Modal Test Of Simple Supported Bridge And The Test Of Cable Force

It is necessary to detect whether the newly built bridge meet the safety criteria.Also, inspection and evaluation of the existing bridge is important. It is verydifficult to precisely evaluate the security and service life of the newly built bridgeand the existing bridges using the static method only. And then the dynamic methodis required to detect the bridge. However, the modal test is inevitably when usingthe dynamic method. So far, modal test is the most reliable method to identify theinherent mode of bridge.Large span bridge is the trend of modern bridge. The span of Jinmen Bridgefinished in 1937 is 1050m, and Shihai Bridge finished in 1990s in Japan is 1991m.From 1980s until now, large span bridges develop rapidly in our country. The spanof suspension bridge, cable bridge, continuous rigid -frame bridge, arch bridge andso on has set the pace in the world. The characteristics of the modern large bridgestructure are large, light, flexible and sensitive to wind. Therefore, the resistingwind analysis has become one important work for the design of large bridge. Inaddition, seismic load is one important factor that must be taken into account. So itmust consider the resisting wind and aseismatic in the design of modern largebridge. The exact dynamic characteristics of bridge structure are indispensable inresisting wind and aseismatic design.The original data of structure vibration can be derived from modal test(generally the time-domain of acceleration). Accordingly, based on the original data,inherent mode of bridge structure can be gained. Bridge vibration test mainlyincludes three types: (1) forced vibration test;(2) free vibration test;(3) pulse test.The third type is to measure the dynamic characteristics excited by environment.This method has attracted more and more attention due to the characteristics ofconvenience, feasibility and not intermitting traffic and so on. The modal test incivil engineering usually has output only according to the environment encourage.Because the environment encourage has characteristics of small response, greatrandomicity, noise disturbance and a lot of data acquisition and so on. Therefore,modal test excited by environment is different from the traditional mechanismmodal test.The random decrement technique is used to deal with the response data exitedby environment in earlier days. This technique mainly translates the structuralrandom response to structural free response. However, free response only includesvibration parameters of structure itself, and it is independent of random load. Atpresent, on the basis of different uses, different researchers put forward to somemethods to identify the environment encourage system. For example, the peakpicking method based on the power spectral density, the ARAM model based onthe discrete-time data, the method of stochastic subspace and single modalidentification using the environment encourage as imagine input and so on. Above–mentioned methods each has his strong point. And they can complement andcheck mutually in practice.If large complex bridge requires modal test, generally many test points arerequired arranged. Accordingly, a lot of measurement channels are required.However, the measurement channels are usually not too many in practice. Ifembedded sensor in advance is used to inspect bridge real time, the position andnumber of the embedded sensor in advance is paramount. On the basis of the abovetwo points, the paper uses the effective independence method to select the testpoints of one simple-supported rigid beam, 4m in length. And selecting one pointas reference point (fixed point), vibration test is carried out in the selected pointwith stretching group by group. Because the excited signal is not collected duringthe test, the paper identifies the modal parameter of the rigid beam with the peakpicking (PP) method.Finally the paper adopts the vibration method to measure the cable force of some lightstandard rail without back cable in ChangChun. Measuring the cable force with vibrationmethod is to use the environment random as encourage. After dealing with the accelerationsignal collected from seismometer by the anti-filter, magnification and analysis of spectrum,then the cable force is gained according to the relation between the frequency and cable force.The critical problem is how to identify the inherent frequency of the cable bridge preciselyaccording to the vibration test data.