RSS Feeds
Structurally Deficient Bridges in the United States
Though the conditions that triggered the catastrophic collapse of I-35W are not yet known, speculation surrounding the rest of the country’s bridges has begun
Sean Snyder, Associate Editor -- Design News, August 3, 2007
Check in with our I-35W bridge collapse coverage page for the latest news, videos and photos covering the failure.
In the wake of the I-35W bridge collapse, individual states all across the country are re-evaluating their bridges. A number of recently published findings on bridge and transportation safety offer information on the evaluation process for bridges.
Approximately 12.4 percent of all bridges rated by the Federal Highway Administration have been declared structurally deficient. What exactly does this mean? “It doesn’t mean you have cancer or anything like that, it could mean you have acne,” says Sam Schwarts, former chief engineer for NYC bridges. “It means that you have a condition that has to be paid attention to, it doesn’t mean that the bridge has to be shut, but it could in the very worst case mean that you have terminal cancer; structurally deficient is a very broad term, and there are some bridges that need to be in the ICU.”
In Massachusetts, two days before the collapse of the Minneapolis bridge, the Pioneer Institute released their findings regarding all of the commonwealth’s assets including all of its bridges. Based on a November 2006 report they noted 501 structurally deficient bridges in need of repair and maintenance. According to David Westerling of the Pioneer Institute, “each inspection report evaluates different components of the bridge … and each component of each category is then rated from zero to nine; zero means that it has failed, one means imminent failure and nine is excellent.”
“If some components are given a one, in most cases they would close the bridge,” Westerling says. “It depends on the component too. If curbing on the sidewalk is in imminent failure, they’re not likely to close the bridge, but if it has a main structural member that is a two or a three, they may close the bridge.”
According to many analysts and engineers, bridge maintenance appears to be one of the most important methods for preventing a collapse such as that in Minneapolis, but other options including fatigue sensors and strain gages. Companies like MATECH have been working on fatigue sensors for awhile now that can detect micro-cracks in surfaces of bridges and other metal structures.
MATECH’s electrochemical fatigue sensors (EFS) monitor the current flow at the surface of a metal while it is being mechanically strained using an electrical test method, which the company claims operates similar to an EKG machine. The EFS can detect minute cracks “at a resolution of a few microns,” according to the MATECH website.
When Shultz was asked which types of bridges were better, he made no indication to design type, but design time period. “The problem was after WWII and we began using computers to do some analysis, we were able to make sleeker bridges, use less material, have a lower safety factor and we thought as a profession that we were just being more efficient,” he says. However, Shultz says, the reality was “we were just ignoring the fact that bridges may not get treated well over the years or that they can suffer from fatigue, and then if you don’t have redundancy and you do have a crack, you have catastrophic failure.”
“Your biggest concern is generally in the north states where salt is being used; that has such a corrosive effect on bridges,” says Shultz. “I would say the northern states more than the southern states, but there is salt in the air in Florida and places like that, that are near oceans.”
Talkback
DN's Resource Center Get Free Information, Made Easy
-
2008 Adhesives Sourcebook - Loctite Products for Design, Assembly, Manufacturing and Maintenance
Henkel Corporation | Design Guide
Request Item -
Solutions for the Appliance Industry
Henkel Corporation | Design Guide
Request Item
