Calculation Method And Experimental Study On Impact Coefficient Of Simply Supported Bridge Span Structures

The so-called impact coefficient of bridges can be defined as the ratio of dynamic deformation or internal force of the bridge caused by moving vehicle loads to the static deformation or internal force caused by vehicle static loads.Bridges must undertake live loads(the loads caused by moving vehicles) during the couse of using,but moving vehicles have impact action to bridges.Because of the existence of impact action, internal force of bridges caused by live loads is larger than that caused by static loads.Therefore,Impact effect of moving vehicles to the bridge must be taken into consideration during the course of the bridge design and using.The bridge impact coefficient is usually used to express the impact effect of moving vehicles to the bridge.Because vehicles and the bridge formed in a coupling vibration system during the course of vehicle moving, influencing factors of bridge impact coefficient is complex.Based on vibration theory analysis, the influencing factors of bridge impact coefficient is as follows.(1)Dynamic characteristics of vehicles,which mainly includes vehicle suspension stiffness, tire stiffness, damping coefficient, wheel base and wheel base of each part.(2) Dynamic characteristics of bridge structures,which mainly includes section shape of bridge span structures,mass, stiffness distribution and damping characteristics of bridge structures.(3)Vehicle running speed.(4) Unevenness of the bridge deck and subsidence of bridgehead,etc.(5)Position and number of moving vehicles on the bridge.However, the computing specifications and regulations about bridge impact coefficient at home and abroad at present,only take one or some certain parameters of bridges or vehicles into consideration,without thinking of the complex vehicle-bridge coupling vibration.For example, bridge impact coefficient is calculated by bridge span length stipulated in 1989 version bridge design code of our country. Bridge impact coefficient is calculated by bridge fundamental frequency stipulated in 2004 version bridge design code of our country.Because of without taking the complex vehicle-bridge coupling vibration into consideration, the existing specifications or regulations can not be used to calculate bridge impact coefficient accurately.However, bridge impact coefficient directly control the using status of bridge design load and impact.Therefore, it is a signality to calculate bridge impact coefficient accurately.In order to calculate bridge impact coefficient accurately, the complex vehicle-bridge coupling vibration must be taken into consideration.Because solution of vehicle-bridge coupling vibration system is complex,current calculation methods of bridge impact coefficient have not formed in theoretical system,and even there are no effective technical standards and regulations.Therefore,it's very necessary to carry out the calculation method and experimental study on impact coefficient of bridge structures.Values of bridge structure impact coefficient used in design code of countries is analyzed deeply at first part of this paper.Based on structure vibration theory, various vehicle-bridge coupling vibration models are established.The established models can be used to analyze the bridge structure impact coefficient.A certain inclined plate bridge in Changchun is taken as examples,and the calculation method of impact coefficient proposed in this paper is verified finally.Research work carried out in this paper is mainly as follows:1.Calculation method and values of bridge structure impact coefficient used in design code of our country and part of other countries are summarized and analyzed deeply. Values of bridge structure impact coefficient used in design code of countries are compared,and it provides a meaningful conclusion for the value of impact coefficient used in design code of our country.2.Vehicles are simplified as moving loads,moving mass and half vehicle model,respectively.Based on different simplified forms of moving vehicles, moving load-bridge model, moving mass-bridge model and vehicle-bridge coupling vibration model are established respectively.Based on half vehicle-bridge coupling vibration model,space vehicle-bridge coupling vibration model is established.Based on different vibration modes, dynamic response is calculated respectively.Based on the calculated dynamic response, calculation method and influence law of bridge impact coefficient under different vehicle model loads is obtained by using the concept of dynamic magnification factor.3. A certain inclined plate bridge in Changchun is taken as examples,and the calculation method of impact coefficient proposed in this paper is verified. Dynamical load test was carried out to the certain inclined plate bridge in Changchun.Based on the tested dynamic response, dynamic characteristics such as the bridge impact coefficient are analyzed. The bridge impact coefficient was theoretically calculated by using space vehicle-bridge coupling model.The tested results were compared with theoretical results and the value recommended by our design code,and then the correctness of bridge impact coefficient calculation method proposed in this paper is verified.It provides some theoretical guidance and measured datas for improvement of impact coefficient in design code.