A Study On Time-varying Aerostatic Reliability Of A Long-span Cable-stayed Bridge

Reliability theory is an important method for bridge structure analysis,which allows systematic investigation of the uncertainties for structural resistance and load effects,and is widely used in the evaluation and optimization of structures.However,it is difficult to quantitatively describe the randomness of wind load and its effects in the study of wind resistance performance of long-span bridges,and the failure mechanism and limit state equation of structures are also complex,which limits the application of reliability theory.At present,limited wind tunnel laboratory resources cannot meet the needs of such research.Therefore,in this paper,the computational fluid dynamics(CFD)method was used to study the random distribution characteristics of static wind load for the Sutong Bridge.Then,the simulation results were used for structural response calculation,aerostatic instability process analysis and reliability evaluation,and the following conclusions were obtained:By combining CFD analysis with MCS,full-scale computational domain batch modeling,structured meshing and the calculation of aerostatic coefficients were handled for 510 random cross-sections using ANSYS CFX.The aerostatic coefficients were calculated through two meshing partitioning accuracies,which had a significant effect on the results.The variational coefficients of the lift coefficient and lift moment coefficient were slightly reduced by using the fine mesh splitting method.The accuracy of the aerostatic coefficient samples in the paper was confirmed by comparing the calculated results with wind tunnel tests and field measurements of the Sutong Bridge.Based on several theoretical probability distribution models and maximum likelihood estimation method,the distribution model of three-component force coefficients was fitted.According to the results of the K-S test and the chi-squared test,the lognormal distribution model was recommended to estimate the overall distribution of the aerostatic coefficients.When predicting the extreme value of the aerostatic coefficients,the Gumbel distribution or a“hybrid model of lognormal and Gumbel distribution”could significantly improve the accuracy of the prediction.The ANYS APDL command stream was used to calculate the displacement response of the bridge deck under dead load,lane load and aerostatic wind loads.On the basis of the traditional internal and external double iteration method,the torsional target wind speed method and torsional increment method were proposed,and the corresponding wind velocity indexes U _a and U_δwere defined.The stability of the bridge deck could be reflected by the variation trend of wind speed with wind attack angle.Under the condition of positive wind attack angle,torsion divergence might occur when the torsion deformation of the bridge deck reaches 4°.Under the condition of negative wind attack angle,the torsional deformation of the structure required an increasing wind speed as the condition.The impact of cable cross-section damage on the critical wind speed for static wind instability was studied.The critical speed decreased gradually with the increase of the damage rate of the cable cross-section.The failure of the mid-span leeward cable would have a significant adverse effect on the structure in the case of a positive wind angle of attack.After the failure of the most unfavorable cable,the increase of cable force in the other cables was not obvious.In the case of a negative wind angle of attack,after the failure of the most unfavorable cable,the cable forces of the two adjacent cables on the same side of the failed cable increased significantly,but the cable forces on the leeward side of the mid-span decreased significantly.The first-order reliability theory combined with finite element methods was utilized to calculate reliability index for aerostatic stability of long-span cable-stayed bridges.It was found that the standard deviation of the coefficient term in the limit state function had a large effect on the iterative results of the value,while the standard deviation of the aerostatic coefficients had little influence on the value.When the coefficient terms were considered to be constant 1.0,the valueβwas negatively correlated with the initial wind attack angle.The reliability index of aerostatic stability of the structure at 8°was 10.035.Considering the failure condition of a single cable on the leeward side in the mid-span,βdecreased in different amplitudes at each positive wind attack angle.At this time,the reliability index of aerostatic stability of Sutong Bridge at 8°was 8.553,indicating that the structure was safe.