Bridge Safety

With the help of sensitive microphones, scientists are inspecting bridges for cracks and structural flaws by listening to them. I'm Jim Metzner and this is the Pulse of the Planet.

"Bridge inspection has traditionally been done by human visual inspection. That type of inspection, while it can be fairly effective with a highly trained, dedicated inspector, leaves a lot to be desired."

David Prine is a research scientist with the Birl Industrial Research Laboratory at Northwestern University.

"There's a lot of problems in bridges that you can't see, so a visual inspection won't really find them, or if it does find them it doesn't find them until they progress to the point where the damage has become very severe, and now we're looking at instead of minor remedial repairs, major structural damage has been done so we've got to do big time repairs. So what we're doing here at Northwestern is using some of the new technology that's being developed for non-destructive testing, non-destructive evaluation. One of the tools we're applying to bridges, in this case steel bridges, is acoustic emission monitoring."

Acoustic emission monitoring means listening for high frequency sounds which indicate the bridge structure is under stress.

"When a piece of steel cracks high frequency sound is emitted. If we have the proper sensors attached to the piece of steel we can pick up those sound waves."

"Most of these cracks, though, are benign. They're not being driven by the live loading on the bridge, so they're not likely to propogate and cause any harm. However, there are cracks that do grow under the live traffic loading. And those are the ones we want to catch, those are the ones you have to spend the money on to fix. And one of the very powerful things you can do with acoustic emission is determine whether or not a crack is in that category."

Acoustic Emission Monitoring of North East Trunnion Shaft on Oregon DOT Bridge 1377A I-5 over the Columbia River, Portland, Oregon by David W. Prine and Jerome E. Oleksy, Northwestern University, BIRL Industrial Research Laboratory, May 1996.

Acoustic Emission Monitoring of the Trunnion Shafts on Oregon DOT Bridge # 1377A, I-5 (Interstate) Columbia River Bridge East Lift Span, Portland, Oregon by David W. Prine, Northwestern University, BIRL Industrial Research Laboratory, Project G-101, November 30, 1994.