Structural Vibration Diagnosis of Scour Damage in Coastal Bridges

This dissertation presents a research program to continue the development of a vibration based damage detection technique to improve its practicality in identifying the existence and location of scour of coastal bridges, and to predict the effective height of supporting piles based on this selected damage feature of the superstructure. An analysis of recent bridge failures in the United States showed that the leading cause of bridge failures are due to hydraulic damage. In coastal regions, scour is a serious type of hydraulic damage resulting from a coastal storms. Scour occurs when strong water current removes compacted sediment around bridge pilings. Scour damage can be very difficult to detect since it is not always visually observable. Monitoring methods that involve underwater instrumentations are difficult for operators to perform and may result in damage or loss of equipment. Vibration based damage detection (VBDD) techniques have been developed to successfully detect scour damage in the laboratory and have also been implemented on an in-service bridge. The VBDD method based on the change in flexibility deflection of the superstructure has shown success in identifying the existence and location of scour damage, and has the potential to predict the effective pile height. But more research was desired to evaluate the practicality of VBDD techniques on more typical coastal bridge, such as bridges with multiple spans, or with several possible scour locations. The most crucial research of VBDD scour diagnosis that needed to be accomplished is the prediction of scour extent, or the effective pile height.;The first part of this dissertation focuses on the evaluation of the accuracy and practicality of the implementation of the selected VBDD technique on a chosen multi span coastal structure. In order to apply this technique, the dynamic characteristics must be obtained from healthy and scoured bridge condition. Acceleration and impact loading during the vibration testing were collected in the field monitoring and signal processing was applied to determine the dynamic characteristics. The desired dynamic characteristics include the natural frequencies, the horizontal mode shapes, and the flexibility deflection of the superstructure. In this study, two similar portions of a coastal structure, except for the pile heights, are used to represent the healthy and scoured conditions. Thus, the problem becomes a comparison of the flexibility deflection from two similar structures with different pile heights. The results have shown that this VBDD technique can identify the existence of scour around the piles. Impact locations were also found to have an influence on the extracted horizontal mode shapes. The fundamental mode is also found to be the most important mode to be considered in the calculation of flexibility deflection.;The second part developed a mathematical relationship between the flexibility deflection and effective pile height. Stiffness ratio of the substructure to superstructure was found to be the primary parameter determining the dynamic behavior of a bridge superstructure. Numerous FE modal analyses were performed in order to establish the mathematical relationship. The connection is considered as part of the substructure that provides the horizontal resistance to the vibration of superstructure. Thus, the development of the mathematical relationship is first established for the case of a rigid superstructure-tosubstructure connection. Then this relationship is adjusted to allow for different levels of connection stiffness. Lastly, a practical solution is proposed to be used in the field to predict the effective pile height. Through validation against finite element analysis, the proposed mathematical relationship shows reasonable prediction of effective pile height.