The year 2017 shouldn’t have had any major projects slated for the Multnomah County-owned Morrison Bridge in Portland, but the 1958-built bascule bridge spanning the Willamette River will get one much-needed repair to its roadway this year, a completely new lift span deck.
The original open steel grate deck that had lasted 50 years, but proved dangerously slick in the often-wet weather of the Pacific Northwest, received an overhaul in 2011. The county then installed a fiber-reinforced polymer deck to replace the steel, limited in choice during the design phase as engineers needed to keep the lift span deck weight in balance with the two 1.9 million pound concrete counterweights that help operate the movable bridge, the largest mechanical device in Oregon. Plus, engineers had concerns about the ability of the bridge to withstand additional weight from a heavier deck.
While in 2011 that meant selecting FRP panels as the best option, similar to what was installed on the county’s Broadway Bridge a year prior, the Morrison deck began to show signs of failure in 2012. An in-depth investigation confirmed panels were loosening and deteriorating. The award of roughly $3 million in settlements, so far, with the companies involved in what they have called faulty material—the FRP started peeling while screws loosened—has allowed the county to start moving forward on a replacement for the replacement. A new round of engineering means April 2017 marks the start of yet another new deck style for the Morrison Bridge, this time an open steel grating deck filled with a lightweight concrete.
The concrete layer will come in at two inches thick and with a top layer of polymer for improved vehicle traction. This style wasn’t originally chosen in 2011 due to weight, as it will come in at 44 to 47 pounds per square foot, more than double the 20 pounds for the original open steel grating deck and the 2011 FRP deck. Recent analysis, though, the county reports, says that the bridge structure can handle the additional weight.
Portland engineering firm David Evans & Associates completed the design work on the new lift span deck and Hamilton Construction of Springfield, Oregon, was awarded the $6.5 million contract to install the new deck from April to October.
Follow Tim Newcomb on Twitter at @tdnewcomb.