Assessment of the structural integrity of timber bridges using dynamic approach
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In this study, a systematic approach was adopted to investigate, numerically and experimentally, localised defects and/or damage in timber bridges, such as rot, using modal based damage detection techniques. An existing damage detection method namely damage index (DI) method that utilises modal strain energy before and after damaged state was adopted. One contribution of this study was to modify the Dl method by an additional step of normalising the modal curvature, which would minimise the dominance of higher modes. In the numerical models, a comparative study of the effects of numerical integration techniques used in a damage detection process was carried out. The results show that when mode shape curvature integrations use the rectangular rule for the numerical integration, it yields better results than the trapezoidal rule. In the numerical examples using a finite element model of timber beam, the modified DI (MDI) methods were found to perform better than its original form for locating'" single and multiple damage scenarios. For the DI methods, two types of formulations were adopted and modified, and they are denoted as modified damage index I (MDI-I) and modified damage index II (MDI-II). Another modal based damage detection method, namely changes in flexibility (CIF), was adopted for locating damage. It was found that the ClF method performed reasonably well for single damage but not multiple damage scenarios. As part of the study, the modified damage index methods were utilised for evaluating severity of damage. For the :MDI-I method, the formulation was not derived to evaluate damage severity directly. Instead, a hybrid of the MDI-I and CIF methods (HMC), was proposed for evaluating severity of damage in terms of loss of '1' (moment of inertia). Using three levels of damage, i.e. light (L), medium (M) and severe (S), the HMC method is able to predict the medium and severe damage quite well, but it is less efficient for light damage scenarios. For the MDI-II method, further manipulation of the algorithm can predict the severity of damage in terms of loss of'I'. This method is able to predict the medium and severe damage quite well but is not as good for the light damage. Both methods, HMC and MDI-II, for predicting severity of damage, required some adjustment using a weighting factor in order to obtain reasonable results. An experimental modal analysis (EMA) test program of timber beams was undertaken. This was done to verify the robustness of the modified damage index methods for detecting location and estimating severity of damage. The laboratory investigation was conducted on the corresponding changes of modal parameters due to loss of section. The MDI methods were used to detect location of damage and to evaluate the severity of damage in the test beams. A mode shape reconstruction technique was utilised to enhance the capability of the damage detection algorithms with limited number of sensors. The test results and analysis show that location of damage is quite accurately estimated with the available sensors. The methods demonstrate that they are less mode dependant and can detect damage with a higher degree of confidence. The MDI methods also show that they are able to predict the severe damage well, but it is less accurate for the medium damage and not as good for light damage. The damage index II (DI-II) method extended to plate-like structures (DI-II-P) was adopted and evaluated for detecting damage. Based on finite element analysis (FEA) results of a laboratory timber bridge, the DI-II-P method which utilises two dimensional (2-D) mode shape curvature was employed to detect location of damage. The results show that the tnethod based on 2-D mode shape curvature is able to locate damage quite well, numerically. A supplementary work using the DI-II-P method in a timber plate model was carried out. The results also show that the method was able to predict the damage location well. A process of updating a laboratory timber bridge, analytically, is presented. A finite element model was developed and updated with experimental modal data. Material properties of timber beam (girders) and plywood (deck) as well as the screw connection between deck and girder were experimentally investigated. These test results were then used for the finite element modelling. The model has been developed sequentially starting with a preliminary model having very simple features. It followed by the advanced model calibrated with the experimental modal data employing a global objective function, consisting of errors of natural frequencies and modal assurance criterion. The calibrated finite element model shows a good correlation to the experimental model with minor adjustments to the real material properties and boundary conditions. The calibrated model can reasonably be used to study the damaged behaviour of the laboratory timber bridge. The bridge model was then used to verify the numerical results for detecting damage. The bridge was inflicted with various damage scenarios with loss of section similar to the timber beam models. The limited number of data was expanded using the 2-D cubic spline. Using the reconstructed data for detecting damage yields better results than just using 'as is' data. Using the undanlaged and dmnaged modal data, the D I-II -P method was employed to detect the location of damage. The results of using the first nine modes showed that generally the severe damage is able to be located by the method. It performs reasonably well for the medium damage but does not perform as good in the light damage scenarios. However, in some cases the method can present some problems in identifying severe damage, which may be due to lack of normalisation of mode shape curvature. Complementary work was undertaken using the method 'On a timber plate, experimentally. The results showed that the damage detection process in the timber plate is less efficient compared to the laboratory timber bridge. A comprehensive comparative study was carried out based on the results of the numerical and experimental investigation of damage detection on timber beam, laboratory timber bridge and timber plate. For the timber beam, both damage detection methods, MDI-I and MDI-II, were capable of detecting medium and severe damage in the numerical and experimental studies. However, the light damage was not identified well using the experimental data in the presence of noise. To estimate damage severity in the timber beam, the HMC method performed well for the medium and severe damage. The method did not work well in estimating severity of light damage. Similar conclusions can be drawn in using the MDI-II method to estimate the damage severity. The results of applying the DI-II-P method (using 9 modes) to locate damage in the laboratory timber bridge showed that numerical and experimental data are capable of detecting all severe damage for damage cases with less than three damage locations. While for light and medium damage, the experimental data did not work well as compared to the numerical one. For the timber plate (a complementary work), the numerical and experimental results also showed that they are able to detect the severe damage well. However, there were serious false positives appearing in the light damage cases in the experimental results.
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