This image shows the ground supports of the bridge before the
collapse:

http://www.wmur.com/2007/0802/13805989_240X180.jpg

taken from:

Nation's Bridges Face Immediate Inspection.
Fifth Victim Found; President Bush To Visit Minneapolis
UPDATED: 9:20 am EDT August 3, 2007
http://www.wmur.com/news/13801620/detail.html

A video of the collapse is also available on this page.

Note the ground supports are slender concrete columns. Note also the
steel arch only extends to the top of the support. It does not extend
down into the ground.
In contrast note the arches of the 10th Avenue bridge next to the
collapsed bridge extend into the ground:

10th Avenue Bridge.
http://en.wikipedia.org/wiki/10th_Avenue_Bridge

The strength of an arch extends from its curvature. Note that an arch of the
I-35W bridge not extending into the ground means the arch is shorter which
necessarily makes the arch straighter, and therefore
weaker.
The 10th Avenue Bridge also has supports in the middle of the river
while the I-35W bridge did not. Compared to the supports of the 10th
Avenue Bridge, the ground supports of the I-35W bridge can only be
described as flimsy.
This is a bridge that carries the most traffic in the state of
Minnesota. Moreover the 10th Avenue bridge only has to carry 2 lanes, while
the I-35W carried 8.
A professor at Northwestern argues the failure was likely due to the
joints connecting the bridge to the concrete supports:

Investigators in bridge collapse focus on chilling video.
By Jon Hilkevitch | Tribune transportation reporter
9:41 PM CDT, August 2, 2007
"The bridge must have been near a state of collapse for some time, and the
construction might have contributed to its failure," said Zdenek Bazant, a
professor of civil and environmental engineering at
Northwestern University. Bazant said he suspects there may have been a
hairline crack or fatigue in the steel joints near bridge supports,
leading to the buckling"
http://www.chicagotribune.com/servic...6.story?page=2

This page on the I-35W describes it as a truss bridge:

I-35W Mississippi River bridge.
http://en.wikipedia.org/wiki/I-35W_M...i_River_bridge

These are among the cheapest and flimsiest of bridges. They lack the
redundancy of many other types of bridges:

Investigators in bridge collapse focus on chilling video.
By Jon Hilkevitch | Tribune transportation reporter
9:41 PM CDT, August 2, 2007
"Other engineering experts said that the 1960s-design of steel-arched
bridges did not contain structural redundancies, meaning that if one
component fails, the whole structure is in jeopardy because the weight does
not shift to other points on the bridge.
"We know that we would not build a bridge like this today,'' said Kent
Harries, an assistant engineering professor at the University of
Pittsburgh."
http://www.chicagotribune.com/servic...6.story?page=2


Bob Clark

Not only are we abusing our bridges with more traffic but also with increased wheel loading.

Although alloy wheels have helped slow the growth in 'unsprung' mass, the heavier tires we use on our SUVs and light trucks combined with heavier suspensions have more than overcome any 'alloy wheel' benefit.

Probably time to markedly increase our design criteria. We aren't driving 1966 Beetles anymore.

jm2cw ... pete

What? How is that? Got somewhere for me to go to understand this?

Most people look at the "cool look" benefit and don't go any farther. The rest is academic.

__________________
Numbers are not case sensitive. (me)

I always assumed a 1.5 ton vehicle imparted the same weight on a bridge regardless of wheel and suspension type.

Different tires could have obviously different effects on wear/tear of the road/driving surface...but that shouldn't have anything to do with a bridge's structual integrity.

__________________
I'm like one of those idiot savants...well, except for the savant part.

Poor design?
This bridge had stood for forty years or thereabouts.

Of course here in the UK we have bridges that have stood for four HUNDRED years. But they don't carry motorways!

John

I was on a business trip when I heard the news of the collapse. In the confusion of the moment, a lot of things get reported that later turn out to be incorrect. One thing I heard yesterday was that a freight train was passing over the bridge at the time of collapse (I've seen some combination auto/train bridges but I have no idea if this bridge was one of them). If that's true, there may be some other possibilities as to what triggered the collapse. For example, if the train somehow derailed at that point or had some load shifting, it might have struck a support. Several years ago, I read about a bridge that collapsed when it was struck by a barge. If there was a train going at the same time, perhaps there is a connection to the collapse. Of course, it could've just been a coincidence or a completely inaccurate news story in the first place.

Nope. No train on the bridge. 8 lanes of road vehicle traffic only -- at the time, 4 lanes of traffic and 4 lanes of being-resurfaced.

A train was moving (edit: some accounts say standing) under the northern end, parallel to the Mississippi, at the time of the collapse.

__________________
0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 1 0 0 1 0 1 1 0 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 0 1 1 0 1 0 0 1 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 ...

A rogue seismic wave?

__________________
PW -- Plant Whisperer

i wouldn't say it was a poor design- the thing stood for 40 years with ever increasing traffic loads- but it was an outdated design with no real redundancy. one little thing breaks, and the whole thing comes down. it could have been as "minor" as a single rivet rusting out or getting sheared off.
they don't build bridges like that one any more. the world has moved on from the days of steel truss bridges.

__________________
"blacker than the blackest black... times infinity."- Nathan Explosion
The.. Best.. Thread..Ever...

In Scotland the Forth road bridge is slowly collapsing. Water has found its way into the Cables and they are expected to be scrap withing 15 years.

__________________
'The eye can only see what the mind is prepared to accept'

Same weight but differences in vertical accelerations.

Kind of like running in worn out running shoes versus new ones. Same weight, but remarkably different impact on one's feet and legs. Translate that to the bridge structure.

I'd go with the old design and inadequate maintenance approach as the primary cause, rather than a poor design. A poor design and deferred maintenance as the cause are best exemplified by the Mianus River bridge collapse.

Re deferred maintenance, I could point out that using a rather small percentage of the funds spent on a certain foreign operation in one day, if applied immediately to repairs after that 50/120 score in 2005, would have prevented this mess and very likely would have indicated the need for an upgrade. But that would get political. So, no more on that.

The same applies to a huge percentage of all bridges across the US.

__________________
A person's name, or a mark representing it, as signed personally or by deputy, as in subscribing a letter or other document.

Yeah, but that also gets into, what's the state responsible for and what's the federal government responsible for?

If that daily expenditure wasn't going overseas, I'm sure it would have gone somewhere else other than bridges and this still would have happened.

We are agreed that it would be considered a poor design today, because of no redundancy.
The question: should it have been considered a poor design in the sixties when it was built? Certainly it was known then that these type of truss bridges were among the weakest types of bridges. It was also among the cheapest. It's pretty clear that the intent was to get an inexpensive, easily built bridge in service quickly.
BTW, I just saw this NY Times article that had this alarming fact:

Engineers See Dangers in Aging Infrastructure.
By JOHN HOLUSHA and KENNETH CHANG
Published: August 2, 2007
"Still, a study by the Federal Highway Administration found that visual inspections, the primary method used by bridge inspectors, only rarely detect cracks from metal fatigue.
"In the study, completed in 2001, 49 bridge inspectors from across the country examined test bridges in Virginia and Pennsylvania. Only 4 percent correctly identified a fatigue crack. Worse, many inspectors identified nonexistent problems, suggesting that bridges sometimes undergo unnecessary repairs while some serious conditions are not detected.
Inspectors now sometimes employ tools like ultrasound, but those add time and cost to their work."
http://www.nytimes.com/2007/08/02/us...=1&oref=slogin

Clearly, better methods need to be employed routinely in inspecting bridges.

I attached a blow-up of the image showing the concrete support of the I-35W bridge before the collapse.
I also attached the image accompanying the NY Times story. You can see the supports for both the I-35W bridge and 10th Avenue bridge in the image. (note: this image shows the 10th Avenue bridge actually has 4 lanes not 2.)
The I-35W supports look puny in comparison, and this is for a bridge that had twice as many lanes and did not have the extra supports in the middle of the river.


Bob Clark

Attached Thumbnails

   

Not at all. A vehicle with four large tires has tremendous weight loading around those tires. Spread the load out over more tires, and the point stress on the bridge is lessened.

Point stress translates into bending moments in the roadway supporting structure.

HOWEVER, as clearly shown on the video, it was the roadway structure itself which gave way, but the main structure of the bridge, which has very little to do with the distribution of weight from vehicles.

__________________
I am Mugs, of the Alien clan of Usa, Nordamerica, a Terran, of Sol. A human.

Whoever says "perception is reality" is daft. It's merely an abstraction, and often not a very good one.