjected to various environmental loads,such as wave slap,water flow shock,seismic wave disturbance,etc.In addition to the same dynamic loads as the land bridge,the deep-water bridge will be subjected to hydrodynamic pressure caused by waves,currents(ocean currents)and earthquakes,and the structural stress state is very complicated.The deep-water environment is bound to increase the concealment of the substructure of the bridge,and the structure is difficult to repair after the damage.This further highlights the importance and urgency of seismic analysis for deep-water bridges.In this paper,the hydrodynamic forces and responses of the deep-water bridge substructure(including single pile,hollow pier and pile group structure)are studied.The main contents and conclusions are as follows:(1)A numerical analysis method for fluid-structure interaction between bridge substructure and surrounding wate under combined loadings of wave-currents and earthquakes in deep-water environment is constructed.Several validation examples show that the numerical method has fairly high precision and reliability.Its advantage is that it is not limited by the size of the structure and the form of the section,so it has a wide range of applications.It is an ideal research supplement for the current difficult experimental research in this field.The Morison method is conservative when the water depth is relatively small,and it is unsafe when the water depth is relatively large.(2)Based on the validated fluid-structure coupling analysis method,the interaction for wave-current,earthquake and pile structure system elements is studied.The combined cation mechanism of wave-current and earthquake is revealed.The method of determining the maximum dynamic force is given from the point of view of probability.The vertical and horizontal components of seismic wave play a major controlling role in seismic free flow field and seismic wave field respectively.The action frequency of wave-current increases with the presence of earthquake irregular oscillation flow,thus deviating from the already resonant zone.The influence of strong current and weak wave on seismic response is more obvious than weak current and strong wave.In the many factors that influence the seismic response of the structure,the wave period is the most significant,so it is very important for the safety design of deep-water bridge to correctly estimate the effective period of wave.(3)An analytical algorithm for solving the nonlinear bending deformation of a thin-walled hollow bridge pier under the action of water flow considering the fluid-structure coupling effect,is proposed by using the united Lagrangian-Eulerian(ULE)method.The comparative study is conducted between the analytical solutions and the numerical solutions computed by ANSYS combined with CFX(two-way coupling)in ANSYS Workbench,and the analytical algorithm based on the ULE method for the problem of pier-water coupling is verified to be feasible.by using the united Lagrangian-Eulerian(ULE)method.The hydrodynamic problems of circular hollow piers with holes(not confined to thin walls)under the action of current,wave and earthquake are investigated by the fluid-structure coupling numerical method.Numerical experiments show that it is not suitable to open holes on the wall of hollow piers in deep water for the engineering design.(4)Through numerical simulation,the pile group effect for different pile groups in seismic wave field is studied.The approximate expression for the effect coefficients of pile groups is given,and the influence of current-wave on the pile group effect of drodynamic force generated by earthquakes is analyzed.In the seismic wave field,the pile group effect can not be ignored when the pile spacing ratio is less than 4.5.For a specified deep-water pile group foundation,the pile group effect coefficients of the free pile groups and the one with cap are 0.95 and 0.69 respectively.The action of pile caps has great influence on the effect of pile groups.The hydrodynamic force of the corner piles is greater than the one of the center pile.Consequently,strengthening section reinforcement for the corner piles should be accepted to resist the larger hydrodynamic forces.The current has a certain promoting effect on the oscillatory flow,thus increasing the interference degree of the pile groups in the seismic wave field.(5)The dynamic response of a typical sea-crossing bridge for the pile-cape-pier system in deep water under conbined loadings of wave-currents and earthquakes(wave motion is described by using Airy linear wave and Stokes fifth order wave)is studied by the finite element dynamic method,considering the different wave-current elements and real seismic time-history wave.The coupling effects of complex environmental loads are further summarized,and the causes that the response of the pier structure is significantly affected by the wave flapping on the cap and above are discussed.It is necessary to select elevated pile cap for sea-crossing bridge.For the bridge in this study,the earthquake load has a greater contribution to the dynamic response of the structure.But the influence of wave current can not be neglected.the influence of wave current on seismic response is-31.6%~63.5%,so it is necessary to analyze the combined action of wave and earthquake.