DAMAGE DETECTION USING OPERATIONAL MODAL ANALYSIS

Abstract

Civil engineering strucutures undergo progressive deterioration due to aging effects. In addition to this, hazards like earthquakes, floods, blasts, etc will increase the causes of damages. Hence Structural Health Monitoring (SHM) has become an area of immense research as it is important to ensure the safety and servicebility of these structures. Even though several non destructive techniques are available for the same, vibration based approach has earned popularity due to its advantages over conventional techniques. In this dissertation modal parameters based approach is being illustrated with the help of experiments on 2 test models. Modal parameters were extracted with the help of Ambient Vibration Testing (AVT). Test models were of 1.8m high four storey frame structure with bolted connections. Free vibration tests were conducted in two measurement sets. Two test models were constructed to examine the accuracy of the damage detection mechanism to a damage at differnet levels of the structure. The separation of modal parameters from the free vibration signature was carried out by using ARTeMIS Extractor v3.4. Frequency Domain Decomposition technique was used for the same. Mode shape scale factors for mass normalization of modal vectors were found out by using mass change method. Damage state analyses were done by intoducing a crack of 10 mm depth on columns connecting first floor and second floor in Model 1 and second floor and third floor in model 2. All the cracks were made in the stronger direction of the model and the modal properties in the weaker direction were considered for damage detection. Modal parameters which could be used as damage indicators were identified by comparing the modal properties of the undamaged and damaged structure and were suitably used to detect and locate the damage. Further a level 3 damage detection was achieved by quantifying the damage. Analytical models of the test models were also prepared to predict the dynamic behaviour of the model. On comparing the modal parameters of the analytical model with that obtained from experimental data, it was found that modeling errors were present in the analytical model. Hence model updating was done by using Inverse Eigen-sensitivity based Method (IEM). Thus with the help of model updating, analytical models were developed which can predict the behaviour of the model more accurately. The damage detection mechanism proposed involves the detection, location and quantification of a damage at the storey level. In order to detect the damage at member level it is required to get the vibration signature of the structure at each Degree of Freedom (DoF). Practically it is not possible to instrument sensors at each DoFs. Hence in this study each storey is assumed to have one DoF in one direction. Thus the damage iii detection mechanism proposed in this thesis will considerably reduce the effort of Structural Health Monitoring (SHM) by accurately predicting the storey where damage has occurred and there by alleviating the need for visual inpection at each and every floor level.