The Bridge Collapse…Two Years Later. What Do We Know?

August 6, 2009 at 8:22 am 1 comment

By Carol Metzner
President, The Metzner Group, LLC
Managing Partner, A/E/P Central, LLC home of

On August 2, 2007, the entire span of I-35W (officially known as Bridge 9340) in Minneapolis collapsed into the Mississippi River. Vehicles, concrete and metal crashed into the water below.  Lives were lost and many more were changed that fateful day.

Immediately following the collapse, federal officials ordered an inspection of all steel deck truss bridges like the I-35W bridge.  Investigative reporter, Bill Dedman reported the following:

“At first, officials thought there were 756 steel deck truss bridges like the one that fell. That’s how many they found in the official federal database of bridges, the National Bridge Inventory, which gets its records from the states. Then state engineers found 32 more to add to the list. But when states started the inspections, they found that 280 of the bridges weren’t steel deck trusses at all — including 13 bridges made of wood timbers. Another 16 no longer existed; a bridge in Pennsylvania had been closed in 1982. Another 11 were private bridges, not subject to federal inspection. One in New Mexico was a pedestrian bridge. And a bridge in Pennsylvania had been double counted; federal officials had placed an identical ghost bridge in Maryland. By the time the survey was finished, the count of bridges of the same type as the Minneapolis span was down to 479, or 277 fewer than initially reported, according to internal e-mails from the Federal Highway Administration received Thursday by under the Freedom of Information Act.”

The Federal Highway Administration recommended framework for a bridge inspection QA/QC program is comprehensive.  In 2008, they cite six (6) state DOTs that have “existing QC/QA procedures that address specific aspects of the “Recommended Framework” in a manner the FHWA considers commendable.”  Six? Out of all of the DOTs in the US? I do realize that all states must have existing QC/QA procedures.  But only six are “commendable?” What is the status of the remaining state inspection programs? Adequate? Average? Acceptable?

We know that bridge construction has changed over the years.  Improvements in technology for use in bridge design, materials and construction have allowed engineers to project increased longevity of bridges. Structural engineers now describe bridge lifespan in terms of 100 years, instead of 20-50 years. Building new “improved” bridges, are we going to have 50 DOTs with commendable QA/QC inspection programs?

With the ability to build with an eye to sustainability, how do we fix what we have? Where does this leave us with our decaying bridges?  Many of those bridges now require billions of dollars for rehabilitation or replacement. How can we financially repair them if we don’t even have an accurate count of where they are and what type of bridge they are? What do you think?

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Entry filed under: Civil Engineering, Civil Engineering Issues, Failing US Infrastructure, US Infrastructure. Tags: .

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1 Comment Add your own

  • 1. Stephen McNally, P.E.  |  August 6, 2009 at 11:04 am

    Hi Carol

    A very interesting and relevant article you’ve written. I have been involved in the inspection, load rating and rehabilitation of large truss bridges throughout the US and have a story for you.

    One truss bridge in particular comes to mind. We were asked in 1989 by a State DOT (to remain nameless) to provide an in-depth inspection of a 9 span deck truss with a 275 ft through navigation span. We recommended that a load rating be performed prior to completing the inspection of the bridge to confirm which members and gusset plates are most heavily stressed. Although the State DOT protested, they eventually went along with the request, since there hadn’t been a load rating performed on the bridge since it was built in 1936 and the NBIS would be asking for a load rating soon regardless.

    The load rating revealed that there were four identical built-up truss members consisting of channels and plates, missing some plates that were in the design plans, but never fabricated or installed. This reduced the bridge rating from H15 to H6. We confirmed during inspection that these members in fact were missing the plates and noted that many laces were bent, which confirmed that the bridge had been very close to being in jeopardy.

    The bridge was promptly closed.


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