Research On Application Scope Of Morison Method In Deep-water Bridge Pier Seismic Analysis

Under earthquake, deep-water bridges are exerted with not only seismic forces, but also dynamic pressure from water. The dynamic water pressure has been proven to be very affective to the seismic response of the bridges. So, the dynamic pressure from water should be highly considered while designing deep-water bridges.For the seismic response analysis of deep-water bridges, the Morison equation is widely used for its simplicity and effectiveness. But the equation takes no account of the interaction between water and structure, and then its application scope is limited in bridge seismic analysis. Until now there has been no quantitative research on the application scope of Morison equation in deep-water bridges seismic analysis. In this paper, to define the scope, three structural parameters:water depth ratio, slenderness of bridge pier and the mass ratio between pier top and pier body were used as the control variables to analyze the difference between seismic analysis results of bridge piers with Potential Fluid Method and with Morison Equation. The major contents of this paper are as follow:1) With both the Potential Fluid method and Morison Equation Method to calculate the dynamic water pressure, the common law for the influence of structural parameters to the deep-water bridge seismic response was found.2) Compare the modal and dynamic analysis results of the same bridge pier models under the same control variables but with two different dynamic water pressure calculation method and define the scope of application of Morison equation with the difference between these two sets of analysis.3) A set of SDOF were utilized to try to explain the relationship between dynamic water effect and the bridge pier seismic response. Harmonic force and seismic force were both used to test the effectiveness of application scope of Morison Equation Method with two different type of force. Meanwhile, the near-fault and far-fault seismic record were used to analyze their different influence on deep-water bridge pier seismic response.After all the analysis above, a series of conclusions were attained, as follow:1) When the water depth ratio R is no bigger than40%, the error of Morison Equation Method is fairly small and hardly influenced by changes of the cut face shape (among circular and rectangular) and of pier slenderness. On the other end, while R passes40%, the error will be highly detennined by those changes.2) The slenderness ratio of piers is a very effective indicator for the Morison Equation application scope in deep-water bridge seismic analysis. Results in the paper shows when the slenderness is smaller than16, the error of Morison Equation Method is too high to tolerate. Meanwhile the bigger slenderness is, the smaller error could be.3) When the mass proportion of pier top to pier body is bigger than the error of Morison Equation Method will bear little changes around a value set by other elements of the pier, like the slenderness ratio.4) Morison Equation Method use added mass to consider the dynamic water effect. Whether the dynamic water effect will enlarge the seismic responses of piers is very much affected by the spectrum of the loads.5) Near-fault seismic records will increase the dynamic response of a deep-water bridge pier and the effectiveness of the dynamic water effect comparing with far-fault seismic loads.