Damage Identification in Engineering Structures from Changes in Measured Dynamic Response

  • Victor Richards

    Student thesis: Doctoral Thesis

    Abstract

    Identifying the structural characteristics of buildings, and the structural components, using measured dynamic response could provide engineers with an early warning system of the occurrence of damage. The key to such an approach is to understand how the processes of damage, which may be caused as a result of a single specific event or as part of natural deterioration, influences the dynamic response properties. By understanding how the processes involved in damage affect the latter, engineers can make use of this information, which could provide a basic for remedial action.

    This research project was initiated to study where damage identification, using measured dynamic characteristics, could be of particular advantage over more commonly adopted visual based methods. To achieve a thorough investigation into the beneficial aspects of this non-invasive and non-destructive process, a comprehensive study has been compiled and presented in this thesis, which aims to provide detailed information of those circumstances when the technique is most appropriate. The research examines how natural frequencies, mode shapes and damping characteristics are affected by damage, and provides detailed results of investigations carried out to determine the sensitivity of these properties to deliberately induced structural defects. The study focuses on a full-scale eight-storey steel-framed building, which was constructed for the purposes of research in structural engineering.

    To commence the work, a study is presented that compares natural frequencies representative of the whole-frame behaviour of the building during specific stages of construction, to similar characteristics determined from calculations. This comparison reveals that calculations prepared using detailed analytical model of the structure do not represent the information found from measurement, which is thought to be a general observation that would be encountered in other similar studies. A method is presented that allowed calibration of the prepared numerical model against the measured values, which is vital if changes in the structure, as a result of damage, are to be interpreted from natural frequency measurements collected over time.

    Similar comparisons were carried out on one of the floors within the above-mentioned building. In this case, a more refined numerical model of the structure was prepared and was used to calibrate calculated natural frequencies of the floor against those measured directly from the structure before and after damage. To achieve the calibration, the methodology formulated from studies on the whole-frame was utilised, with additional rigour being introduced into the procedure to allow the imposed damage to be quantified in further detail.

    A detailed laboratory investigation was carried out, which examined the changes that occurred to the natural frequency, damping and mode shapes of localised slab panels. These panels were prepared to represent localised sub-elements of the aforementioned floor slab, and were tested under both static and dynamic load conditions to allow imposed damage and dynamic characteristic changes to be identified and compared. The former of these loading conditions was considered to provide a controlled means of introducing damage, which was quantified by recording strain and displacement data during the procedures associated with the test. To interpret the damage caused by this process, a number of numerical models are presented, which model the characteristics thought to be attributed to the defects due to the static loads. Comparing the information to that obtained from the dynamic load tests reveals the sensitivity of the dynamic characteristics and how they are influenced by the development of the progressive damage.

    Finally, due to certain characteristics that were observed from the measurements recorded during the dynamic load testing above, a number of mathematical models are presented, which show how the adjusted dynamic behaviour of the damaged structure can be calculated. This last phase of the research provides additional detail of the way the dynamic response of the panels changes after damage had occurred, which has been considered to offer an additional means of recognising the onset of structural damage.
    Date of Award2000
    Original languageEnglish
    SupervisorB.R. Ellis (Supervisor), R. Wiltshire (Supervisor) & I. Feltham (Supervisor)

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