I-35W bridge collapses in Minneapolis
A disaster has struck Minneapolis, Minnesota. A major bridge, the I-35W, has collapsed into the Mississippi River.
From the Wikipedia entry attached to Google Earth:
The I-35W Bridge is a deck-arch truss bridge that spans the Mississippi River in Minneapolis, Minnesota. It was built in 1967 by the Minnesota Department of Transportation. The bridge is notable for not having any piers in the water. Instead, the main support piers are located on the banks of the river, and are built of tubular-shaped concrete pillars. This allows for a wide, clear span across the river, making river navigation easier. Although not very decorative, the bridge is one of the widest bridges in the Twin Cities area and provides an important link for Interstate 35W traffic.
You can see what a major thoroughfare the bridge represented for the city of Minneapolis. The bridge, of course, did not follow the ground level and water level. This is an artifact of the Google Earth perspective rendering.
Here is what the I-35W bridge looked like from below before the collapse:
Designed by the Minnesota Department of Transportation, this bridge has a high water clearance and no mid-river piers in order to facilitate navigation of large vessels up to the two locks at St. Anthony Falls, which were opened four years earlier. The single cross-river arch has a span of 458 feet.
This photo makes clear the long span of the bridge. A truss, the triangular structure, can resist both tension and compression, but not twisting. That's why truss bridges are invariably straight.
Perhaps a failure in one of the four piers pulled the bridge truss over in one direction off the centreline, and the failure progressed along the truss length, unable to resist the progression of the failure until the next pier was reached, which in this design was at the far side of the span instead of at the mid-river point.
We'll learn soon whether that inherent property of trusses had a role to play in this disaster.
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Comments
I was just there on I 94,...
Oh well it's still safer than the trans Canada.
Posted by: DrWright at August 2, 2007 06:35 AM
Me and my big keyboard, I heard that there were 6 dead on the radio. Seems the number is far larger.
Posted by: DrWright at August 2, 2007 06:55 AM
Terrible tragedy.
I don't remember which one of Ayn Rand's novels it was - The Fountainhead or Atlas Shrugged - but similar collapses of infrastructure were depicted therein.
How is it that millennial structures are still standing, and a bridge built in the 60s can collapse like that?
Posted by: Gabby in QC at August 2, 2007 07:27 AM
Scary....like a pack of lemmings. What's worse is, over 10 years ago I read of the US of A's deteriorating bridge infrastructure. I bet there's over 400 more shaky bridges like this one, down there....
Posted by: Feldwebel Wolfenstool at August 2, 2007 11:27 AM
A few points.
These bridges go through scheduled inspections so I have doubts that one of the pier connections failed, as this an obvious location to inspect and look for weathering.
I would be looking for steel connection failures higher up inside the truss and maybe 10-20ft from the piers towards the river. These connections are still under huge relative stresses and are harder to inspect.
Posted by: James McKenzie at August 2, 2007 01:10 PM
I just heard a report of stress fractures being found during an inspection in may 2006.
The fractures were found on girders and their joints just below the bridge deck.
The bridge was going under some deck repair a the time of collapse but no word whether these girders were to be repaired as well. Either way, stress fractures in those girders is a critical problem which could definitely lead to this sort of catastrophic failure.
Posted by: James McKenzie at August 2, 2007 01:51 PM
Latest I heard was a concern about vibration. Normally that ought not to be a problem, but if there were pre-existing cracks (I found out about those after I wrote this piece), vibrations from traffic or the railroad the runs under it (you can see the railroad in the Google Maps photo in the lower left corner) might have played a part.
Posted by: Steve Janke at August 2, 2007 02:06 PM
I'm not an engineer but from what I have read and inferred, truss bridges are not designed for exceptionally long spans unless supported by other principles (i.e. truss is attached to cantilevered arms, truss is supported by intermittent piers, etc.). The 35-W bridge was a deck truss supported soley by its anchorages to shore piers 458' apart. This 458' distance is apparently exceptional for a truss bridge, because Wikipedia makes a point of discussing it, explaining that the absence of mid-channel piers allows for an unobstructed water passageway. As such, the 35-W bridge's mid-span rested entirely upon the integrity of the deck truss. Questions: How common is the 35-W's truss/span specs? How many deck truss bridges in the USA/world have otherwise unsupported spans of at least 458' between piers?
Posted by: Dan Drew at August 4, 2007 02:35 AM
 
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