Study Of Dynamic Impact Factor For Medium And Small Span Beam Bridge

Medium and small span girder bridges are those superstructures that have clear force transmission paths and are easy to construct. The procedures of design and construction for this type of bridges are highly standardised, which make them the world's most widely used bridges.By the end of 2010, the number of highway bridges has exceeded 650 thousands while the majority is medium and small span bridges. However, highway transportation demand has been expanding extremely quickly in the past serveral decades. The vehicle loads, quantity and speed are rapidly increasing, simultaneously. As a consequence, bridge failure events occur at times. Thearefore, obtaining accurate bridge dynamic impact factor(IM) in order to evaluate the bearing capacity of existing bridges and to rationally guide design of new bridges becomes an significant problem to be solved urgently in bridge engineering.Researchers around the world(including China) have pointed out that the dynamic impact factors that the bridge codes specify may underestimate the actual dynamic effects of vehicles.Moreover, the studies of some important factors that influencing IM, including bridge support,cross-section types, material of girders, vehicle load rating, are rarely studied in-depth.The present thesis has developed a methodology which is base on the vehicle-bridge coupled vibration theory to deal with bridge dynamic impact factor. The IM of medium and small span bridge under truck loads were analyzed. The influencing factors, such as bridge cross section, bearing support, road surface condition, bridge span length, vehicle speed, were investigated. Some interesting conclusions were drawn. The results from this study can be used as additional references for current bridge codes by bridge engineers and researchers when dealing with related matters. The following contents are included:(1) Elaborated finite element models of trucks and bridges were built. The dynamic equations of vehicle-bridge coupled vibration were derived. Based on mode superposition method and Newmark-? integrattion method, a step-by-step iterative numerical algorithm was deduced.Computer routine flowchart was also given.(2) The IMs of various types of medium and small span girder bridge under complex driving cases were investigated. The numerical simulation had been used to examine Chinese and U.S bridge specifications. The results indicates that the JTJ021-89 and JTG D60-2004 may underestimate the bridges' real IMs.(3) The IMs of bridge models with serveral frequent used sections were studied. The paremeter analysis of road roughness, vehicle type and vehicle velocity were implemented.The results show that there exists significant variation of IM for bridges with the same span and fundamental frequency but different cross sections, indicating that engineers should pay attention to the difference in IM due to cross-section types.(4) FEM models were built to simulate simply-supported and continuous bridges concrete bridges, and composite steel girder-concrete deck bridges as well. Comparing analysis were performed to study the difference between dynamic IMs of simply-supported and continuous bridges, bending moment IMs and shear IMs. It was found that the current bridge code doesn't consider thoroughly the influence of IMs due to bridge bearing support types and superstructure materials. Caution needs to be taken when applying the current bridge code to continuous bridges and composite bridges.