| In the past several decades, the truss structure has been widely used as an important structure form for large span building. The truss structure of long-term using may be damaged by load effect, ambient factors, natural calamities and incidents, its loading capability will decrease gradually, and its security has attracted people attention. Therefore, it is an imperative problem to detect truss damages so that its security can be evaluated and monitored nowadays. As a results, the health monitoring and damage diagnosis for large structure have been regarded as a key science and technology project for China.Compared with the response of structure static force, the response of structure dynamic force contains more structure dynamic character, so the damage diagnosis methods based on vibration have become a research focus for many scientists, and many methods have been proposed by them in the past years. However, the most of these methods are based on the modal analysis, and modal parameters are regarded as the basic variables of damage detection. Since the effect of the noise, structural non-linear and artificial factors, etc, some errors will arise from modal analysis when modals are fitted using measured frequency response functions (FRF). Moreover, since we can only obtained partial data of whole modal using current test method, testing data are not integrated, therefore it is difficult for truss damage diagnosis using the method based on modal analysis. Considering the reason given above, a damage diagnosis method based on measured FRF and principal component analysis (PCA) is brought forward in the paper. By means of theory analysis and deduction, the basic theory and method for truss damage diagnosis are formed, and by dynamic experiment of a whole size truss in the laboratory, the proposed method is validated. The experimental result shows that the proposed method is feasible and reliable for truss damage identification. In brief, main research achievements and conclusions in the paper are as follow:(1) By means of basic concept and practical meaning of FRF, original data matrix, which contains the most of truss modal information, is formed based on measured FRF. Thus, using original data matrix we can constitute the damage identification matrix, which is the basis for truss damage identification.(2) Since measured FRFs contain large numbers of data, and damage identification matrix has numerous dimensions, a damage orientation method, based on PCA and multivariate control theory, is proposed for truss damage identification. With PCA technology, each principal component of damage identification matrix can be obtained. Then the first several order principal components, which contain the almost all information of damage identification matrix, can be obtained by means of the contribution ratio of each principal component, and the data of damage identification matrix can be reduced. Thus, by analyzing the data character of first several principal components, separating singular data and extracting truss damage information, truss damage elements can be oriented successfully.(3) Using the Mahalanobis Distance theory of Multivariate Statistical Analysis, an evaluation method for truss damage extent is proposed based on the damage distance. Original data matrix is formed using measured FRFs of undamaged and damaged truss, and its dimensions are reduced with PCA technology. By constituting observation sample using principal component data, calculating the distance between damaged samples and undamaged samples, truss damage extent can be evaluated successfully. At the same time, some problems, which can not be solved very well only using the damage orientation method, also be solved using the damage distance mothed, so damage distance method is a very good supplement for truss damage orientation method.Using twice measured data of the FRFs from undamaged truss, the minimal damage distance can be obtained. The minimal damage distance is a index for quickly judging truss damage in truss on-line health monitoring. In the other hand, the minimal damage distance can be used as a quantity unit for truss damage extent evaluation.(4) The dynamic test signals obtained from practical truss have been contaminated by the noise, and existing truss has complicated boundary condition, big mass and damp, so the reliability of dynamic test signal noise is reduced. Therefore, a method for reducing or eliminating FRFs noise is proposed based on the PCA in the paper. By analyzing the PCA of original data matrix, we can reconstruct original data matrix using first several principal components, which generally contain all most information of original data. Thus, the original data noise can be reduced or eliminated, which is the basis for next damage identifications.(5) In order to validate the reliability of the method, a whole size truss was tested with 20 kinds of damage case. The experimental result shows that the proposed method is feasible and reliable for truss damage identification. At the same time, the measurement points selection, exciting signal control and response signal collection, are also searched for the SISO dynamic test method, as well as testing process and result evaluation. In the last, the basic dynamic test methods and process with manual exciting manner are suggested and summarized in the paper.The damage diagnosis method proposed in the paper directly uses FRFs for truss damage identification, does not need any modal parameters and integral modal data, so damage diagnosis results are not affected by modal errors and incomplete data. Since the damage diagnosis, which is performed by analyzing FRF data character, does not need truss mechanics mode, so it has not any special requirement for truss structure types, restricting manner and boundary condition. And the dynamic test method using manual excitation and SISO is simple and convenient. Therefore, the damage diagnosis method proposed in the paper has very important theoretical and practical value for health monitoring and damage diagnosis of existing truss.
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