Vibration Based Damage Detection of Scour in Coastal Bridges.

Abstract

INTRODUCTIONIn its latest report, the National Bridge Inventory revealed that 603,168 bridges currently exist in the United States.1 The Federal Highway Administration (FHWA) rated 25 percent of these bridges as deficient. The American Association of State Highway and Transportation Officials (AASHTO) announced in their 2009 Bottom Line Report that 50 percent of US bridges are more than forty years old without sufficient information on their current condition.2 And although these represent 50 percent of the bridges, they are responsible for 80 percent of structural deficiencies and about 75 percent of total deficiencies.Wardhana and Hadipriono collected 503 cases of bridge failures that occurred from 1989 to 2000 in the United States. They concluded that the dominant types of failed bridges are steel beam/girder and steel truss bridges, which represent 50 percent of the total bridge failures. The next significant cases involve failures of concrete beam/girder and concrete slab bridges, representing 11 percent. It should be highlighted that the leading cause of bridge failures are flood/scour (48 percent).3Further, the visual inspection of bridges regulated by the FHWA has limited capabilities in detecting all possible damages in bridges and there is the potential to miss damage that appears between inspection intervals. Based on the current condition of bridges, the need for global damage detection methods that can be applied to complex structures has led to the development of methods that examine changes in the vibration characteristics of the structure.DiscussionThe primary objective of this research is to develop and evaluate a rapid vibration-based damage detection (VBDD) technique capable of evaluating the condition of a bridge following an extreme hydro-meteorological event. This technique relies on the effect of scour at submerged piers on the response of the bridge superstructure without any underwater instrumentation.Research SignificanceExtreme natural hazards, particularly hydro-meteorological disasters, are emerging as a cause of major concern in coastal regions. From a structural point of view, hydraulic loading due to flood and scour is responsible for about 50 percent of the failed bridges in the United States.4 Scour failures tend to occur suddenly and without prior warning or signs of distress to the structure. The nature of the failure may lead to collapse of the bridge. During floods, the flowing water tends to excavate pits in front of the submerged bridge piers. As the velocity of floodwater decreases, the suspended sediments precipitate and fill the excavated pits. Consequently, following inspections and measures cannot furnish indications on the maximum depth reached by erosion during flood.5 Moreover, the suspended sediments that fill the excavated pits do not provide good confinement for the pile since they are not as compacted as the rest of the soil. Therefore, rapidly assessing the condition of bridges after an event will inform the decision making process in the context of assessing evacuation and first responders' routes. It should be mentioned that, after meeting with NCDOT Division 3 maintenance engineers, the most reliable way to measure the level of scour is by underwater investigation using divers. Further, underwater investigations are performed once every four years, or when a bridge is overtopped.Research MethodologyVBDD techniques have shown success in detecting several types of damage, such as fatigue cracking in steel beams. Since the primary objective of this research is to develop a damage detection framework capable of rapidly detecting critical damage of the superstructure as well as scour, VBDD techniques were selected to study their sensitivity for detecting scour. For this investigation, an idealized structure representing a two-span continuous bridge was selected. …