Get ready for a return to old-fashioned steel on Portland’s Morrison Bridge.
Plastic just didn’t work out on the six-lane Morrison Bridge deck. It was such a lackluster option that the material installed in 2012 started breaking up mere months after installation and a jury has already awarded $5.6 million in damages based on the faulty decking.
But that never answered the question of what Multnomah County engineers would come up with to fix the deck they reduced sped on and set weight limits on in order to keep it functioning until a fix was found.
Due to the initial designs of the bridge planning for an open-grate steel deck and the draw span working as a counterweight, engineers had to find a relatively lightweight solution to the decking. The original open-grate steel option was changed in 2012 due to high occurrences of accidents on the slippery surface—Portland does get its fair share of rain. The fiber-reinforced polymer from North Carolina’s ZellComp Inc. was chosen for its expected strength in a lightweight model. But within months of installation, the polymer started pulling away and breaking apart.
A jury awarded the county $5.6 million in a trial, but did also assign 22 percent of the negligence to the county, reducing the payment to the county by that percentage. That left $2.3 million (40 percent) assigned to ZellComp, $1.2 million (21 percent) to engineering firm Hardesty & Hanover of New York and $959,990 (17 percent) to Conway Construction Co., the contractor on the project.
But with the polymer failing, a lightweight solution was still out of reach until county engineers announced a steel grating design filled with a two-inch layer of lightweight concrete. The concrete will provide traction, something the open-grate system couldn’t provide. Originally thought too heavy an option—hence the earlier polymer selection—engineers now believe that the bridge can support the material more than twice as heavy as the open-grate or polymer.
The $7 million project will require adding more tonnage to the counterweight and researching motors to make sure the functions of the bridge can handle the extra stress.
Tim Newcomb is Engineering News-Record’s Pacific Northwest contributor. He writes for Popular Mechanics, Sports Illustrated and more. You can follow him on Twitter at @tdnewcomb or visit his website here.