The Influence Of Local Flushing And Flushing Depth On Structural Dynamic Characteristics Of Cross-sea Bridges Was Studied

In recent years,with the rapid development of China’s transportation network,cross-sea bridges,as an important part of the transportation network,have been developed rapidly,and many world-class cross-sea bridges such as Hangzhou Bay,Hong Kong-Zhuhai-Macao have been built one after another,while human beings have gradually moved from offshore to deep sea.With the increase of wading depth of cross-sea bridge,various complex pile groups appear one after another.The maximum depth of the main pier of Xihoumen Rail-cum-Road Bridge to be built on Yongzhou-Zhoushan Railway reaches 65 m,and the pile diameter reaches 6.3m.Such complex pile foundation brings a severe test to the safety and stability of cross-sea bridge.Pier local scour is one of the important factors leading to the destruction of bridges.It affects the design of pile length in the design stage,affects the construction safety in the construction stage,and affects the service life of the bridge in the operation stage.Therefore,the study of pier local scour is of great significance.Based on the project of a sea-crossing bridge which has been built in Ningbo and the Xihoumen Rail-cum-Road Bridge which will be built in Ningbo,The following researches are carried out by using numerical simulation method on the method of predicting bridge local scour,the related factors affecting pier local scour,the maximum scour depth of Xihoumen Rail-cum-Road Bridge to be built and the identification of bridge scour depth by using the natural frequency of the structure.(1)At present,there is a lack of research on local scour of cross-sea bridges,and how to accurately predict the local scour depth of cross-sea bridges still needs to be studied.In this paper,several commonly used methods of scour prediction are summarized: theoretical formula method,numerical simulation method and physical model test method.It is found that the HEC-18 formula of the United States is too conservative for the prediction of cross-sea bridges,and the formula in the Code for Hydrological Survey and Design of Highway Engineering of China is not suitable for cross-sea bridges.The local scour calculation formula of Hangzhou Bay SeaCrossing Bridge is relatively accurate in predicting the scour depth of the Sea-Crossing Bridge,and the numerical simulation method can use the original scale modeling to predict the scour well,but the numerical simulation needs to determine the correctness of the modeling method,which is usually combined with physical experiments;The results of the physical test are affected by the geometric scale of the model,the test conditions and the model sand used in the test.The three methods have their own advantages and disadvantages,so it is appropriate to use a combination of methods to predict the scour depth of the bridge.(2)There are many factors affecting the local scour of piers,and the factors affecting the local scour of cross-sea bridges still need to be studied.In this paper,Fluent software is used to analyze the influence of flow velocity,flow depth,different pile diameters and pier layout on scour,and from the perspective of scour,the numerical simulation method is used to provide the basis for the selection of piers by taking the No.5 complex pier of Xihoumen Rail-cumRoad Bridge as an example.The results show that the scour depth increases with the increase of flow velocity and flow depth,and the influence of flow velocity on scour depth is greater than that of flow depth.The scour depth of quincunx arrangement is greater than that of determinant arrangement.The influence of pile diameter is closely related to pile spacing.The quincunx arrangement should be avoided as far as possible when selecting the complex pile foundation based on scour characteristics,and the pile group foundation with fewer piles should be used.(3)The maximum water depth of Xihoumen Rail-cum-Road Bridge to be built reaches65 m,and the pile diameter reaches 6.3m.Considering the safety of the bridge,it is particularly important to predict the scour depth of the bridge.In this paper,through the analysis of the geological environment of the piers of Xihoumen Rail-cum-Road Bridge,it is concluded that the maximum scour depth is located at the No.6 pier,and the Fluent software is used to predict the scour of the pier,and the prediction results are compared with the theoretical formula values and physical model test values.The results show that the formula calculation < flume test <numerical simulation,and the formula calculation is close to the flume test,which further proves the correctness of the local scour formula of Hangzhou Bay Sea-Crossing Bridge,and the error between numerical simulation and formula calculation and flume test is about 10%,which not only verifies the correctness of the model,but also proves the influence of geometric scale on flume tests.It is considered that the numerical simulation results are more accurate,and the maximum scour depth during the construction period is 11.58 m,and the maximum scour depth during the operation period is 12.23 m,which provides a reference for the design,construction and operation.(4)The identification of scour depth of cross-sea bridge during operation period still needs to be studied.In this paper,the relationship between the natural frequency of the structure and the scour depth is studied.Based on BP neural network,a full-bridge finite element scour depth identification model of pile-soil interaction is established by using Midas Civil and Matlab software,and the model is tested by different structures of the bridge.The results show that the local scour depth of the bridge pier can be well identified by the natural frequency of the structure.The identification error is about 15%,and the method has strong applicability to cablestayed bridges and is simple and convenient in actual operation;Although it is simple and feasible in theory,it is a problem to determine the higher order modal frequency in the actual operation process,so it is of great significance to use this method to monitor and identify the scour depth of cable-stayed bridge and continuous beam bridge with large stiffness.