Studies On Vehicle Loads Of Highway Bridge And Reliability

As one of the basic variable loads of the highway bridge,vehicle load plays an important role in design.The vehicle load on the highway bridge has strong randomness,due to the wide variety of vehicles on the road and the uncertainty of vehicle load locations in each lane.Thus,the statistical method is adopted to analyze and study the vehicle load.The vehicle load in the current highway bridge load specification of China is obtained through statistical analysis based on the actual measured results in the 1990s.However,in recent decades,the traffic flow of China has changed significantly with the rapid economic.Especially as the overloading phenomenon increases and the bridge collapse occurs frequently,the applicability of the vehicle load standards in the current highway bridge load specification has attracted much attention.Moreover,the available reliability based bridge design method,which has been applied since the 1990s in China,is far from complete in many aspects.In this dissertation,several related aspects,such as the probability model of vehicle load,partial factors of vehicle load,transverse reduction factors of vehicle load,reliability evaluation of bridge structure and reliability analysis of long-span bridge structure,are studied.The main contents are listed as follows:(1)Based on the vehicle load data measured in 69 road sections of 23 provinces and regions,statistical analysis is conducted on the vehicle types,gross vehicle weight,time interval,inter-axles distances,and axle weight proportion in each lane(fast lane,middle lane and slow lane)of non-controlling area,toll-by-weight area and overload controlling area respectively,and then the probability models of vehicle load in accordance with China's current situation are developed.(2)In view of that the sensitivity of highway bridges with different spans to the vehicle load is dissimilar,the simulation motorcade is generated based on the proposed probability model of vehicle load,and applied to the single-lane prestressed reinforced concrete T girder bridge,then the bending moment and shear force of the girder are calculated.In accordance to the principle of the same exceeding probability,influence factors of characteristic value of vehicle load effects with various spans are obtained.It's found that,for a given exceeding probability of the vehicle load or reliability index of the bridge,the partial factor of vehicle load increases with the decrease in the span of bridge.On this basis,the calculation formula for partial factor of vehicle load accounting for the bridge span is established.(3)It's highly unlikely that the multi-lane bridge is covered by vehicles or many heavy vehicles appear at the same time.In view of this,the characteristic value of lane loading should be reduced to ensure the consistency of the reliability of highway bridge with different lanes.The traffic flow is generated using the proposed probability model of vehicle load and applied to the simply supported multi-lane prestressed reinforced concrete T girder bridge in different lanes,then the corresponding load effects are calculated under a certain guaranteed ratio,and the multi-lane transverse reduction factor of vehicle load is obtained,i.e.?the two-,three-,four-,and five-lane of multi-lane transverse reduction factors are 1.00,0.75,0.65,and 0.55,respectively.It's indicated that,if the multi-lane bridge structures are designed by using the multi-lane transverse reduction factor obtained in this paper,the reliability indexes of members can keep consistent.(4)Based on the proposed probability model of vehicle load,the load effects of prestressed cored slab and T girder bridge in Chinese General Drawing Collection for Highway Bridges are calculated.The arbitrary-time point distribution of the load effect and the maximum distribution of the load effect in the design reference period are obtained.On this basis,the reliabilities of the bridges in the non-controlling area,toll-by-weight area and overload controlling area are analyzed.It's found that,the reliability indexes of flexural strength and shear strength of superstructure of the bridges under current vehicle load,designed according to the Chinese General Drawing Collection for Superstructure of Highway Bridges,can't satisfy the target reliability index required by Unified Standard for Reliability Design of Engineering Structures(GB/T 50283-1999).The reliability index in the non-controlling area is the lowest,the reliability index in the overload controlling area is the highest,and that in the toll-by-weight area falls in between.(5)Based on the proposed probability model of vehicle load,the stress histories of truss members are calculated for a steel truss bridge.The stress range and probability distribution of its equivalent range are obtained by means of rain flow counting approach and linear damage accumulation criterion.On this basis,the fatigue reliabilities of truss members are analyzed.It indicates that the equivalent stress range of stuss member follows lognormal distribution.The reliability indexes of truss members are in the range of 3.612-7.789 for the steel truss bridge under vehicle loads.(6)Based on the probability model of vehicle load proposed in this paper,the probability model of stay cable stress of long-span cable-stayed bridge is developed with respect to Sutong Yangtze River Bridge.The probability function of maximum stay cable stress in the design reference period can be determined using up-crossing theory.Then the reliability of stay cable under random vehicle loads is analyzed.It's found that the stay cable stress can be modeled by a Gaussian stationary process.Moreover,the reliability index of stay cable is from 7.93 to 14.55 for the long-span cable-stayed bridge under vehicle loads.The reliability index of the stay cable closing to the cable tower would be larger and vice versa.