Vehicular Loading Spectrum And Fatigue Investigation Into Orthotropic Steel Bridge Decks On Heavy Freight Transportation Highway

The orthotropic steel bridge deck(OSBD)has the advantages of light weight,great overload capability,fast erection,easy maintenance and factory prefabrication,which make it widely used in bridge design with various structural types and different span length.However,due to its complicated connection details,significant stress concentration,strict requirement on welding control,the fatigue crack initiation and expansion may occur under the repeated vehicle loadings if the connection details were improper designed and prefabricated.This situation may become even worse if the bridge locates on the heavy freight transportation highway under the passage of trucks of large volume and seriously overweight.Systematic fatigue investigations into the OSBD on heavy freight transportation highway are carried out based on the weight-in-motion system,FEM analysis and field monitoring.The paper focuses on the following work and presents some important conclusions as follows:The traffic data from a weight-in-motion system,which is located on a steel bridge in Guangdong Province,is used to present ten representative vehicle models,their corresponding axle spacing and weight,vehicle gross weight and overweight information,from which the vehicle load spectrum and the corresponding fatigue truck models are proposed,which can be employed to evaluate the fatigue performance of steel bridges.It is found that the volume of average daily truck is significantly high on this heavy freight transportation highway,with part of serious overweight gross weight and axle weight,and the passage of trucks is concentrated on part of the traffic lanes.It is also found that the fatigue loading effect from the two-axle vehicle should not be ignored because a high fraction of then are overweight.The girder sectional model and sub-model of the OSBD are established respectively based on the FEM,as to investigate the stress response of connection details and fatigue performance of the cutout detail.The research finds that the stress local effect under wheel loads is significant high at the connection details,while the stress influence line in both the transverse direction and longitudinal direction are short.The deck plate splice and rib-to-deck weld detail can identify the individual axle,but the stress responses at the floorbeam cutout,rib-to-floorbeam weld detail and rib splice can only identify axles group.The stress response at floorbeam cutout is complex,of which obvious stress concentration occurs at the free edge of the critical section on floorbeam web,and the normal direction of its maximum principal stress is in agreement with the fatigue cracking direction.In addition,the stress response at the floorbeam cutout is dominated by the in-plane component although a certain degree of out-of-plane distortion exists at the detail,which means that the increase of the floorbeam web thickness can effectively reduce the stress level at the cutout detail.The thermal elastic-plastic FEM model of the OSBD is built to simulate the welding process of rib-to-floorbeam connection,and it is shown that the residual stress at the cutout detail is of nearly 300MPa and is in tension,which satisfy the fatigue cracking condition due to its superposition with wheel loading effects.The estimated fatigue life at cutout details is in accordance with the actual fatigue life observed on the bridge under the most critical load scenario.Long-duration of stress time histories and typical stress response under the truck loading at the details are obtained,based on stress monitoring on the OSBD under the passage of random traffic flow.The stress spectra at each detail are obtained through the rain-flow counting method,and the effective stress range and loading cycles are presented after applying the Miner's rule,which help evaluate fatigue performance at the detail per AASHTO LRFD specifications.The research finds that the evaluated fatigue life at the cutout detail based on the fatigue categories A and B are both less than the design life of 100 years,and the expected fatigue life with categories B is consistent with the fatigue life observed on the bridge.In addition,the stress at rib-to-floorbeam weld,rib-to-deck weld and rib splice are high under typical truck loading,and the evaluated fatigue lifes at these details are also less than the design life per the category C.In order to investigate the effects of enlarging floorbeam cutout on the stress and fatigue performance at the cutout detail,the multi-scale finite element models based on the ANSYS are respectively established as to accommodate the original and enlarged cutout shape.Comparison study of stress response between the two types of cutout shape is carried out through structural analysis based on fatigue truck from WIM,and through simultaneous stress monitoring under random traffic flow.The research finds that the enlarged treatment will help alleviate stress concentration at the cutout detail,and reduce the nonlinear stress peak caused by notch effect.However such treatment will reduce the net section of floorbeam web,and further weaken the stiffness floorbeam web,resulting in increase on the in-plane stress while no significant decrease on the out-of-plane stress at the detail.Hence it may eventually increase the total stress at the cutout.Accordingly,enlarging floorbeam cutout will not help improve the fatigue performance at the cutout detail under wheel loads.Gages are horizontal arranged outward from the free edge of the critical section on floorbeam web,which is under the heavy lane line,to monitor the stress response of cutout detail under truck loadings,and obtain the stress distribution around 2.5t wide of the cutout detail,then the fatigue assessment of the cutout detail is carried out based on the hot-spot stress metheds.The research find that the notch effect induce a remarkable nonlinear stress,the radius of cutout detail determine the nonlinear stress range,the specification recommended methods are not suitable for the assessment of cutout due to the cutout radius.This paper presents two hot-spot stress extrapolation methods with suitable extrapolation points for cutout,the methods exclude the influence of nonlinear stress peak,and the assessment fatigue life is accordance with the cracking time of the bridge.It is concluded that the thin floorbeam web,large residual stress at the cutout detail,high volume of trucks and high fractional of serious overweight trucks contribute to the premature cracking at the cutout of the OSBD on the heavy freight transportation highway.The vehicle loading spectrum and fatigue truck models presented in this paper will provide important information for those needed on China highway.The present results will also provide valuable suggestions for the design and fabrication of the OSBD,especially for the fatigue evaluation and rehabilitation on the floorbeam cutout detail.