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[Rick Denney]

I sent this email to a well-connected/recently-retired close friend:

"Close friend"? Is it who I think it is?

My call to my sources hasn't been returned yet, but salt and reinforcing steel do not mix and I'm sure that's a contributing factor. But there are much older bridges than this one made the same way that have survived salty winters for far longer, so I doubt that's the only issue.

Usually, these things are a combination of several factors that sum up to a failure. Inspectors sometimes consider these factors separately and don't realize the effect of their interaction. This bridge was inspected only a year ago.

I also note that the bridge was closed down to one lane during construction. Maximum loading in the outboard lane of a bridge is a severe loading condition because of the torsion it applies to the bridge deck. It was specifically considered in the segmented winged T bridges in Texas that used a single row of center piers supporting a 50-foot deck, but maybe they didn't consider it for this more conventional design.

Rick "thinking some Minnesota lawyers are about to get richer" Denney

[Chuck(G)]

Here's a UofM 2001 bridge inspection report:

http://www.lrrb.org/pdf/200110.pdf

From what I've seen and heard, it wasn't the concrete that failed, but the steel. The report seems to indicate this as a source of concern.

[Rick Denney]

Rick, will you or any friends/colleagues be heading out to help plan the battle against the added burden to the Minneapolis rush hour? When you lose a major route in or out of a city like this (meaning for years) how do you get around it? Re-do light timings on the city streets in the affected area?

Very likely, but probably not me. MnDOT has their own experts and there are good consultants in Minnesota. MnDOT has an excellent reputation in their Twin Cities control center for high-quality freeway management.

We consult with MnDOT on other issues, however, so we might be involved in some ways. Usually our involvement is in helping them to support their system operation rather than offering advice on what actions to take with their system.

They will undoubtedly reconfigure their ramp metering plan, and they will definitely make revisions to signal timings. I'm sure they have been working on those things since last night, and will be making changes in real time over the next many days.

But you can't overcome a major loss of capacity in a network that is already congested. They will gain much more benefit by persuading local business and travelers to shift their demand to other times and modes in addition to shifting to other routes.

Were it me, I would have requested that companies with many employees who cross that bridge immediately encourage them to work from home for a while. And then I would encourage them to allow flexible work hours so that their employees can avoid the heaviest parts of the peak periods on the remaining routes. I would encourage the use of company vanpools, and I would put a temporary moratorium on transit fares for a week or so on the routes that will serve the shifted demand.

Rick "congestion = demand > capacity" Denney

[Rick Denney]

Here's a UofM 2001 bridge inspection report:

http://www.lrrb.org/pdf/200110.pdf

From what I've seen and heard, it wasn't the concrete that failed, but the steel. The report seems to indicate this as a source of concern.

Not really. They pointed out that some details were not designed in a way that was sensitive to fatigue, but they also determined that no fatigue cracking had occurred and that they didn't expect it to. Fatigue cracking of the exposed steel would have been first on the list of inspection items, especially with the sub-optimal details identified.

And this report isn't a bridge inspection report, but rather a research report looking at a specific potential failure mode. A bridge inspection would be less deep and much more comprehensive, looking at all potential failure modes. And it would likely not be done by grad students at UofM, but rather by consultants or MnDOT employees who are experts in bridge inspection. That doesn't mean the bridge inspections done recently (and much more recently than this research) aren't faulty--after all, the bridge fell down, so some failure mode or combination of failure modes were obviously missed.

Fatigue in steel, for those who don't know, occurs when loading reaches a high percentage of the yield strength, without yielding, for enough cycles to cause a crack to form and propagate. It's cause by stress hardening, which makes the steel brittle. A poor construction detail can provide a stress riser which can start a crack, and then repeated stress cycles cause the crack to travel. Steel is actually highly resistant to fatigue cracking at normal temperatures. Cast iron (used in some historic bridge trusses and truss components) and aluminum, for example, have much lower fatigue resistance. That's why airplane inspectors are so interested in evidence of fatigue--most of the structure of modern airplanes is made from aluminum.

My (specifically ignorant but generally educated) suspicion would tend more along Bloke's line. All concrete cracks, and cracks in the concrete can allow salty moisture to reach the reinforcing steel. That sort of corrosion is buried down in the concrete and a little harder to see. But that's not the issue raised in this report, at least I didn't see it in my quick scan, which was more concerned with the exposed steel components.

Rick "sharp inside corners in fatigue-prone structures = bad" Denney

[Chuck(G)]

Here's a clip of the bridge going down:

http://extras.twincities.com/video/web_ ... index.html

I spoke with a retired CE friend of mine this morning about the disaster (he's done a few bridges in the Northwest himself). His comment?

Bridges are a lot like teeth. If you ignore them, they tend to go away.

.

Oregon is replacing some bridges on the I-5 corridor. One, near me over the Willamette, is very instructive. A "temporary" bridge is first constructed and the freeway diverted over it. The old bridge is next demolished and a new one built; then the temporary bridge is removed.

The temporary bridge began to go up in 2005. We're now at the stage where demolition of the old structure should happen in the autumn. Construction of the new bridge should start in 2008 and be complete by 2010.

This infrastructure upgrading thing is slow and expensive. And Oregon has a lot of bridges.

[dtemp]

Here is the latest from my source:

The bearings that allowed the bridge to expand/contract were all rusted up.

There have been a flurry of reports as to what caused it. I'm waiting to hear it from the officials.

What really pissed me off is *edit - something political...nevermind...*

d(who HONESTLY lives less than .5 miles from the bridge and felt the ground shake when it fell...)