Bridge structure is the main load structure that under constant load,vehicle load and crowd load.When the vehicles passing over the bridge at a certain speed,the bridge structure will vibrate due to the jitter of engine,irregularities of the deck.In addition,the crowd load,wind power, the action of earthquake force can also cause the bridge vibration.With the continuous development of transportation enterprise,on the one hand,the number and load of vehicles have a rapid growth, the vehicle's speed also had a very big enhancement,on the other hand,with the popularization and application of new structure, new materials and new technology,bridge structure gradually trends to light-duty,for the large span and large span bridge structure, earthquake and wind vibration are often the control factors of the design and construction.Therefore, vehicle loading and other dynamic loading for shock and vibration of the structure of the bridge, has become one of the important issues in bridge structure design, calculation, construction, operation, maintenance and curing process.Vibration problem of the bridge structure has a lot of influence factors,involving complicated theory,we can't get the research method of combining theoretical analysis and field testing only by theory analysis,therefore,it is not practical results the to address the issue of structural vibration,we usually use the method of combining theoretical analysis and field testing,therefore,the vibration test is essential to solve the problem of engineering structure vibration.Over the past 20 years,with the development of electronic computer and automation technology,vibration testing technology has made great progress,On the one hand, performance in the wind tunnel test, simulated earthquake shaking table test and pseudo dynamic test has been widely used,on the other hand performances for engineering structure under seismic load, wind load, the dynamic responses under the action of vehicle load of field test method have also been greatly improved.

Dynamic load test of bridge is a test item that use some of the structure of the bridge excitation method arouses the vibration of bridge structure, the determination of natural frequency, damping ratio and vibration of the structure of the bridge type, dynamic impact coefficient, dynamic response (acceleration, dynamic deflection) parameter,thus judging the overall stiffness of the bridge structure on the macro stage.Although there are some differences on test purpose,test content in dynamic and static load test of the bridge structure,while for the overall analysis of bridge structure performance they are equally important.In terms of test steps,it is basically the same as the static load test.The dynamic load test should pass three stages,namely preparation stage,test stage and analysis summary stage.In terms of the the test nature,dynamic loading test can be divided into production and scientific research test,generally speaking,the dynamic load test usually test on the actual structure testing,we can also start the dynamic load test of the structure model in indoor experiment according to the characteristics of the bridge structure and actual need,such as the test of large span bridge wind-induced vibration in wind tunnel,study on seismic response of the structure of the bridge in a simulated earthquake vibration bench etc.The basic task of the bridge structure of the dynamic load test can be summarized as the following respects:

(1)Determination of the structure dynamic characteristics,such as determination of bridge structure or component natural frequencies,vibration damping characteristics,type etc.

(2)Determination of the structure's forced vibration response under dynamic loads, such as the determination of the bridge structure or component in vehicle load, wind load amplitude under dynamic stress and acceleration.

(3)Determination of dynamic properties of dynamic loads,such as determination of structural vibration caused by force like size, direction, frequency and function law etc.