Engineers Make Third Attempt To Fix Bridge in Portland, Ore
Portland's troubled Morrison Bridge, which spans the Willamette River, will receive its third deck in four years after Multnomah County engineers settled on an open-grid steel deck with a 2.5-in. layer of lightweight concrete to replace a faulty polymer decking system.
Opened in 1958, the six-lane bridge featured an open-grid steel deck for its bascule drawbridge opening, but the slippery-when-wet deck caused a high rate of accidents. In 2012, an experimental fiber-reinforced polymer deck from North Carolina's ZellComp Inc. was chosen for its expected lightweight strength. Within months of installation, the polymer started breaking apart.
An 11-day jury trial in Oregon's circuit court awarded the county $5.6 million in damages but assigned 22% of the negligence to the county. That decision left $2.3 million (40%) assigned to ZellComp, $1.2 million (21%) to New York-based engineering firm Hardesty & Hanover and $959,990 (17%) to Ridgefield, Wa.-based contractor Conway Construction Co.
The new $7-million project will use roughly four inches of open-steel grating filled 2.5 in. with a lightweight concrete mix and topped with 3/8 in. of polymer.
"This lightweight, concrete-filled steel grid provides the traction that the old steel grid by itself didn't provide," says Mike Pullen, county spokesman. "That is the big advantage: You get the safety benefit in a relatively light weight-and it isn't experimental."
Portland firm David Evans & Associates continues researching needed bridge modifications to support a span twice as heavy-about 45 lb per sq ft, up from 20 lb-as the past two decks.
The bridge was over-engineered and likely can handle the additional weight without major changes, except for new motors to lift the span. The 950-ton counterweight was designed to take on additional concrete-block weights-"like a game of Jenga," Pullen says.
Construction will start in spring, with the job wrapping in late 2016.
