Study On Dynamic Impact Factor Of Short- And Medium- Span Simply Supported Highway Bridges Suitable For China

Bridge is an important part of highway transportation facilities.In recent years,the number of vehicles and vehicle weight have been progr essively increased and the safety issue of bridges under the action of vehicle dynamic loading has always been concerned.Up to the end of 2017,the number of bridges in our country is over 800000,in which the proportion of short-and medium-span bridges is larger than 88 percent.The deadweight of short-and medium-span bridges is light and thus the ratio of live load is large,therefore,their safety under the action of moving load has always been the focus of interest.The dynamic impact factor(IM)is an important index to reflect vehicle dynamic effect on bridges.However,due to the complex mechanism of vehicle-bridge coupled vibration,scholars from different countries still have different ideas on taking the value of the IM.The calculation formula for the IM in the current bridge design code in China is regarded as a function of bridge fundamental frequency,which was obtained based on the measured data in the 1990 s through probabilistic method.However,compared with traffic load when the specification was made,the actual traffic load has changed a lot today.Therefore,the IM specified in the bridge design code may not be able to accurately reflect the actual vehicle dynamic effect on bridges.In addition,a large number of research results on th e IM conducted by domestic scholars were obtained based on foreign classical vehicle models and few researches on vehicle-bridge coupled vibration were carried out using vehicle models able to present China's actual traffic conditions.Therefore,based on the three dimensional vehicle-bridge coupled system,the IM of short-and medium-span bridges under the action of vehicle load specified in the bridge design code in China have been investigated and the sugges ted value of the IM has been proposed.The main work conducted in the thesis is as follows:(1)the vehicle dynamic model that can well represent the dynamic effect of the design load has been derived,which can be used as the vehicle model for domestic scholars to conduct vehicle-bridge coupled vibration analysis.(2)Finite element models of thirteen representative bridges were built based on the ?General Design Atlas of Highway Bridges and Culverts?,and combined with the vehicle model derived in previous part,a three dimensional vehicle-bridge coupled system was established.The dynamic and static responses of three bridges,including a simply supported T-girder bridge,a hollow slab bridge and a small box girder bridge all with a span of 20 m,have been investigated under different cases.(3)Based on the established vehicle-bridge coupled system,the effects of four key parameters,including bridge fundamental frequency,road surface conditions,vehicle speed and vehicle weight,on the IMs of those thirteen bridges have been investigated.(4)Statistical properties of those obtained IM have been studied through probabilistic method,based on which the suggested value of the IM has been proposed.The rationality of the proposed IM has been discussed by making comparisons with the IMs specified in bridge design codes of many foreign countries.