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.
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