Posted on 08/03/2007 5:40:58 PM PDT by traumer
Engineers are trying to understand what caused the catastrophic collapse of the bridge over the Mississippi river in Minnesota.
Resurfacing work was taking place, but the bridge was last inspected in 2006 and no significant structural problems were found.
Such complete bridge collapses are a very rare occurrence.
If they happen, it is either because the load is too heavy, or the connections between the bridge's structural elements are too weak, Keith Eaton, chief executive of the UK's Institution of Structural Engineers, told the BBC.
"The engineers will have to see where the collapse started. Clearly a failure occurred somewhere which imbalanced the whole thing," he said.
Speculation that hot weather contributed to the accident by weakening the concrete or expanding the steel framework was not a likely explanation, he added, as modern bridges are built to cope with extremes.
A crack in the steel making up the bridge's structure was the most likely explanation for the disaster, he said.
Corrosion
The I-35W highway bridge (Bridge 9340) was built using a framework of rafters, posts and struts - a structure known as a truss bridge.
In 2005, it was one of thousands across the US rated as "structurally deficient" on the federal National Bridge Inventory database.
It rated 50 on a scale of 100 for structural stability in that study, White House press secretary Tony Snow said.
About 140,000 cars are thought to have used the bridge every day, but a 2001 report by University of Minnesota's civil engineering department found traffic levels were below those the bridge was designed for.
See graphic of the bridge collapse
The report went on to express concerns that a single crack in the main truss could "theoretically" lead to the entire bridge's collapse.
However, it also said that even if there was a crack, the load could "theoretically" be redistributed along the steel trusses or the concrete deck of the bridge, keeping the bridge aloft.
It added that no fatigue cracking had occurred, and that the bridge "should not have any problems with fatigue cracking in the foreseeable future".
File photograph of the Minnesota bridge The bridge crossed the Mississippi River near downtown Minneapolis
The state need not "prematurely replace this bridge because of fatigue cracking, avoiding the high costs associated with such a large project".
The truss bridge was built in 1967, with eight lanes over a span of 581 meters (1,900ft). It had no piers in the water, allowing easy passage for river traffic.
While no longer the cutting edge of bridge design, truss bridges are relatively cheap to build, and were a very popular structural choice in the US in the 1960s and 1970s, Mr Eaton said.
They have a downside, however.
"They are made of lots of complex pieces of metal, interconnected bolts or rivets," Mr Eaton told the BBC.
"They have little corners between two pieces of steel where water can collect and cause corrosion."
Nesting pigeons could also be an issue.
"Their droppings are very corrosive, which can be a problem," he said.
The pigeon droppings also got my attention...
Congressman Billybob
Lame excuse.
A structural engineer burst the global warming bubble on one of the interviews on FOX news a few days ago. It was her assertion that heat isn't a likely cause at all and that extreme cold would be more likely to cause damage.
One possibility that hasn't been mentioned too often is washout. They have a term for it but it escapes me.
bttt
“Speculation that hot weather contributed to the accident...”
Did Al Gore make any comments ?
None to date that make any sense.
Come to think of it...
It is clearly President Bush’s fault.
The economy is so good that more individuals are able to buy heavier and bigger cars. And on their comute back and forth to their jobs where they are so over-paid, these (over-fed and sedentary, too!) workers get stuck in rush hour traffic which, contrary to its name, leaves people stuck in their cars and on the bridge.
All the weight, people and cars, made the bridge fall down.
If the economy weren’t doing so well, that bridge would still be up. Bush’s fault.
Oh come on Brits, give it up...structural failure is the long and short of it.
I thought this bridge was built without “in the water” footing to prevent the wash away (term eludes me also) problem?
The term you’re looking for is “scouring”, and the bridge got a “more than adequate” grade last year in that dep’t
I've seen reports that the the bridge's initial designation of "structurally deficient" was the result of a 1991 inspection.
ahh...yes, that’s it.
Anyway, I'm confident they will be able to figure it out.
I wonder if the concrete footings are still in place on both sides. In the video it went straight down quickly, no bending that I could see. My theory is erosion around the base. In the before pictures it looks like there was fast moving water.
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