August 2, 2007

Was The Bridge Deficient?

Normally, in the wake of a catastrophe, the details become clear only slowly and fitfully. Last night we heard that the 35W bridge had passed all of its inspections and that the collapse completely surprised everyone. Today, the Pioneer Press reports that inspectors had warned of a problem with this particular bridge, although the state overall had done an excellent job in bridge maintenance:

Bridge inspectors had noted structural problems over the years in the Interstate 35W bridge over the Mississippi River that collapsed Wednesday evening, but it was unclear whether obvious warning signs had been ignored. ...

In 2005, inspectors from the Minnesota Department of Transportation deemed the bridge "structurally deficient," in data submitted to the Federal Highway Administration's National Bridge Inventory.

Inspectors gave the bridge a sufficiency rating of 50 percent on a scale of 0 to 100 percent. A rating of 50 percent or lower means the bridge might need to be replaced.

The condition of I-beams, girders and other components called "structural members" was rated four on a scale of 0 to nine. A rating of 0 means failed, nine means excellent. Inspectors look at these for signs of distress such as cracking, deterioration, section loss and malfunction and misalignment of bearings. The deck received a rating of five and the substructure rating was six.

The design of this particular bridge made its inspection an even greater necessity. It had a span of almost 500 feet with no struts. When it was built, the city did not want to impede river traffic; barges still carry agricultural goods south on the Mississippi River. Viewers of the television coverage will see that the newer bridge that paralleled the collapsed section had a different design, with a support in the center of the river, and one would expect the replacement to mimic that.

That long span relied more heavily on the engineering of the steel supports, and for 40 years, it worked. Yet the 2001 study by the University of Minnesota noted "many poor fatigue details on the main truss and floor truss system", the very portions on which the entire bridge rested. The same study also said that Minnesota probably would not have to replace the bridge in the near future. The Pioneer Press report suggests that later inspections reached a different conclusion, or at least gave a stronger warning of problems.

State legislators expressed surprise at that news. Rep. Melissa Hortman (DFL-Brooklyn Park) noted that Minnesota's Department of Transportation (MnDOT) never mentioned that to the state legislature. If MnDOT knew of a poor rating on this bridge, they apparently never considered it a high enough priority to ask for special funding for repairs.

It's a bit ironic, as Minnesota has a good record for bridge maintenance. The US Department of Transportation rated only 3% of Minnesota's bridges as "structurally deficient", which put us as the eleventh-best state in that efforts. Rhode Island, in contrast, has 23% of its bridges rated structurally deficient, and seventeen percent of Michigan's bridges have that rating.

More details will come in the days and weeks ahead. If someone ignored a warning on this, expect Minnesotans to demand some housecleaning.


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Comments (6)

Posted by chuckR | August 2, 2007 7:32 AM

Writing from RI, I find your statistics unsettling - and possibly optimistic. An elevated section of I195 in Providence has been buttressed with wooden columns for years and is only now being replaced. Several years ago, a few foot long crack was found in a main girder web of a Providence I95 overpass. There but for the grace of God......
My deepest sympathy to those who have lost loved ones. Mourn them and then honor them by insisting on proper maintenance and/or replacement as necessary. There's a price to be paid to do the work, but its less onerous than price paid by those who died.

Posted by AnonymousDrivel | August 2, 2007 7:33 AM

Interesting observation on the bridge design in view of its failure. The supporting girders and framework probably saved lives. You'll notice from pictures that large sections, while cracked and segmented, stayed somewhat intact and rested above the waterline providing a pad upon which the traffic could rest after collapse. Imagine another material and its disintegration into the depths of the river. Surely more vehicles would have gone in or been washed off the road leading to even more drownings. Also, the iron/steel maintained its integrity (sort of) as it collapsed and compressed, even if minimally, and ever so slowly diffusing the energy over a bit more time, perhaps just enough to reduce acute, percussive impact.

Just trying to note any silver lining.

Posted by wolfwalker | August 2, 2007 7:53 AM

The design of this particular bridge made its inspection an even greater necessity. It had a span of almost 500 feet with no struts.

As best I can tell from before and after photos (look here for a decent 'before' photo), it wasn't even a suspension bridge. Just an ordinary highway-overpass-style bridge. I get a distinct feeling that in twenty or thirty years, this will be one of those "famous engineering disasters" that engineering students learn about right alongside the Tacoma Narrows bridge collapse and the Kansas Hyatt Regency Hotel walkway collapse.

Posted by Wise Ol Bird | August 2, 2007 9:20 AM

This was a non-redundant structure. That is there are only 2 main tension members and a failure of either one would result in total collapse of the structure. Fairly common design practice 40 years ago for large span bridges. That is why it was inspected so frequently; not the span length. This is not the way it is typically done today.

If there were 3 chords along the bottom, then the lose of any one would (probably) not lead to a total collapse.

Posted by dvr | August 2, 2007 10:32 AM

-Viewers of the television coverage will see that the -newer bridge that paralleled the collapsed section -had a different design, with a support in the center of -the river, and one would expect the replacement to -mimic that.

The "newer bridge" mentioned in the post is the Cedar Avenue Bridge and was built in 1929, thus being older than the 35W bridge. Just wanted to make a clarification.

Posted by Will Allen | August 2, 2007 11:26 AM

I often thought it amusing that when the Henepin Avenue crossing was replaced in the early 80s that they used a suspension design for such a short span. To a layman such as myself, it seemed extremely over-engineered.

However, at the same time, my running route used to frequently take me under the 35w bridge, along the river's shore, and as I looked up at the truss system, it always seemed to me to be in poor condition, obviously suffering from the ravages of mositure, salt, and temperature extremes.

I guess overengineeering is the path to take.

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