| Highway transportation network is the lifeblood of national economy.Bridges engineering cross natural barriers,which is the critical node and hub project of interconnection of transportation infrastructure.Once the bridges are destroyed in the earthquake,it will directly affect the normal operation of the transport system,hindering the timely arrival of relief supplies and rescue workers to the stricken area,resulting in greater loss of life and property.The repair of bridge engineering after earthquake is difficult,and seismic fortification for structures is very important.In the guidelines for seismic design of highway bridges,earthquake actions include the action of ground motion,seismic soil pressure,seismic hydrodynamic pressure,etc.Seismic hydrodynamic pressure of bridge pier is the key point of considering water pressure in aseismic design.However,the understanding and studying of seismic hydrodynamic pressure are still not enough.The guidelines for seismic design of highway bridges in China directly draw on the relevant provisions of Japanese specifications for highway bridges.With the development of China's economy,large-scale bridge construction is increasing in high seismic intensity zone,and deep-water bridges are also built more and more at the same time.When some reservoirs start to impound water,submerged depth of bridge piers can reach dozens of meters,and even more than one hundred meters.The computing method of seismic hydrodynamic pressure in previous guidelines is mainly applicable for the water depth within 15 meters.Whether it is still applicable for deeper water,and whether it needs to be modified become more and more concerned in the engineering field,and a large number of related researches are done in the theoretical field as well.In order to provide reference for the revision of the Guidelines for Seismic Design of Highway Bridges(JTG/T B02-01-2008),this paper mainly answers two important basic questions through summing up the existing references and carrying out numerical analysis.one is how the seismic hydrodynamic pressure increase when submerged water depth reaches tens or hundreds of meters,the other is what is the distribution law of the seismic hydrodynamic pressure with underwater depth.First of all,according to calculation results of seismic hydrodynamic pressure with various water depths in the 14 collected references,this paper uses an improved influence coefficient,providing a comprehensive summary of difference on seismic responses in different input ground motions,pier types,pier diameters,pier heights,submerged water depths and other working conditions,comparing with the curve of influence coefficient with the depth according to the formula in the current guidelines.The results show that the relative changes obtained according to the guidelines are genrally consitent with trend of the calculation results of the 14 references.Both ofthem manifest the linear segment of increasing and the nonlinear segment of slowdown in growth,and there is sufficient redundancy for the regulation of the range above 80 meters water depth.In this paper,an actual deep-water bridge is selected and a single pier model is established,the water-pier dynamic interaction is expressed by the added mass and the added damping of moving water calculated by Morison equation.The spring parameters for expressing the interaction of soil-pile-pier are estimated by “m” method.Three ground motion waves of four fortification intensities are input into the model respectively,and the earthquake response of a single pier model in six conditions of various submerged depths is calculated.This paper uses the shear force at the pier bottom in the working conditions of various submerged depths minus the shear force at the pier bottom without water working condition and lets the maximum shear force difference represent the corresponding seismic hydrodynamic pressure.The results show that the values of seismic hydrodynamic pressure increase with the increase of depth and the increase slows down gradually when submerged water depth reaches over30 metes,and with increase of each fortification intensity,the seismic hydrodynamic pressure increases exponentially.This paper utilizes the bending moment at the pier bottom in the working conditions of various depths to minus the bending moment at the pier bottom without water working condition and lets the maximum bending moment difference divided by the corresponding seismic hydrodynamic pressure,getting the acting position on pier of the seismic hydrodynamic pressure.The results show that the ratio of the action position on pier of the seismic hydrodynamic pressure and the submerged water depth is between 0.29 and 0.58,and the average value is about 0.437,which is between 1/2 in the provision in Chinese guidelines and 3/7 in the provison in Japanese Specifications.Relatively speaking,the provision of the acting position on pier of the seismic hydrodynamic pressure in Chinese guidelines is conservative.The difference of shear force between the upper and lower pier units with water working condition is subtracted from the corresponding difference of shear force difference without water working condition.The further difference is regarded as the seismic hydrodynamic pressure in the unit center of the pier upstream at the corresponding depth.The results show that the seismic hydrodynamic pressure changes obviously with underwater depth and the curve shape is generally similiar when the submerged water depth reaches over 10 meters.At the 25% of relative submerged water depth,the seismic hydrodynamic pressure start to increase rapidly with underwater depth,then increase slowly and reach the maximum value,gradually decrease with underwater depth,at the 75% of relative submerged water depth,the seismic hydrodynamic pressure tend to be a stable value,sharing the same distribution law with the results of the existing references.The stable value of the seismic hydrodynamic pressure at pier bottom is multiplied with the increase of fortification intensity,that is,linearly related to the maximum amplitude of the input ground motion.Finally,compare the calculation results of this paper with the results calculated bynewly corrected suggetion for the current guidelines for seismic design of highway bridges.The results show the same law as the increase of the seismic hydrodynamic pressure with increase of submerged water depth.Within 15 meters of water depth,there is a tendency for the seismic hydrodynamic pressure increasing linearly with the increase of water depth,and the calculation results of this paper are very close to the results calculated by the newly corrected suggestion.At depths of more than 20 meters,the increase of the seismic hydrodynamic pressure has increased rapidly,and the difference between them is close to one time.It shows that the newly corrected calculation formula of seismic hydrodynamic pressure has obvious improvement,but whether it is still too small and whether the added mass and the added damping of moving water should change with depth in the model need to be further studied. |
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