Seismic Analysis Of Deep Water Bridge Pile Foundation Considering Hydrodynamic Pressure And Scour Effect

With the rapid development of China’s transportation infrastructure,bridges lay astride river and sea have become a key node in the national traffic arteries.Co MPared with the land bridges,seismic analysis and design of deep-water bridges will face greater challenges: on the one hand,during the service period of bridge,the deep-water bridges may suffer multiple disasters,among which scour is the main factor leading to damage.Excessive erosion leads to soil loss and pile exposure,which changes the dynamic characteristics of pile foundation and ground motion response of local site.It increases the risk of earthquake damage of bridge.On the other hand,the deep water is another important factor affecting the seismic response of Bridges.However,the research on dynamic characteristics of scoured pile foundation is not enough.The hydrodynamic pressure is still calculated in the form of "added mass",based on the empirical formula of circular column(Morison equation),in the current seismic design code of Bridge.The research on the hydrodynamic pressure of other cross section piers is not sufficient.In view of the above problems,this paper focuses on the fluid-bridge structure interaction,the seismic response mechanism of pile bridges under scour conditions and the ground motion characteristics of the scoured site,and puts forward the seismic analysis method of deep-water Bridges on this basis.The main research contents and conclusions include the following:(1)The analytical solution of hydrodynamic pressure on the inner and outer surface of rectangular hollow bridge pier is proposed.Based on the radiation wave theory,the series solution of the hydrodynamic pressure on the rectangular pier is derived The accuracy of the formula is verified by co MParing with the numerical results.The effects of load frequency,pier cross section size and pier movement mode on hydrodynamic pressure distribution are also discussed by the three-dimensional numerical method based on potential-based fluid elements.A simplified formula is given for the hydrodynamic added mass of the rectangular pier,which can be used for seismic analysis of deep water pier.(2)The modal characteristics,seismic response,and hydrodynamic pressure distribution of deep-water pier under earthquakes are studied based on the centrifugal seismic tests.It shows that the hydrodynamic pressure leads to the decrease of pier frequency,especially for the high order modal frequency;while the damping effect on the pier is small,and can be ignored.It also shows that the hydrodynamic force is mainly characterized by inertia,and it is reasonable to describe it by the way of "added mass".(3)Centrifuge tests on the seismic response of the pile-soil-superstructure system in local scoured site were carried out.The scour effects on modal character and seismic response of bridge,pile-soil interaction,and pile group are discussed systematically.The results show that the scour increases the period and the seismic response of the bridge(especially the second-order modal),which is not conducive to the earthquake resistance of bridge;The damping of pile-soil interaction system is mainly contributed by near-field soil,while the far-field make little contribution;As scour depth growing,the pile bending moment increases,the distribution pattern changes,and the failure probability of bridge pile increases significantly.While the failure probability of pier tends to decrease,which may lead to the change of bridge failure mode from pier bottom to pile.The scour effect should be considered in the seismic design of bridge pile foundation.(4)Three-dimensional numerical modeling and centrifuge shaking table test were carried out to study the ground motion characteristics of local scoured site and general scoured site.The results indicate that as the scour depth increasing,the period and ground motion of general scour site both decrease;The period of local scour is almost unchanged,while the ground motion response decreases more.The ground motion of the local scoured site is similar to location of the original un-scoured site at the same depth.Based on this,the ground motion of the local scoured site is corrected.(5)Based on the test results of the bridge pile group foundation and scoured site,a simplified seismic analysis model,based on two-degree-of-freedom system,is presented,which can be used for quick calculation of seismic response,risk assessment of earthquake damage,and determination of critical scour depth of bridges failure at local scoured site.Furthermore,a seismic analysis model considering pile-soil-structure-water interaction is given,which is a powerful tool for seismic design of deep-water bridge and accurate calculation of internal force and deformation of pile foundation.