Vibration Test And Mode Parameter Identification Of Continuous Box Girder Bridge With Three-span

Long span is the development trend the modern bridge. The development of the span is from 1050m of the JinMen Bridge to 1991m of the MingShiHai Bridge in Japan finished in 1990's. In our country, since 1980's, long-span bridge construction develop greatly rapidly and suspension bridge, cable -stayed bridge, continuous rigid frame bridge, arched bridge etc. have already entered advanced row in world while. The characteristics of the modern structure are long, light, flexible and sensitive to the function of the wind. The wind loads have already become to the dominate load. Therefore, the wind-resistant analysis of the bridge has already become an important job. In addition, the earthquake load is also an important factor, which is must considered in the large bridge design.The dynamic characteristics that are obtained from the dynamic model established based on the theory only have already not been satisfied the request of the wind-resistant and earthquake-resistant. And the theoretic model needs to be checked by actually measure. Thus the dynamics model can be perfected. So far, the mode test used to identify the intrinsic modes of the structure is the most dependable method.The dynamic nondestructive detect is a kind of fast, high effective, exact method. And the mode test is the base of the dynamic nondestructive detect.The mode test analysis technique is a method that makes use of the vibration test, signal processing and parameters identified. Under the condition of the physics parameter unknown, the modes are established through putting in the excitation and putting out the response, which are treated by the computer signal and parameter identification to find out the mode parameter of the vibration system. Therefore, the method can provide an intuitionistic view for understanding the mode and provide the basic dynamic parameter of the structure. Based on this, the vibration characteristic analysis, the optimums design of the dynamic characteristic and modification etc. can be carried on. The vibration test mainly include three types:(1)Forced vibration test;(2)Free vibration test.(3)The pulsation test. Among them the first is excited by the artificial heavy falling or excited equipment to encourage. The second is to release the heavy suddenly to make bridge vibrate freely. These two kinds of methods need to break off the transportation and are hard to realize or hurt to cause damage. The third method encouraged in the natural environment such as the pulsation pattern and its combination etc. to measure the dynamic characteristic. This method has already been thought much of more and more by the people because it is simple and it does not break off the transportation etc.For large civil engineering structure, the structure response is to be obtained by installing the sensor in some parts of structure. However, it is not easy to acquire the input (encourage) especially on the spot in the bridge test. Therefore, the mode test of the civil engineering usually encouraged in the environment to putout the mode only. The environment encourages have the characteristics of response value small, strong random, big noise interference and the great quantity data to collect. So the mode test encouraged by environment differs from the traditional mode test.This paper carries on the vibration measurement and mode parameter identification of some practical prestressed steel concrete continuous box girder with three-span. In the course, the main contents are as follows.(1) The paper uses the group test method because of the relative large span and the restriction of the line length. Also, the paper introduces the measured points selection and optimum methods in order to improve the efficiency and maximize the information.(2) The whole bridge adopts totally two fixed measure points. Because of the vibration test carried out in the environment encourage can't measure the encourage size, the paper introduces the method of summarization the total modes in the whole bridge.(3) Because no encourage signals are collected in the test process, the intrinsic modes are identified based on the transmission rate of the measure points. The calculation method of the transmission rate is similar to the frequency response function but only replace the acceleration of reference points by the encourage signal. The frequency response function is obtained through encourage spectrum divided by the response spectrum. The transmission rate is obtained through the response spectrum of moving measure points divided by that of the reference points.(4) Because the transmission rate is the response spectrum ratio of two points. If the encourage value of every group is different and the influence of the excited force to the two points is the same, the influence caused by different encourage value to mode parameter can be eliminate. Resonance frequency usually locates"flat area"of the transmission ratio curve. Therefore, it usually appeals to the self-power spectral density and cross-power spectral density to determine the resonance frequency.(5) The validation to the bridge vibration types which are obtained by analyzing the intrinsic modes is also included in the paper.