A Time-domain Method For The Hydrodynamic Pressure On Deep-water Cylindical Structure With Large Diameter

For across-sea long and large bridges and offshore buildings, the structures are indeep water. Under earthquake, the interaction between water and structures cangenerate the hydrodynamic pressure that can not be ignored for structures. With theincrease of deep-water bridge span and the increase of water depth, the cross-sectionaldimension of the bridge underwater part increases. For structures of the deep-watercylinder of large diameter. When calculating the hydrodynamic pressure on largediameter cylindrical structure, the results that adopted compressible or incompressiblewater model are significant different. At present, incompressible water is oftensimplified as an added mass, which could be conveniently carried out in time-domainanalysis. While in time-domain analysis it is difficult and time-consuming to evaluatethe analytical solution of the hydrodynamic pressure for compressible water.Therefore, considering the compressibility of water, a time-domain method that canrepresent the action of hydrodynamic pressure on the large-diameter cylindricalstructure is proposed in this study. Specific researches are as follows:1、Based on the radiation wave theory, the detailed derivation process ofanalytical solutions of hydrodynamic pressure about compressible and incompressiblefluid was given. The dynamic stiffness coefficient of compressible fluidhydrodynamic pressure was related to frequency, in structural time-domain nonlinearanalysis, it involves convolution calculation, and the calculation has high cost; Themass coefficient of incompressible fluid hydrodynamic pressure was not related tofrequency, so the hydrodynamic pressure can be calculated by the added massmethod.2、Using the analytical solution of hydrodynamic pressure of compressible fluid,there was a further discussion about the influence of water compressibility on thehydrodynamic pressure. Regular pattern which under different cross-sectiondimensions and water depth conditions was summarized. Studies have shown thatwith the structural section size increases, the influence of the water compressibility onthe hydrodynamic pressure that structures suffered under earthquake is moresignificant; The hydrodynamic pressure considering the water compressibility did notchange significantly with increasing water depth, but the hydrodynamic amplificationfactor peak was more and more concentrated in the seismic frequency band multiplewith the depth. So it was necessary to consider water compressibility in calculatingthe hydrodynamic pressure of large diameter structures.3、A time-domain method considering the compressibility of water is proposed tocompute the hydrodynamic pressure on the deep-water cylindrical structure of large diameter. The compressible fluid hydrodynamic is equal to the product of the dynamicstiffness coefficient and the displacement. The dynamic stiffness coefficient wasrelated to the loading frequency, it is need to calculate the global convolution integralin time domain. In order to avoid the time convolution, the rational functionapproximation of dynamic stiffness coefficients was introduced, and the specificmethod of parameter identification was given. Then in the time domain, byintroducing an auxiliary variable, the high-level order linear system of time canconvert into the low-level system. The system was a spring-damper-quality model,and the calculation method of the model parameter was given.4、The hydrodynamic pressure of Japan’s Akashi Kaikyo Bridge main towercylindrical foundation under the dynamic loads was calculated. Three ways——added mass method of incompressible water、analytical method of compressible waterand the time-domain algorithm the compressible water was adopted. Calculationresults show that this method that proposed in this study has a high computationalaccuracy and computational efficiency.