Risk Analysis Of Accidental Load In Reservoir Area On Steel Truss Arch Bridge

The high-frequency microseismic induced by the impoundment of the Three Gorges Reservoir and the frequent winds in the canyon area make the bridge construction in the reservoir area facing safety risks.This paper takes the construction of Xiangxi Yangtze River Highway Bridge in the Three Gorges Reservoir area as the background.The main span of this bridge is 519 m,and it is constructed by the diagonal pull-hook-cable lifting method.Combined with accidental load risk sources often encountered in the reservoir area-reservoir induced high frequency microseisms and frequent winds in the canyon area,this paper analyze the four key construction stages of bridge construction.The four key states are the maximum cantilever state,the initial closure state,the bare arch state and the completed bridge state.Using Midas to establish the bridge construction model,and apply micro-seismic action and wind load to calculate.The calculation results show that the maximum cantilever state of main arch rib construction is the most disadvantageous stage of Xiangxi Yangtze River Highway Bridge construction.The main risk scenarios are the large displacement of the maximum cantilever end and the large stress at the arch foot of the main arch rib.Frequent microseismic loads in the reservoir area have a greater impact on the stress of the main arch rib,while the effect of wind loads in the canyon on the displacement of the main arch rib is more obvious.Based on the study of probability distribution characteristics of frequent microseismic loads and gorge wind loads,this paper assumes that the peak acceleration of frequent microseismic loads obeys lognormal distribution,while the basic wind speed of gorge wind loads obeys extreme type I distribution.Based on the finite element calculation results of the maximum cantilever construction state under the action of frequent microseismic loads and Canyon wind loads,the maximum cantilever end offset and arch foot stress are taken as the risk limit state control factors,and the Monte Carlo risk probability solution is carried out.The Monte Carlo method for solving the risk of failure risk requires a large number of sample points for finite element calculation to be randomly sampled.To simplify the calculation,BP neural network is introduced and trained by finite element calculation data.Establishing BP Neural Network-Finite-Monte Carlo Joint Risk Probability Assessment Model by Using Matlab Programming.This method can significantly reduce the computational storage and time requirements and achieve efficient calculation of risk probability under the premise of satisfying the calculation accuracy.The calculation results show that the probability of risk under the action of frequent microseisms is 1.52%,and the probability of wind load is 6.53%.Under the coupling of frequent microseismic and wind loads,the probability of construction risk is 8.69%,and the probability level is 4,which should be paid attention to.During the construction period of the Xiangxi Yangtze River Bridge in the diagonal pull-hookcable hoisting,the frequent micro-seismic effect has less impact on the construction safety risk,and the wind load has a greater impact.The construction monitoring data shows that the Xiangxi Yangtze River Highway Bridge experienced micro-seismic and canyon winds during construction,but the maximum cantilever offset and arch stress are within the controllable range.The results are in good agreement with the trend of structural safety risk calculation,and the main arch circle has been successfully combined.This paper can provide some reference for analyzing the influence of accidental load on the safety risk of cable-stayed-cable lifting construction.