Research On Automobile Load Model For Extra-Large Span Suspension Bridges

With the rapid transformation of China’s economic development stage,the construction phase of China’s infrastructure has undergone new changes,and in the future,more long-span bridges that cross rivers,seas,and canyons may continue to be built.Among all types of longspan bridges,suspension bridges are increasingly favored due to their strongest crossing ability,and the span of their construction repeatedly breaks records.The current specifications of automobile load models mainly address medium and small span bridges,and they have poor adaptability for ultra-long span suspension bridges.Therefore,reasonable evaluation of automobile loads is of vital importance for the design and operation of ultra-long span suspension bridges.This paper is part of the sub-project "Design Load Model and Formulation Method for Ultra-Large Span Cable-Stayed Bridges" of the National Key Research and Development Program 2021YFB1600302.Based on a summary of existing research on automobile loads,this paper analyzes and calculates the automobile loads in detail on a large-span suspension bridge with a double eight-lane highway by establishing a random traffic flow model.The specific work content is as follows:1.Conducted a survey of related regulations on car loads in China and abroad,summarized the research status of domestic and foreign lane load models and fatigue load models in a review,and systematically discussed their research methods.2.Based on a large amount of automobile load data measured by a dynamic weighing system,the distribution characteristics and parameters of vehicle types,traffic flow rates,vehicle weights,vehicle speeds,vehicle axle distances,axle loads,and headway distances on different lanes were statistically analyzed.Then,a random traffic flow model suitable for this type of large-span bridge was established in a lane-by-lane manner.The finite element model of the Zhangjinggao Bridge South Channel Bridge was subsequently established,and the influence area of the suspension cable was calculated to provide data support for the study of the lane load model and fatigue load model.3.A calculation program for the effect of automobile loads on cable forces was compiled.The cable force history of the suspension cable under random traffic flow loading was calculated and compared with the results of the calculation of the highway grade-I lane load specified by the regulation.The data of the automobile load effect ratio history of the suspension cable force was obtained.The overall and tail data fitting and analysis were carried out for this data.The standard value of the automobile load effect ratio was calculated using the method consistent with the regulation.Based on the generalized Pareto distribution fitting of the tail data of the load effect ratio,an extreme value extrapolation of the load effect ratio for the design base period was carried out.The effect of automobile load under congested road conditions was calculated considering the characteristic of large span of cable-stayed bridges,and the difference in load effect on the suspension cables on both sides of the cable-stayed bridge considering the characteristics of upward and downward traffic flow on the cable-stayed bridge was analyzed and calculated.Finally,based on the above results,some parts of the lane load model for ultra-large-span cable-stayed bridges specified in the regulation were supplemented and revised.4.Based on the cable force history data,the stress amplitude and cycle number of the suspension cable were calculated by using the rainflow counting method,and the development of the fatigue damage degree of the suspension cable within one year was predicted using the linear damage criterion.Considering two situations-whether the growth of traffic flow rates was taken into account or not,the development process of fatigue damage of the suspension cable during the service life was predicted.Finally,a fatigue vehicle load spectrum suitable for the region was compiled based on the statistics and calculations of the measured traffic flow,and the fatigue effect of the suspension cable was calculated using the fatigue load spectrum.