Basic Failure
Federal Highway Administration is rushing to get states to look at a basic component of bridges that is routinely overlooked in calculating bridge loads. This in the wake of the announcement that the Minneapolis bridge collapse was a result of a failure of these basic components called gusset plates. This is a serious issue and could potentially cause many bridges to fail if the gusset plates are not thick enough.
A weakness built into that bridge went undetected for 40 years because it involved a part so basic that highway departments and bridge contractors seldom considered it even when they reanalyzed a bridge’s capacity, experts said Tuesday.
As the National Transportation Safety Board announced its findings in the bridge collapse, the highway agency recommended an analysis of the flawed parts, known as gusset plates, every time major work was done on a bridge. There are about 12,600 bridges nationwide that could be vulnerable to the kind of design error found in Minneapolis, although the problem may not exist anywhere else.
Engineers found broken steel plates in 8 of 122 spots where they held together steel beams in the latticework of the 1,000-foot span that collapsed, each plate half an inch thick when they should have been an inch thick, said Mark Rosenker, the chairman of the transportation safety board. The plates are customarily the strongest part in a steel truss.
“In the history of this organization, we have not seen anything like this before,” said Mr. Rosenker, briefing reporters on Tuesday on the fatal flaw in the bridge’s design.
In Minnesota, Gov. Tim Pawlenty said state workers would immediately recalculate the maximum loads on the 23 highway bridges in the state that shared design similarities with the bridge that collapsed. Workers had begun such reviews on 7 of those bridges even before the board made its recommendation, said Mr. Pawlenty, a Republican. The reviews will cost about $500,000, he said, and will probably take until June to complete.
Although the problem may not be widespread - or even exist in any other bridge at all - it needs to be looked into. In the type of bridge design that was used in Minneapolis, the failure of a single gusset plate could cause the collapse of the entire structure. Investigators found eight of them broken out of the 122 used. Inspections have already begun on the most critical bridges with the highest potential for problems. Unfortunately, replacing these plates is a very challenging operation - simply because they are so basic to the integrity of the structures.
By Lars Walker, Wednesday, 16 January , 2008 @ 9:29 am
One irony here is that right after the collapse, state Democrats were quick to jump out and blame Republican tax cuts and supposed reduced maintenance for the disaster. Turns out the real cause is government cutting corners back in the “good old days” when Democrats were in control. Oops.
By Sam, Wednesday, 16 January , 2008 @ 11:42 am
I doubt that cost cutting had anything to do with it, or which party was in control of the White House or Congress, or Minnesota government for that matter. Bridge design techniques are pretty standardized and likely don’t change much from year to year. They are more likely to change with changes in technology (computerized design) or materials (high strength steel and concrete) or with changes in design philosophies (truss bridges out of style, suspension bridges in style).
I work for an gov’t electric utility that builds electric transmission lines, and the design of the lattice steel tower we use is basically the same as the design used in the 1960s. The conductors are newer types, the insulators are composite rather than ceramic, and there may be a fiber optic cable in the groundwire, but the tower design is basically the same. The designs are not influenced by the cost of steel or who is in the White House or Congress. Neither is the frequency and type of maintenance done on them.
Having said all that, claims that the bridge collapse was due to tax cuts or some malfeasance of a particular administration is stupid on the face of it, and just partisan mudslinging. I try to ignore such claims, and the people claiming them because I know the statements are made from ignorance and spite, and mostly reflect poorly on the person(s) making them.
By feeblemind, Wednesday, 16 January , 2008 @ 3:12 pm
Sam: Interesting comments on the transmission lines. I have an uncle that has put up those towers for 40 yrs.
By Mwalimu Daudi, Wednesday, 16 January , 2008 @ 4:53 pm
You mean that having US troops in Iraq had nothing to do with the collapse?