ABSTRACT
Most vibration-based damage detection theories and practices are formulated based on the assumption that failure or deterioration would primarily affect the stiffness and therefore affect the modal characteristics of the dynamic response of the structure. If this kind of changes can be detected and classified, this measure can be further implemented for a bridge monitoring system to indicate the condition, or damage, or remaining capacity of the structures. It can also be used to evaluate the seismic behavior of the structures. However, conventionally defined modal parameters have been shown to be mildly sensitive in the detection of various types of bridge damages. Furthermore, the modal parameters of conventional modal testing such as frequencies and modal damping are global parameters, which cannot locate the damages. Many damage detection schemes rely on analyzing response measurements from sensors placed on the structure. Research efforts have been made to detect structural damage directly from dynamic response measurements in the time domain, e.g. the random decrement technique, or from frequency response functions (FRF). Also, some damage detection methods have been proposed to detect damage using system identification techniques. In this paper, an algorithm based on changes in Power Spectral Density (PSD) is presented. The algorithm is used to detect damage, locate its position and monitor the increase in damage using only the measured data without the need for any modal identification or numerical models. The method is applied to the experimental data extracted from a steel bridge after inducing some defects to its members. The release of some bolts from one stiffener located on the web of the main girder of the bridge introduced the damage to the bridge. A future goal of a comprehensive bridge management system is to have a self-monitoring bridge where sensors feed measured responses (accelerations, strains, etc.) into a local computer. This computer would in turn apply a damage identification algorithm to this data to determine if the bridge has significantly deteriorated to the point where user safety maybe jeopardized. In such case, the dynamic properties of the structure have to be identified from ambient, traffic-induced vibration. The damage identification method presented in this study requires the excitation forces used for the undamaged and damaged structures to have the same waveform, amplitude and location. Therefore, ambient vibration cannot be used as excitation source in this case. In order to find an excitation source that can be used for continuous health monitoring of the bridges (without interrupting the traffic) and at the same time can provide equal forces, the implementation of piezoelectric actuators as a local excitation source for large structural elements is presented. The advantages of using piezoelectric actuators than using shakers, hammers or ambient vibrations are discussed.
