Study On Fatigue Life Of Orthotropic Steel Bridge Deck Under Coupling Effect Of Temperature And Vehicle Speed

With the rapid development of China’s transportation industry,there is an increasing demand for long-span steel box girder bridges and a wider range of applications.Orthotropic steel deck is widely used by Chinese engineers in the design of long-span steel structure bridges in order to reduce the bridge weight and shorten the construction period.However,the subsequent fatigue problem has become the focus of this kind of structure.Understanding the variation law of dynamic modulus of asphalt pavement and studying the internal fatigue crack propagation mechanism of orthotropic steel bridge deck are very key to promote the design and maintenance of steel bridge.In this paper,the influence of the coupling effect of temperature and vehicle speed on the fatigue life of orthotropic steel bridge deck is studied,and this problem is discussed by means of experimental research and numerical analysis.The main work and conclusions are as follows:(1)Combined with the basic theory of linear elastic fracture mechanics and extended finite element method,the fatigue crack mechanism was analyzed.Based on the basic concept of fracture mechanics and the classical Paris formula,the theoretical formula of crack propagation criterion was deduced.The propagation finite element method was used to solve the discontinuous problem of fatigue crack propagation.(2)Considering that the composite stiffness produced by asphalt pavement and orthotropic steel deck has an impact on the fatigue life of the overall structure,it is necessary to conduct experimental research on asphalt materials.Using asphalt mixture SMA-13 and sup-20 as test materials,the dynamic modulus tests under various temperatures and loading frequencies were carried out to find out the variation law between the dynamic modulus of asphalt mixture and ambient temperature and loading frequency.According to the test results,the transformation relationship between loading frequency and vehicle speed was established,and the dynamic modulus of asphalt mixture at the selected vehicle speed was obtained.(3)Taking the three standard sections of Anqing Yangtze River Highway Bridge as the research object,the semi elliptical crack was used to simulate the fatigue crack at the welding details of roof and U-rib,and the finite element model with three-dimensional fatigue crack was established.Under the loading conditions of different temperatures and vehicle speeds,the fatigue crack propagation on the weld root and toe at the connection between U-rib and roof was numerically simulated.It was found that the fatigue crack propagation at the weld root and weld toe is not always in the same plane,but slightly deflected;When the crack propagation depth is the same,the total value of cumulative strain energy release rate of mode I crack at the same speed is similar,but the number of loading cycles is different;For the fatigue crack at the connection between the top plate and the U-rib,under the same driving speed,the crack propagation life increases with the increase of temperature;When the temperature is constant and the temperature is low,the driving speed has little effect on the crack propagation life;When the temperature is in the range of 20℃～45℃ and the driving speed is 100 km/h,the crack propagation life is the minimum.(4)Based on the linear elastic fracture mechanics analysis method,the fatigue life of orthotropic steel bridge deck under the coupling effect of temperature and vehicle speed was evaluated.The real bridge data(daily traffic volume,axle load,etc.)were counted by rain flow counting method,and the vehicle load frequency spectrum that can be used for fatigue calculation was obtained.The finite element software was used to calculate the stress near the fatigue crack tip under different loading conditions.According to the calculation results and the fatigue life calculation formula,the fatigue life of steel bridge deck was evaluated.It is found that the fatigue life caused by the change of temperature is more obvious,and the maximum change range is 32.3%;The variation range of fatigue life caused by vehicle speed is small,ranging from 1.7% to 2.2%.