Study On Mechanism Of Water-pier Interaction Under Earthquake Excitations And Coordinative Similitude Law For Underwater Shaking Table Tests

The piers of cross-river bridges,cross-sea bridges and brigdes in west reservoirs are generally located in deep water,and are subjected to the interaction between the pier and water under earthquake,which has a significant influence on the response of bridge with a complicated mechanism.Serious damages have been observed to the deep water bridges with seismic design at high earthquake-intensity region during previous earthquakes.To ensure the seismic safty of large bridge structurs and improve the the design theory for deep water bridges,the mechanism of the interaction between the pier and water under earthquake and the coordinative similitude law for underwater shaking table tests are investigated in this dissertation.The main conclusions are as follows:(1)A series of water-pier interaction shaking table tests are conducted.The mechanism of the interaction between the pier and water under earthquake,and the hydrodynamic added mass changing with excitation frequencies,excitation amplitudes,water depths,diameters of cross section,and shapes of cross section,are examined.The results show that the hydrodynamic added mass changing rules with water depth,excitation frequencies and diameters are different with those described by the traditional Morison equation.The added mass first decreases and then increases with the increasing action frequency,and the changes are more than 20%,which disagrees with the traditional Morison equation.The added mass changes slightly with the action amplitude,and the changes are less than 10% and thus can be neglected,which agrees with the traditional Morison equation.The added mass increases with the water depth,and the increase ratio of added mass is obviously more than that of water depth,which disagree with the traditional Morison equation.For the circle specimen,the added mass increases with the diameter,and the increase ratio of added mass is obviously less than that of the square of diameter.For the rectangle specimen,the added mass increases with height and the width of the upstream face of cross section,and the influence of section height is less than that of the width of the upstream face.(2)Based on the characteristics dynamic responses of pier structures under earthquake,a modified Morison equation is proposed to calculate the hydrodynamic added mass of the pier under earthquakes.The modified inertia coefficient and transformation coefficient are obtained by the least squares fitting method according to the test results.The results show that the fitting inertia coefficient and transformation coefficient match well with the test results and the maximum error is 9.9%.The inertia coefficients of circle and square pier increase with water depth,decrease with width of the upstream face,and first decrease and then increase with the action frequency.The inertia coefficients are 0.064-0.374 for circle pier,and 0.186-0.447 for square pier.For the rectangle pier with an aspect ratio less than 1,the transformation coefficients change slightly with water depth,decrease with the aspect ratio,and first decrease and then increase with the action frequency in the range of 1.021-1.871.For the rectangle pier with an aspect ratio more than 1,the transformation coefficients increase with water depth and the aspect ratio,and first increase and then decrease with the action frequency in the range of 2.237-15.315.(3)A coordinative similitude law considering fluid-structure interaction is proposed for underwater shaking table test to solve the problem that the artificial mass model can't ensure the precision of underwater dynamic test.The design steps of the model are given for earthquake excitation,wave excitation,and combined earthquake and wave excitation.The piers of viariou parameters are built in ADINA,and the responses under earthquake excitation,wave excitation,and combined earthquake and wave excitation are investigated.The results show that the coordinative model can precisely predict the hydrodynamic pressure,displacement,shear force,and bending moment of the prototype with a maximum error of 5%,which is significantly smaller than that for the conventional model of 80%.The coordinative similitude law considering fluid-structure interaction can be used to design the model of underwater dynamic tests.(4)A series of underwater shaking table tests on pier are conducted.To verify the modified Morison equation and the coordinative similitude law,the responses of three different specimens are studied under earthquake excitation,wave excitation,and combined earthquake and wave excitation.The results show that modified Morison equation matches well with the test results by a maximum error of 10%,and it can be used to calculate the hydrodynamic added mass under earthquake.The water around the pier decreases the vibration frequencies of the pier,and the decrease grows with water depth.Comparing with the results without water,the vibration frequency decreases by 18.2% and 25.2% for the first-and second-mode,respectively,when the water depth is 0.65 m.The maximum error of coordinative model less than 10% for all conditions,while that of conventional model is 50%.The precision of coordinative model is obviously higher than that of conventional model,and thus the coordinative similitude law can be used to design the model of underwater dynamic tests.