This sensitivity lends itself to both medical and any non-destructive testing application. This includes inspection of bridge decks and bridge structures. After the fatal collapse of the bridge in Minneapolis our attention as U.S. citizens was focused on this event, but as usual, only for a short time. We tend to forget about events like these: Hurricane Katrina and Rita for example. I bring up Rita because it devastated my home town of Beaumont, Texas. Bridges are critical to our every day lives, but they are costly to repair, maintain, or replace. It is discouraging for our U.S. bridge inspectors to note the same discontinuities year after year and see nothing done about them. It is a shame, but it sometimes takes a catastrophic event to really get our attention.
My wife’s business www.tastefullysimple.com is based in Alexandria, Minnesota. Coming from Texas I never realized people in Minnesota actually doive trucks and cars on their lakes in the winter. They tow their fish houses and set them on the ice. Temperatures can reach -40 F or -40 C which is the only point at which these two temperature scales are the same.
I bring this up because it applies to the topic, I am looking out of my office window at a lake that’s still 90% frozen and its April 29th. What these cold temperatures mean for bridge structures is a huge coefficient for thermal expansion when summer comes around.
AWS D1.1 Structural Bridge code and ASME codes reference -20 C as a critical temperature for brittle fracture for many steels. In Minneapolis and much of the Northern U. S., these temperatures are a yearly reality. In the summer, temperatures rise to over 100 F (38 C). This means the molecular structures are shrinking in the winter and expanding in the summer. It is the molecular vibration of these molecules that cause all materials (everything) above -273 C and -459 F (absolute zero) to emit infrared radiation. Our cameras see this emitted infrared radiation and the better the sensitivity and image quality, the more likely they are to find a potential crack in a concrete or steel bridge structure or a delamination in a concrete bridge deck. We have published a paper that will be in the June issue of the “Concrete Today” magazine. This article describes inspections that companies like “EarthTech” and guys like Tim Crowley do on a regular basis. I believe we need to recognize guys like these and inspectors working for our U.S. Department of Transportation who are behind the scenes making our bridges safer. The following is an excerpt taken from an article from CNN after the Minnesota bridge collapse.
For more on this article, see this link: http://edition.cnn.com/2007/US/08/02/bridge.structure/index.html“As of 2003, there were about 160,570 bridges deemed structurally deficient or functionally obsolete, according to the American Society of Civil Engineers. The number represented 27.1 percent of the nation's bridges.
The American Society of Civil Engineers also reported that the number of bridge deficiencies had steadily declined from 34.6 percent in 1992 to 27.1 percent in 2003.
Minnesota Department of Transportation bridge engineer Dan Dorgan said the term "structurally deficient" is a Federal Highway Administration rating.
Inspectors rate sections of the bridge on a 1 to 9 scale, with 9 being in excellent condition, he said.
"A structurally deficient condition is a bridge that would have a rating of 4 either in the deck, the superstructure or the substructure," he said. "Any one of those in condition 4 or less is considered structurally deficient."
But, he noted, out of 13,000 state and local bridges in Minnesota that are 20 feet and more in span, 1,160 of them -- 8 percent of the state's bridges -- are considered structurally deficient.
Tom Everett of the Federal Highway Administration's National Bridge Inspection Program said the structurally deficient rating was a "programatic classification rather than an indication of safety."
"It does not indicate a bridge is dangerous or that that bridge must be replaced," he said”.
Gary Strahan
Infrared Cameras Inc.
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