Fatigue Reliability Study On Partially Prestressed Concrete Bridges Under Heavy Traffic Load

The structural performance of partially prestressed concrete(PPC)bridges is between ordinary steel bars and full prestressed concrete bridges.It has the advantages of high strength and good ductility.Generally,strength and stiffness damage will not occur.But the fatigue problem has caused concern.With the continuous development of China's transportation industry,the road traffic volume and vehicle load gradually increase.The long-term effect of the continuously increasing random heavy-duty traffic will cause the PPC bridge to work in a cracked state,thereby amplifying the fatigue damage effect.Based on the measured vehicle flow data and structural fatigue reliability theory,the fatigue safety level of PPC bridge in service period can be evaluated,which can provide theoretical basis for PPC bridge design,reinforcement and traffic control,and lay a foundation for the popularization and application of PPC structure in bridge engineering.This paper relies on the National Basic Research Program of China(973 Program)from the "Super large-span bridge safety design theory and method"(2015CB057706)and the National Natural Science Foundation project“Fatigue evolution mechanism and evaluation method of main beam of long-span concrete cable-stayed bridge"(51678068),considering the key scientific problems of fatigue stress spectrum simulation and fatigue reliability analysis of PPC bridge under heavy-duty random traffic flow,considering the effects of the actual vehicle parameters,vehicle impact effect,traffic density parameter,road roughness condition,traffic growth and load limiting measures on the fatigue reliability of the PPC simply supported beam bridge and the PPC main beam of cable-stayed bridge.The main research contents and conclusions are as follows:(1)A nonlinear numerical simulation method for fatigue stress and crack width of PPC beams under fatigue loading is studied.Based on the bond-slip relationship between steel and concrete under fatigue loading,the stress distribution formula between ordinary steel bars and stress steel bars after concrete cracking is derived,and the formula for calculating the maximum crack width of PPC beam under fatigue loading are proposed.And the accuracy and feasibility of the analytical method are verified by numerical examples.The numerical analysis shows that the nonlinear method proposed in this paper can accurately calculate the fatigue stress of PPC beam after cracking,and can capture the crack width expansion law.(2)Based on a large number of weigh in motion(WIM)data for long-term monitoring of the China highway,a heavy-duty random vehicle flow load model is established.The difference between the random vehicle flow load model and the typical fatigue flow model at home and abroad is compared.The probabilistic characteristics of daily traffic flow,vehicle type,vehicle weight,vehicle distance,vehicle lane and vehicle speed are analyzed.Monte Carlo sampling method is used to generate random traffic load model.The established random traffic load model can capture the overload probability,traffic growth,intensity,and load limiting measures of the actual traffic flow,which lays a foundation for studying the influence of traffic flow parameter changes on the fatigue reliability of the bridge.(3)The fatigue stress spectrum of PPC simply supported beam bridge under random vehicle impact is studied.The dynamic fatigue stress time history of PPC girder bridge is analyzed by the vehicle-bridge coupled vibration theory.The fatigue stress range is equivalently calculated based on the S-N curve of European norms.The effcts of vehicle type,vehicle weight,vehicle speed and road roughness condition on the fatigue stress spectrum of the medium and small span PPC simply supported beam bridge are studied.For the operation of a large number of vehicles,the time-consuming problem of vehicle-bridge coupled vibration analysis is proposed via interpolation response surface method;the research shows that for the 40m span T-beam bridge under the condition of "general" road surface roughness,the cumulative fatigue damage of the dynamic bridge is 16.71 times as the cumulative fatigue damage of general specification for the design of highway bridges and culverts(JTG D60-2015).(4)The fatigue damage function of PPC bridge is established.The effects of prestress degree,road roughness condition,traffic load limit and traffic growth on the fatigue reliability of small and medium span PPC girder bridges are analyzed.The results show that the fatigue reliability index of PPC beam increases with the increase of prestress when the prestress is 0.4,0.6 and 0.8 respectively.(5)The fatigue reliability of PPC main beam of cable-stayed bridge considering the traffic intensity is studied.Taking a concrete cable-stayed bridge with 420 m concrete as the engineering background,the static influence of partial prestress on the main beam of the cable-stayed bridge is analyzed.The cellular automaton is used to simulate sparse and dense random traffic.The effect of traffic intensity on fatigue stress spectrum simulation of cable-stayed bridge is analyzed.The influence of partial prestress on the fatigue reliability of the main beam is studied.The research results show that the reliability index of the full prestressed concrete main girder is greater than 5 during the 100-year operation period,and the fatigue reliability index of the partially prestressed concrete main girder is about 4.23.Considering the traffic growth,the fatigue reliability index decreased from 4.23 to 3.10.With the Ro=100%load-limiting measure,the fatigue reliability index can be increased to 3.76,indicating that the load-limiting measures can significantly improve the fatigue reliability of the bridge structure.