Dynamic Response Of Sea-Crossing Bridge Under Combined Wind And Wave Loads

With the promotion of China’s “Building an ocean power” and “Belt and Road Initiative” strategy,the construction of cross-sea bridge plays an important role in serving the national major strategy and becomes an important part of the realization of a transportation power.With the aggravation of global climate warming,the probabilities and load intensities of extreme weather phenomenon are gradually increasing,which could threaten the safety of coastal and offshore infrastructures.The cross-sea bridge is characterized by large span and samll stiffness which suffers from frequent wind and wave load.For the sake of researching the effects of combined wind and wave loads on the dynamic response of sea-crossing bridges,this text sorts the correlative literature and conducts researches from the establishment of the wind-wave joint distribution model,the establishment of the wind-wave-bridge coupling analysis system and the finite element analysis of the bridge.The major research work includes:(1)The establishment of wind wave joint distribution model.In order to analyze the correlation between wind and wave elements,based on the measured wind wave data from Lianyungang Ocean Observation Station in the East China Sea and Vine Copula theory,a joint probability distribution model of wind speed,wave height,wave period,wind direction and wave direction is established.Firstly,the marginal probability distributions of wind and wave data are determined,in which the AIC criteria and RMSE are employed to select the optimal probability distribution model and the maximum likelihood method is used to obtain the model parameters.Subsequently,the optimal two-dimensional Copula function for wind and wave data is determined through using the AIC criteria,and the model parameters are fitted with a Bayesian framework with a residual-based Gaussian likelihood function.To illustrate the goodness of fit,the binary frequency histogram of the original wind and wave data is compared with the proposed two-dimensional optimal joint distribution probability density function.Finally,the multi-dimensional joint probability distribution model of wind and wave data is established with the Vine Copula function based on the AIC criteria.(2)A wind-wave-bridge coupling analysis system is established.In the engineering background of a sea-crossing bridge,this thesis introduces the project profile of the bridge.Based on ANSYS software,the finite element model of the bridge is established and its natural vibration characteristics are analyzed.The three-dimensional wind field of the bridge is simplified into one-dimensional wind field of the main beam and one-dimensional wind field of the bridge tower by reasonable simplification.The harmonic synthesis method is used to simulate the fluctuant wind field,and the fluctuant wind simulation program is programmed based on MATLAB.The buffeting force of girder is calculated based on the quasi-steady buffeting force model.Wave field is simulated based on linear wave theory and random wave theory respectively,and wave force calculation theory of small scale pier is adopted in combination with bridge foundation form.The wave simulation and wave force time history calculation are realized by MATLAB programming.The wind-wave load is used as excitation to establish the wind-wave-bridge coupling dynamic equation,and then the wind-wave-bridge coupling analysis system is established.(3)The dynamic response of the bridge is analyzed based on the bridge finite element model and the wind-wave-bridge coupling analysis system.Firstly,the responses resulted from fluctuant wind lonely is calculated: the root mean square of vibration displacement of the deck in three directions and the displacement time history of the three directions in the span of the main beam are calculated,and the vibration patterns of the main beam at different moments are plotted.According to the obtained displacement time history,the frequency spectrum analysis is carried out to analyze the vibration of the main beam.Secondly,wave load conditions are determined according to the hydrologic data of bridge site.The linear wave theory and random wave theory are used to calculate the dynamic response of the bridge under wave action respectively and the differences between the two wave theories are compared and analyzed.For linear wave,the transverse displacement response of bridge under different wave height and wave period is analyzed.Finally,the dynamic responses of the bridge under the combined action of wave and wind are analyzed,and the dynamic responses of the bridge are compared with those of single wind and single wave.The results show that: the joint distribution model of wind and wave can describe the joint probability distribution of wind and wave elements well,which is of positive significance to determine the reasonable wind and wave load.Under the combined action of wind and wave,the dynamic response of the bridge has coupling effect,rather than simple composition of wind and wave.Both wind and wave loads contribute greatly to the transverse vibration of the girder.The vertical vibration is mainly contributed by the wind load while the wave load has little effect.