Instrumentation, Digital Image Correlation, and Modeling to Monitor Bridge Behavior and Condition Transportation agencies are increasingly focused on asset management. Bridges are quite expensive to replace so bridge managers increasinly focus on the life span of these assets. Bridge Managers have historically relied on visual inspections to determine bridge health. Structural Health Monitoring includes instrumentation and a structural model to provide additional information for bridge management. Prepared by the University of New Hampshire Department of Civil Engineering for the New Hampshire Department of Transportation

How to you measure the structural health of a bridge? Strain gauges attached to the bridge steel Good data Attach to bare metal Must remove protective coating You damaged my bridge paint But what sort of instruments are used to measure structural health? Strain gauges provide good data, but they must be attached to bare metal. That usually means the protective coatings, such as paint, should be removed. Damaging the bridge coating tends to irritate bridge managers because that means additional maintenance to avoid corrosion at those locations. Bridge Maintenance Engineer

How to you measure the structural health of a bridge? Digital cameras track pixel changes in images of chalk on the girders Good data Clamp on - easily deployed and removed Chalk wipes off No damage to my bridge paint This study, led by the Professor Erin Bell at the University of New Hampshire, looked at a different way to measure the response of the structure. Digital cameras were clamped onto the bridge beams and the beam was chalked to create some contrast. This method does not damage the bridge – remove the clamps and wipe off the chalk and Voila! No damage! The camera images are processed to measure changes in the pixels of adjacent images. Very small movements can be detected. Bridge Maintenance Engineer

Does it work? Some data processing is needed to get good correlation with the strain gauges. In this case the sudden jolt from dropping the blocks affected the system. It may not be possible to completely eliminate the use of strain gauges but the cameras can augment a smaller deployment of strain gauges.

Comparing Strain Gauge to Digital Image Correlation The processed data from the images correlates to the strain measurement collected with the strain gauges. The results of strain gauges and Digital Image Correlation were compared and found to be well correlated. This graph shows the strain gauge data in blue and the Digital Image Correlation in red; the two data sources provide very similar results.   Digital cameras are reusable. Many strain gauges are destroyed when they are removed.

What’s next for Digital Image Correlation? Will be used as part of a larger gauging plan on Memorial Bridge in Portsmouth, NH One method in a detailed lab study on the gusset-less connections of Memorial Bridge Perhaps coming to a bridge near you Digital Image Correlation will be used as an additional tool to measure structural response on several projects   It will be part of a larger gauging plan on the Memorial Bridge study currently underway. This truss lift bridge is under study by a team from the University of New Hampshire. In addition, the Memorial Bridge includes a newer form of gusset-less truss connection. The NH DOT is funding a new detailed laboratory study to understand how these connections behave and Digital Image Correlation will be one of many methods used. So if your bridge managers are looking for more ways to measure structural response and assess the health of bridges, perhaps this would work for them.

For more information on Digital Information Beth Klemann, PE Asst. Research Engineer NHDOT eklemann@dot.state.nh.us Prof. Erin Santini Bell,PE, PhD University of New Hampshire Department of Civil and Environmental Engineering erin.bell@unh.edu http://www.nh.gov/dot/org/projectdevelopment/ materials/research/projects/15680l.htm