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.
In the above post, second to last paragraph:
“...with the south end of pier six on top of them...”
should read:
“...with the south end of SPAN six on top of them...”
Sorry for any confusion.
I wonder if its remotely possible to collapse a bridge by deliberately driving obscenely overloaded big rigs across it, I’m thinking of terrorists bypassing scales and hauling a load of heavy material like cast concrete to the point of vehicle axle/drivetrain failure. Instead of an 18 wheeler having a gross weight of under 80,000 lbs what if it was well over 250,000? Please do not nitpick about flat tires or broken axles as I have been a diesel mechanic for 35 years and I know it can be done. I have a three axle tractor with a two axle 24 cubic yard end dump trailer and it routinely hauls over 90,000 lbs.
Jeffers,
Thanks so much for taking the time to do a lengthy, technical post on the bridge collapse. Freepers like you make the Free Republic the outstanding web site that it is.
Ronzo
Resurfacing work was taking place, but the bridge was last inspected in 2006 and no significant structural problems were found.
...
In 2005, it was one of thousands across the US rated as "structurally deficient" on the federal National Bridge Inventory database.
Do these two descriptions seem at odds to anyone else? The best I can make of this is that none of the structural members were found to have problems. That is, no obvious cracks, corrosion, or loose joints. Hence the no problems call.
However, the design itself had little or no margin of safety - it depended on all members of the structure. Should any one fail, there was little or no redundancy in the design to take up the load. Hence the deficient rating.
Still, seems at odds. The gray area stuff that personal injury lawyers make hay with...
Was there ever a final conclusion ?
I didn’t see any news lately...
Not that I heard. I wouldn’t expect one for several months. Probably take weeks to go over the debris, run some computer simulations, etc.
i appreciate the compliment, what i don’t appreciate is that you stole my photographs. please remove them from this and any other site. immediately. i will pursue a legal remedy if this isn’t taken care of by you voluntarily.
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