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 water isn’t very deep in that area, so it could have easily seen hundreds of frozen days.(brittlement?) Also, the bridge wasn’t a straight span all the way across, it had curves, or slight angles, as far as I could tell from the pictures. That may or may not have contributed in some way. I wonder how long the failure analysis will take. I’m sure that they will do a thorough job. I’m not an engineer but admire those who are.
If you look at the structure, note these vulnerable points: 1) the spindly steel structure probably meant a potential for a disastrous one-point failure, 2) the relatively exposed steel structure meant the steel on the bridge could be affected by the wide extremes of temperatures experienced in Minnesota, which can cause corrosion and metal fatigue problems and 3) the entire bridge across the Mississippi River was only held up by four relatively small concrete piers.
Expect the replacement bridge to look like a writ large version of the 10th Avenue (neé Cedar Avenue) Bridge nearby with its thick concrete structure.
Bad news is big news
Good news is bad news.
I first read that in Ben Wattenberg's book "The Good News is the Bad News is Wrong" in the mid-1980s.
Nothing has changed in 20 years.
Wrong war. Probably due to the Vietnam War. Johnson was skimping on domestic projects to fight his war. No doubt the design was the cheapest design and therefore got the contract. The Dims caused the collapse, that’s why the Left is blowing smoke to pin it on Bush. This was Johnson’s baby.
Mild steel was used on this bridge. It is a very forgiving alloy that is used virtually everywhere. High strength steels exhibit more severe cracking as a result of flexing and vibration and is not the desired alloy for bridges.
The link that failed in the 67 Ohio bridge failure was high strength steel and as a result the feds changed the specifications back to mild steel.
Did you ever attempt to build anything with the Korean nails of the 1960s? You could almost bend them with your bare hands.
I was pondering the location point where the video was taken. It definitly shows the structure falling linearly from the end behind that point and then the forward section appears to become unbalanced and fall.. Inquiring minds may figure more things out and get off jackhammers and erosion..
In the video of the collapse in progress, the span falls from the right side (in the video) first and then the left. But it falls flat, which seems to indicate that not one but two of those spindly points failed. If only one had failed, the span would have tilted first. I think it is very possible that it was sabotage. But I’m not an engineer, so I would appreciate it if some knowledgeable person could explain this.
Well, this represents a failure of government. Maintenance deferred to pay for big bloated government unions.
Of course the solution won’t be to trim government so that infrastructure refurbishment can be funded, it will be a gas tax put in a “lockbox” for bridge refurbishment and other infrastructure “improvements”.
This isn’t an engineering problem. Any bridge, left long enough, will fall down. It just takes money to keep it from falling. Governments siphon this money, and give sweetheart contracts to friendly firms that don’t do the job. That’s my prediction as to why the bridge fell down.
Cheap steel also contributed to the Twin Towers collapse on 9-11. Built around the same time. Sounds like we have a lot of ticking time bombs out there waiting for a heavy truck to go over it. No doubt the application of generous amounts of salt put on the bridge in the winter didn’t help. Am I imagining it or is there more truck traffic on the roads. Since they started putting ethanol in gasoline there is much more truck traffic. Imagine what would happen if one of those fuel trucks does a tumble on a collapsing bridge. If the Mexican deal goes thru and they start bringing even more heavy truck traffic up I-35, there will be a lot more disasters.
This is MHO, that the expansion joints were kept from working, combined with the heat, and vibrations from trains,jackhammers, traffic, and the crosion factors,all lead to the failure of this bridge. There is usally not one single factor causing such incidents..
Just my thoughts.....
I would think the 1,000s of pounds of salt they dump on the bridge presents a bigger problem then poop.
My theory is the approach on the right side collapsed, a column or some concrete structure gave way.
When it fell it pulled the left side of the span off its support and, boom.
Yeah maybe, but that bridge is 108 years old I heard...
Is it possible “Slim Jim” was on the bridge when it collapsed?
The bridge had an active de-icing system that ran in the winter time. Little spray heads popped up on the road and sprayed corrosive chemicals all winter long.
First, as others have noted, this structural design has no redundancy in it. If any structural component fails, the whole structure fails. Granted various components are designed with a margin of safety - eg. they need to carry a 10 ton load, they might be built to carry 15 or 20 tons.
If you watch the opening moments of the video you see the whole story. Right at the support you see the metal structure deform and pull down some. This is the initiating event. As the structure pulls down, the span actually shortens. This causes the near end of the span to pull off its pivots. Yes, these structures ride on pivots to allow them to expand/contract and move a little.
However, the failure at the far support allows the deck and structure to move down, pulling the near end off, and the entire deck drops nearly flat.
The 64K question is, why did the structure fail at the far end? Usually in spans, the maximum stress (in tension anyway) is near the center/bottom of the span. At the ends you find the most compressive load on members. Members in compression tend to fail spectacularly and catastrophically. For a simple demonstration, take a soda straw and crush it, end to end. It'll resist mightily and then fail suddenly, buckling. Take that same piece of plastic and pull on it, put it in tension, and it will (generally, depending on the plastic) fail more gracefully, elongating (strain) before finally failing. That elongation in a structure allows an overloaded member to move, potentially giving other structural members more of the load. But if a design has many members in compression, and no redundancy...
Finally, remember that there were two of the four lanes closed for resurfacing work. This presents an asymmetric, concentrated load. Traffic slows, more cars bunch together, they are all on one side, not evenly distributed. Asymmetric loads are bad too, and tend to eat up margins of safety.
Also at the far end of the span, you see the cut across the deck - an expansion joint. Also a great place for water, road chemicals, etc. to seep through the joint in the deck and find their way onto the metal structure.
So for now, I'd put this down to a combination of marginal design - no redundancy; decades of hard use; an asymmetric load; corrosion; and possibly a lack of maintenance (everyone talks about inspections, but what about repairs, painting, etc?)
Very doubtful. Road decks are always graded first in order to remove excess material and weight before re-paving.
My best guess to the cause was 4 decades of harsh winters (salt, chemicals, etc for the snow). This yearly coating of harsh corrosive materials applied to the road deck would do extensive damage to not only the metal structure below, but also tends to cause road material to get into the bridge expansion joints. Add the increased vehicle traffic from 40k to 180k cars per day, it’s a perfect storm for disaster. These bridges almost always fail due to a combination of multiple factors. Not one single factor.
we truely do have a weight problem in America.......
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