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Sensing & Measurement

Sensor instrumentation to monitor bridges

Permanent sensor infrastructure can provide smart capabilities and help monitor aging.
14 August 2007, SPIE Newsroom. DOI: 10.1117/2.1200708.0826

Structural sensor systems can be implemented through the integration of sensor technologies, electronics, wireless nodes, computational techniques, and structural analysis. As permanent elements, these systems may provide benefits that include automation of measurements, access to internal conditions, and distributed data for analysis, as well as related improvements in performance, management, and safety. Sensor systems may provide data for off-site analysis or may be ‘smart’ with in-situ processors intelligently interpreting the information.1 Progress toward instrumented or smart structures involves integrating the component technologies, matching system capabilities to needs, developing field protocols, and validating performance.

Bridges are key components in our transportation infrastructure. They represent an enormous investment with ongoing management activities in terms of inspection, maintenance, repair, upgrade, replacement, and the environment. Many bridges are near or past their intended service life and are carrying unanticipated traffic loads. Infrastructure managers must also prepare for damage due to vehicular accidents, earthquakes, and terrorism. Significant health-related monitoring verifies whether performance meets design expectations, characterizes the extent and significance of damage, assesses strength after service or changes in environment, and determines remaining service life.

Figure 1. Compressive strain at the mid-span surface during dynamic loading of the University of Missouri-Rolla all-fiber-reinforced-polymer bridge. The load vehicle had a front-axle weight of 12.0kN with respect to the 142.4kN rating.
Fiber optic sensors

Accurate assessment of structural health may use load-induced strain or displacement.2 Sensor networks can generate quantitative information at selected locations and can speed the introduction of new structural technologies. A quantitative approach, assuming monitoring is economical and timely, can reduce reliance on such strictures as overly conservative design, qualitative inspections, and risk-intolerant maintenance,. If monitoring can assure safety, implementation of new structural technologies, such as fiber-reinforced-polymer (FRP) materials,3 can be encouraged. Two important types of embeddable fiber optic strain sensors are Bragg and Fabry-Perot.4,5 They are suited for permanent applications due to excellent environmental ruggedness, low profiles, and high sensitivity. In addition, they can provide measurements even during failure events.6

We have investigated the long-term field use of fiber optic strain sensors in bridges.7 Host materials include concrete, rebar, and FRP. Preliminary installation and testing protocols have been performed in the laboratory and on field structures loaded to failure. Long-term implementation on bridges have explored installation protocols to promote sensor survivability (and protection from tampering), selection of sensor locations for measurement, placement of cabling and access points, intelligent processing approaches, and adjustments for temperature. Field bridges include retrofitted reinforced-concrete structures and new FRP construction.

Instrumented bridges

In 2003, we instrumented an aging reinforced concrete bridge that was upgraded with FRP wrap and rebar.8 This in-service bridge is part of the Missouri Department of Transportation system and, after upgrade, was load-tested in 2004 and 2005. The fiber optic network, consisting of Extrinsic Fabry-Perot interferometric (EFPI) sensors, monitored both static and dynamic load-induced strain. Good agreement with design expectations, finite-element computations, and co-located sensors was demonstrated. A health coefficient parameter was proposed as a single measure of load performance, but would require further development related to loading conditions and aging. The dynamic strain signatures may possess more detailed information on the bridge condition and will be the subject of future investigations.

We also instrumented an all-FRP bridge during its construction with an EFPI network.3,7 This on-campus bridge, rated for highway loads, is a field laboratory at the University of Missouri-Rolla (UMR). A full-scale quarter portion was laboratory tested for design loading, fatigue loading (two millions cycles), and failure loading.9 The EFPI sensors provided reliable measurements and survived all tests. Field results on the bridge show excellent correlation with theoretical calculations and companion gauge measurements. The sensors show high sensitivity and noise-free performance. A light truck load at 8.4% for the front axle, with respect to the design load rating, produced a peak compression 8 microstrain. More comprehensive results are in preparation.

Smart systems

Permanent sensor instrumentation can improve bridge management. Fiber optic strain sensors are suited to field environments and provide long-term monitoring even during damage events. Field work demonstrates protocols for installation and testing. Such cooperative investigations with working engineers and managers promote the use of new technologies and capabilities. Ongoing developments are integrating wireless capabilities with the sensor nodes for data acquisition and for intelligent demodulation and processing through dedicated on-site electronics. In sum, smart sensor systems can reduce the management costs, optimize performance, and assure safety.

The work was partially supported by the National Science Foundation and the Missouri Department of Transportation.

Steve E. Watkins
Electrical and Computer Engineering
University of Missouri-Rolla
Rolla, MO

Steve E. Watkins directs the Applied Optics Laboratory and is a professor of Electrical and Computer Engineering at the University of Missouri-Rolla. Since receiving his PhD degree from the University of Texas at Austin in 1989, he has investigated applications of fiber optic sensors and related instrumentation.