Research On Methods Of Structural Modal Parameter Identification Under Ambient Excitation

In recent years,the civil structure health monitoring causes more and moreattentions.A structure whether has been damaged or injuried,could be easily displayedthrough its own modal parameters (frequency, damping ratio, vibration mode), so thesystem modal parameters identification becomes the focus of the structural healthmonitoring. The traditional mode experiment methods are always hard to be achievedbecause it needs to be imposed an external excitation, or because the cost is high, orbecause it affects the normal use. This dissertation is based on the modal parameteridentification under ambient excitation, so it doesn’t need the outside incentive anddoesn’t affect the structure normal use. At the same time, it greatly reduces theexperimental cost. So there is a wide range of applications in the engineering.To sum up, the main work accomplished and conclusions of this paper are asfollows:1A detailed description of the dynamics of the three basic problems and theadvantages and disadvantages of modal parameter identification under ambientexcitation, the definition of the relationship between principles and methods of thekinetic equation decoupling, frequency response function and impulse responsefunction, and the definition of the autocorrelation function and cross-correlationfunction2Focus on two kinds of data preprocessing methods (the random decrement methodand Next method), and five modal parameter identification method (the ITD method,STD method, the complex index method, the ERA method and the StochasticSubspace method).Through the simulation example analysis respectively,weidentificate the frequency and the damping ratio.Finally we come to a conclusionthat the Stochastic Subspace method and the ERA method is the accuratest,and theSTD method and the complex index method is accurater,and the last is the ITDmethod. For the Stochastic Subspace method we have used the stability diagram andachieved the satisfied result.3Put forward the Stochastic Subspace/ITD method, the Stochastic subspace/STDmethod, the Stochastic subspace/complex index method. State its principle in detailand prove it.Through the simulation example analysis, we draw a conclution that itis accurate for the model of damping ratio and frequency identification. 4We did a four-story steel frame model experiment, through the two kinds ofconditions, namely: environmental vibration and hammer excitation. on the ITDmethod,the STD method,the complex index method, the ERA method and theStochastic subspace identification method and also three new methods of thestochastic subspace/ITD method, the stochastic subspace/STD method, thestochastic subspace/complex index method,we find that all the methods areworkable.5Establish a finite element bridge model and use car loads to simulate environmentincentive. Record the displacement of each measuring point,and use Stochasticsubspace method to identify the modal parameters (frequency, damping ratio, modeshape) and use three new methods to identify the modal frequency.Finaly it provethat all the methods can be applied to the bridge structure.