Was I-35 Bridge Pushed Beyond Its Design?

The Interstate 35 bridge was expanded from four lanes to six, and eventually to eight. Some wonder whether that might have played a role in its collapse last week. The interstate highway bridge carried mostly local traffic.

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The I-35 bridge, which collapsed in Minnesota last week was built in 1967. Over the 40 years of its life, the demands on the bridge changed dramatically. Traffic on the span was more than double the initial expectation. Just last year, the Minnesota Transportation Department found some serious problems with the structure, but still deemed it fit for service.

NPR's Jason Beaubien has this report from Minneapolis.

JASON BEAUBIEN: The I-35 bridge was built in the 1960s during a boom in federal highway spending. Don Risk(ph) was on the Minneapolis City Council's public works committee at the time. And he says he wasn't even sure the city needed the bridge, but state and federal officials felt otherwise.

DON RISK: They felt that was imperative that we have this. Now, keep in mind, back in the 60s, there wasn't quite that traffic that we have today. So they, fortunately, foresaw what was happening, and wanted to construct 35W.

BEAUBIEN: It became the most heavily trafficked bridge from North Minneapolis into downtown. When the bridge opened in 1967, it was a four lane road with breakdown lanes along the roadway. As the greater Minneapolis St. Paul area grew, congestion increased and the breakdown lanes were commandeered for regular traffic. And then, finally, the deck was restriped to accommodate four lanes in each direction. The original 1965 plans for the bridge estimated that 66,000 vehicles a day would cross the span. When it collapsed, the traffic volume had more than doubled - to 140,000 vehicles a day.

JOHN ABRAHAM: If traffic loads were as they had planned, that bridge would still be standing.

BEAUBIEN: John Abraham is an assistant professor of mechanical engineering at the University of St. Thomas in St. Paul. A 2006 report prepared for the Minnesota Department of Transportation documented that the bridge was showing signs of excessive wear. Welds were failing and cracks were developing in critical parts of the substructure. And the report suggested that if the bridge was not replaced, steel plates should be added for reinforcement. Again, John Abraham.

ABRAHAM: There are some serious problems. In fact, these fatigue cracks had gone all the way through some of the components. So with that in mind, if you're not going to replace the bridge, you have to increase its stability. And one of the ideas was to put these plates in.

BEAUBIEN: The Transportation Department chose, instead, to inspect the bridge more frequently, this despite the 2006 report stating that some parts of the structure were inaccessible and very difficult to inspect. Since the collapse, state officials have repeatedly said that money was not the reason they chose the inspection route. Kevin Gutknecht, the spokesman for the state department of transportation, concedes that the traffic loads on the 35W had become much greater than when it was built, but he says they were monitoring the bridge carefully.

KEVIN GUTKNECHT: Based on all of the things that we've done, all of the data that we've kept on the bridge and everything else, the recommendations that we had told us that, well, yeah, we should replace this when we can, but it's probably good for a while longer if we do these things. And we don't know why the bridge failed. We don't know why it collapsed. This is baffling to us.

BEAUBIEN: John Adams, an urban geographer at the University of Minnesota, says public infrastructure like the 35W has been taken for granted for decades. He charges that politicians have been trying to do government on the cheap. In a sign that that might be changing, Minnesota's governor, Tim Pawlenty, has signaled that he could support a gasoline tax hike to pay for infrastructure maintenance. Just this spring, Pawlenty vetoed a similar new gas tax.

Jason Beaubien, NPR News, Minneapolis.

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