OIDA White Paper: Bridge and Tunnel Structural Health Monitoring with Fiber Optic Sensors

December 25, 2007

On August 1, 2007, the collapse of the I-35 bridge in Minneapolis reminded us all of the consequences of catastrophic structural failure. As a nation, we need to commit resources to assure the safety of thousands of bridges and tunnels throughout the United States. Technologies are now available that can greatly enhance the ability to monitor changes in structures, indicate necessary maintenance, recommend load changes, and predict failure.

The Interstate Highway System, authorized by the Federal-Aid Highway Act of 1956, encompasses more than 47,000 miles of highway. The system is owned and maintained by the states with significant financial assistance from the Federal government. Some parts of the system are now 50 years old and built to standards, and with materials, which are now outdated. State and local highways are often even older, and built to an even greater variety of standards. Bridges are no exception: the Minneapolis bridge that collapsed was opened in 1967 and did not include the redundant truss structures commonly required in modern bridges.

With an aging infrastructure, constant vigilance and real time monitoring are critical to ensuring public safety. Operating loads, speeds, periodic environmental “shocks,” and stress levels on these structures all create changes from the original as-built characteristics. Inspection protocols are intended to account for these factors, but nothing short of proactive, real time monitoring can provide a true warning system. Of course, such systems didn’t exist when nearly all our infrastructure was built—but they do today.

Current maintenance regimes for bridges use two approaches: periodic visual inspection by trained inspectors, and occasional deep inspection using x-ray, infrared, or ultrasonic techniques. Neither system is real time and both systems require expensive equipment and manned truck rolls. Therefore, the inspection regimes are necessarily limited by cost. Since these inspection protocols are based on the judgment of highly trained inspectors, there is still room for varying interpretations. In addition, a detailed Federal Highway Administration (FHWA) study in 2001 concluded that “in-depth inspections are unlikely to correctly identify many of the specific types of defects for which this type of inspection is frequently prescribed.” The study reported that when inspectors checked welding points, for instance, less than 4% of them correctly identified the presence of cracks.[ ]

Fiber optic-based structural health monitoring offers a substantially improved regime for monitoring bridge and tunnel safety, especially structures with known structural deficits like I- 35W. By placing an optical fiber with special sensing elements built into the glass along or within a structure, an immediate readout of stress and positional changes is available with relatively inexpensive equipment. Fiber optic sensors of this type can be extremely sensitive, detecting strain shifts that would occur, for instance, in cracked welds, as well as providing other useful data such as wind and traffic loading. In addition, continuous monitoring with fiber optic sensors allows a new capability, which far exceeds current ability to detect structure degradation. Dynamic bridge testing, where the bridge is ‘pinged’ with a mechanical jolt and the structure is continuously scanned for responses, can indicate hidden cracks that don’t show positional changes until they fail.

Fiber optic sensing devices are designed so that problems can be located quickly, authorities can be alerted proactively, and traffic load can be altered to avoid a failure. The fault can then be repaired or, if necessary, the structure replaced. Moreover, this equipment can be used both periodically during an inspection (like dynamic bridge tests) and continuously in real time, offering a historical and current view of bridge or tunnel conditions. Best of all, fiber optic-based sensing systems are not just for new structures; existing structures can be retrofitted with these sensors, while repair or replacement requirements are determined. Future structures can have these sensors embedded for maximum performance.

A large number of North American companies are pioneers in ‘smart structures’ technology. These include LxSix, which sells devices that integrate into an optical fiber and can detect minute changes in strain, length, and temperature. Micron Optics sells complete structural health monitoring systems, and Optiphase, makes systems that scan fiber sensors to determine their current status. Some other companies making components or systems include JDSU, Oz Optics, FISO, Redfern Integrated Optics, and Integrated Optical Systems.

Installing a typical real-time fiber optic system can cost from $50,000 to $300,000. This compares favorably to the $100 million plus cost for replacement of major bridges and an average of 10% of that for major repairs. If repair or replacement can be delayed by even one year, the interest charges alone will pay for the system. Furthermore, it is likely that fully-instrumented bridges will have reduced insurance and maintenance costs. Lastly, a bridge failure imposes huge economic costs; the failure of the 35W bridge is estimated to cost $400,000 per day. While these economic arguments are extremely compelling, they pale in comparison to the human costs of bridge failure.

With nearly 600,000 bridges in the national inventory and over 70,000 deemed ‘structurally deficient’ [ ], like the 35W bridge, the U.S. has the monumental task of monitoring and managing repair, replacement, and traffic flow. It is logistically impossible that the frequent inspection, repair, and upgrades of these bridges can happen immediately even with government capital. However, knowledgeable and trained personnel can be focused in their monitoring, management, and repair efforts when aided by modern, practical, real-time systems.

There is no absolute way to prevent tragedies like 35W, but as we rush to react to the disaster, consideration of technologies and techniques that provide a cost effective method to greatly reduce risk and the ultimate cost of such events should be considered. The cost of retrofitting or embedding these sensing technologies into bridges is substantially less than the cost of catastrophic failures. As a complement to traditional inspection techniques, fiber optic sensing provides a real-time, continuous monitoring capability and accuracy that cannot be matched.

Fiber optic sensing technology is proven and commercially available today from many vendors. It is being deployed in China, Japan, and Europe. In the U.S., there have been deployments in Manhattan, Oregon, and New Mexico, but it is still on a limited basis. Let us give our states and cities these tools. Now, with a well-known need to cost-effectively monitor and systematically address our lagging infrastructure assets, the U.S. should not be on the trailing edge in applying these capabilities. We lead the world in developing and producing these technologies—let’s use them for our own benefit.

 

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[1] Phares, Brent M., Rolander, Dennis D., Graybeal, Benjamin A., and Washer, Glenn A., US-DOT, Federal Highway Administration, http://www.tfhrc.gov/pubrds/marapr01/bridge.htm, March/April 2001.

 

[2] National Bridge Inventory, US-DOT, Federal Highway Administration, http://www.fhwa.dot.gov/bridge/fc.htm, updated 2006