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Infrastructure Technology Institute

Tradtional Inspection Techniques: The Foundation of Structural Health Monitoring

An under-bridge inspection vehicle, also known as a “snooper,” has an articulated arm to provide inspectors with close-up access to every part of a bridge.

The current condition of our surface transportation infrastructure - and particular concerns for the condition and safety of bridges - have sparked much discussion about structural health monitoring (SHM), including issues of methods, new technologies, and deployment strategies. This focus on monitoring and preserving the nation’s bridges comes at a time when the capabilities of both sensor and communication technologies are growing rapidly. New SHM technologies are being developed and tested; predictions suggest that large-scale and wide spread use of automated technologies will provide managers and policy makers information that is more timely, more accurate, and less costly.

However, the nature of bridges and their problems and risks suggest that SHM technologies will always need to be deployed in concert with more traditional, hands-on inspection techniques. In the following article, ITI Research Engineer David Kosnik addresses this issue of balance in the bridge monitoring process:

Recent structural failures have highlighted the importance of transportation infrastructure and the consequences of its deterioration. As researchers continue to advance the practice of structural health monitoring (SHM), especially for highway bridges, it is important to understand the role of the spectrum of inspection techniques and to deploy them in a balanced and informed way.

No element of surface transportation infrastructure warrants safety and performance concerns more than highway bridges. Since the beginning of the federal bridge inspection program in 1968, visual inspection has been the primary method for assessing the performance and serviceability of bridges. By federal law, all structures which carry traffic and have a span greater than 20 feet are subject to comprehensive inspection at least every two years. In a routine biennial inspection, trained bridge inspectors check for obvious damage, which may take such varied forms as spalled concrete, corroded steel, and even insect and fungus attack on timber elements; they also examine bearings, deck drains, and expansion joints for proper operation, evaluate the serviceability of bridge substructures, decks, approaches, and appurtenances, and, for waterway crossings, inspect the channel for scour and obstructions to flow.

All of these elements are inspected visually, using basic hand tools where appropriate. The goal of these federally-mandated inspections is to assess and document the condition of essential bridge elements to ensure safety and serviceability and to facilitate the timely programming of maintenance and repairs. This is the essence of structural health monitoring.

Some bridges are also subject to special inspections, in-depth evaluations of the safety and serviceability of particular elements known to have specific problems or present particular risks. For example, special inspections are conducted on fracture-critical bridges: those with non-redundant steel tension components, the fracture of which would likely cause catastrophic failure.

Fracture-critical members (FCMs) are subject to special inspections at least every two years. Bridges containing FCMs of particular concern, such as those with fatigue-prone details, are typically inspected more frequently. FCM inspections often employ special non-destructive testing techniques to detect cracks. Techniques used on accessible surfaces include dye penetrant and magnetic particle testing, which make tiny cracks more visible through the use of brightly-colored dyes and patterns of iron particles in a magnetic field, respectively. Some FCMs have details in which only one side of an element is accessible. In these cases, non-visual methods must be used. A common option is ultrasonic testing, in which a highly-trained, certified operator interprets the reflection of high-frequency sound waves projected into the element. Both the visual inspection and instrument-based non-destructive testing methods require up-close access to the elements in question, and thus, FCM inspections are often called “arm’s length” inspections.

Instrument-based assessment methods can extend the range and depth of hands-on inspection to answer very specific questions about bridge element condition. The most common instrument-based methods deployed on bridges include well-known non-destructive evaluation (NDE) techniques such as ultrasonic testing, eddy-current testing, in which electric signals are used to detect flaws, and radiography, in which high-energy X- or gamma rays create an image of the internal structure of an element on film. These techniques are employed on specific areas of a structure where cracking is suspected. This is done for economy, but also because it is simply not practical to conduct these tests on all parts of a bridge. Therefore, thorough knowledge of the theory and practice of bridge design and maintenance is critical to the effective deployment of NDE and SHM technologies on bridges – engineers must know where to look for trouble.

New sensing techniques suitable for bridges are developing rapidly. From new sensors based on nanotechnology to self-assembling wireless sensor networks, a wide variety of options for sensor-based monitoring will be available to bridge engineers in the near future. In particular, wireless sensor networks promise to enable more widely distributed sensing throughout a structure, increasing the number of sites where measurements may be taken. Still, visual inspection will remain the foundation of structural health monitoring. Visual inspection provides a synoptic view of bridge condition. Inspectors are able to identify a wide range of threats to bridge safety, including damage in unanticipated areas and issues not intrinsic to the structure, such as stream channel changes; furthermore, inspectors can identify details where the application of advanced inspection and testing techniques might be particularly instructive. Comprehensive, integrative assessments by trained bridge inspectors will continue to provide most critical safety and serviceability data for the nation’s highway bridges. Continuous sensor-based structural health monitoring should supplement inspection data by measuring quantities that cannot be otherwise observed and by gauging a structure’s performance over time and between inspections.

No single method or technology can provide the condition information needed to ensure the safety and serviceability of the nation’s bridges. The careful integration of sensing technology with traditional inspection techniques, however, will provide the data that engineers and policymakers need to manage the structures that serve as the connective tissue of
our society.