For the next few weeks, state bridge inspectors will be re-examining Oregon bridges that are similar to the one that collapsed last week in Minnesota.
Governor Ted Kulongoski ordered the inspections after the calamity that killed at least five people. Colin Fogarty reports.
The state of Oregon owns 28 steel deck truss bridges, like the one that fell into the Mississippi River. Local governments in Oregon own 14 or 15 more of those bridges.
Part of the difficulty in nailing down that figure is that Oregon has roughly 5000 bridges all over the state with all kinds of different designs.
Pat Cooney: “Many of these bridges depending on the length of them or the dominant design feature are classified as cantilever or drive through arch or steel deck girder.”
Oregon Department of Transportation spokesman Pat Cooney says it's not easy to narrow down which bridges precisely fall into the category of steel deck truss bridge. For example, Portland's St. John's Bridge is a suspension bridge. But one part of an onramp has a steel deck truss.
Pat Cooney: “A steel truss bridge is where you see steel girders that form diamond shapes or X's and they run parallel to each other underneath the roadbed.”
Cooney says while the agency figures out which bridges have that design, it's scrambling to reschedule bridge inspectors, hire consultants, and prioritize which bridges need the most attention.
Cooney says the inspection is mostly visual. Sometimes they use what's known as a snooper crane.
Pat Cooney: “It's what would be a cherry picker. But it's articulated so that it could go over the side of the bridge and up underneath it.”
Many of Oregon's steel deck truss bridges are rated as either functionally deficient or structurally obsolete. But those labels do not mean the bridges are unsafe. And Cooney adds that steel deck truss bridges are not any more susceptible to problems than other bridges.
Pat Cooney: “In general terms they are not, but because there has been a failure nationally of one of that type of design, it makes sense to look at the same designed bridges just to see if anything was missed. Now, the agency is spending the next few weeks inspecting bridges.”
Just as Cooney says Oregon bridges are safe, he also says they need work. Oregon is midway through a 10-year, $1.3 billion bridge repair project.
One of the structural engineers helping with those repairs is as Oregon State University's Chris Higgins. He's received a lot of press calls in the last few days, and he's answered them with a similar message each time.
Chris Higgins: “The infrastructure is deteriorating over time. And if we don't make the investments to continue to upgrade and restore and rehabilitate that infrastructure, it's matter of time until structurally deficient becomes unsafe.”
Since there doesn't appear to be enough money to replace every bridge that needs work, Higgins experiments with new ways to reinforce old bridges. For example, this is a video of how plates of carbon fiber reinforces a slab of concrete.
As researchers look on, a machine puts increasing amounts of force on the concrete until it breaks.
Higgins says many Oregon highway bridges were built in the 1950s, just as the bridge that collapsed in Minnesota was. And he says engineers back then expected them to last about 50 years.
Chris Higgins: “And if you think about the 1950s, looking out 50 years you assumed cars would be flying by the time 2000 rolled around. In reality what we've found is that trucks have gotten bigger. They've gotten heavier. They've gotten more frequent. And so the loads have changed significantly over time.”
Later this week, ODOT will report on what it knows so far about steel deck truss bridges and how the agency plans to evaluate them.