What the hell is going on? 35W Bridge FAQ
The exact definition of the term "structurally deficient" suddenly seems quite important. Yesterday's bridge collapse is requiring a quick study in bridge engineering and inspections, a subject to which I previously gave zero thought. This afternoon a panel of experts from the state and federal government met with a horde of roughly 100 reporters on the eastern bank of the Mississippi River, across from downtown Minneapolis. Here's some of what we learned:
What the hell does "structurally deficient" mean? It's a federal designation based on annual or bi-annual inspections. The feds use a nine-point scale to rank the structural soundness of bridges in three different areas--the deck (which is what you drive on), the super-structure and the sub-structure. If a bridge ranks four or lower in any of these areas it makes the list.
Are such bridges safe to drive on? So they say. "We view that term as a programmatic classification rather than an indication of level of safety," says Tom Everett, of the National Bridge Inspection Program. "It is not a classification that indicates a bridge is dangerous, nor that that bridge must be replaced."
How many bridges in Minnesota have been labeled "structurally deficient"? According to MNDOT, there are presently 1,160 in the state that carry this designation. That's roughly 8 percent of all bridges in the state.
How does this compare to other states? Nationwide 13 percent of the country's bridges, or approximately 77,000, are labeled "structurally deficient."
How long had the 35W Bridge been deemed "structurally deficient"? Since 1990.
Why was it deemed "structurally deficient"? The 35W bridge initially earned that designation because some of its bearings were corroded, according to Dan Gorgan, MNDOT's bridge engineer (pictured above). Subsequent inspections revealed some additional corrosion in its joints. Finally there were "fatigue cracks" noted in the 1990s on the approach spans of the bridge.
How often was the bridge inspected? It's been inspected annually since 1993. Prior to that it was inspected every two years. "During recent inspections in 2005 and 2006 we found no evidence of additional cracking in the bridge or growth in the pre-existing cracks," says Gorgan. The 2007 inspection had been interrupted by construction on the bridge and was to be completed in the fall. In addition the University of Minnesota's Department of Civil Engineering conducted a study of the bridge's fitness in 2001. The conclusion? It did not need to be replaced in the near future. A subsequent study by the URS Corporation reached similar findings.
Who built this thing? Industrial Construction, a company that is no longer in business. Construction began in 1964 and it was open to traffic in 1967.
How many cars drove over this bridge? Approximately 141,000 daily. "It was our busiest bridge within the state," says Gorgan.
What kind of bridge was it? It was a steel arch truss bridge. "It's an underdeck truss, meaning it's supported from below," says Gorgan. "There were two arches that supported this bridge."
Are there any other bridges of this kind in the state? Yes, three others: trunk highway bridge 23 (near St. Cloud, spanning the Mississippi River), trunk highway bridge 123 (near Sandstone, spanning the Kettle River) and trunk highway bridge 243 (near Osceola, spanning the St. Croix River). All three of these bridges will be inspected immediately.
When will the bridge be rebuilt Nobody knows yet. The city and state have put together a team to study the issue, according to MNDOT engineer Khani Sahebjan. But any rebuilding will obviously have to be put on hold until the collapse of the bridge has been thoroughly investigated. "When we get permission to remove the structure, that's when we can start construction," says Sahebjan.
Who will investigate what happened? The National Transportation Safety Board. Investigators arrived in Minneapolis this morning. In addition, Gov. Pawlenty announced today that the state has hired the forensic engineering firm of Wiss, Janney, Elstner Associates to scrutinize what happened. "We want redundancy and we want to make sure that nothing gets missed," he says.
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