Study On Structural Parameter Identification Under Ambient Excitation

The structural modal parameters are the basis of structural dynamic analysis. They are widely used in many fields, such as the finite element model modification, the structural damage detection, the structural health monitoring, the structural security assessment and so on. So, it’s very important to identify the modal parameters.For the traditional modal parameter identification, both exciting signals and response signals are necessary to establish the frequency response function or the impulse response function. However, with the rapid development of the economy and technology, the civil engineering structures are large scaled and complex, and it’s hard to exciting the structures by artificial methods, which means it’s hard to get the exciting signals. As a consequence, the environmental exiting method is occurred. Compared to the traditional modal parameter identification, the identification under ambient excitation is very convenient because it only needs the response signals and there is no damage to the structures. The identification algorithm is usually on the assumption that the ambient excitation is stationary white noise, which is not completely consistent with the reality. So, the concept of white box is proposed in this paper, and several studies are conducted as follows.1 、 The stochastic subspace identification is chosen as the modal parameter identification under ambient excitation. And a numerical modal is set up to prove that the stabilization diagram algorithm is better than the subspace angle algorithm in the determination of the system order n. Meanwhile, according to the numerical modal, if the excitation is stationary white noise, the stochastic subspace identification method can extract the modal parameters precisely.2、The precise time integration method is used in loading identification, and a formulation is conducted for the method. According to a numerical simulation, the precise time integration method can identify the deterministic load as well as the random load when the sampling frequency is large enough.3、A steel cantilever beam is chosen as the white box, and the hammer exciting method is used to get the frequency response function. Then, a traditional parameter identification method called single modal identification is applied to extract the first to fifth frequency, damping ratio and modal shapes of the white box. And so far the dynamic characteristics of the white box have been calibrated.4、Then, the acceleration of the white box is measured under ambient excitation. Combined with the dynamic characteristics calibrated before, the ambient excitation can be identified using the precise time integration method. Once the ambient excitation is identified, the time period of the excitation which is stationary white noise can be selected. Afterwards, the corresponding time period of the response produced by the white noise can be determined. Finally, the selected response can be used in the stochastic subspace method for modal parameter identification of the white box.5、Based on the comparison between the calibrated dynamic characteristics and the modal parameters, the reasons induced the deviations in the white box experiment are analyzed.Finally, some conclusions obtained from these studies can be the guidance of the subsequent work, and the application of the white box in the real modal test has been previewed.