Expect this spring to hold a complicated fix for the four Rall wheels on Portland’s Broadway Bridge, one of the few remaining lift bridges in the world using the Rall design.

The $12 million project to replace the four Rall wheels, each 8 ft in diameter and able to support 2 million pounds, will move forward this spring with construction on the double-leaf drawbridge, the longest of its kind anywhere in the world.

Mulnomah County, owners of a bridge that was originally built in 1913 and spans the Willamette River in downtown Portland, were able to forgo a traditional bid process and select a contractor to replace the wheels that lift the bridge’s span based on the complexity of the job and the fact that few companies in the world have any experience working on a Rall design. The county hired a consultant team that includes both engineering and construction skills as designers Hardesty & Hanover will work with contractor Hamilton Construction.

The “complex project” requires working when spans are closed. Replacing the wheels will take about three months for each side, likely starting in May. After the Rall wheels, tracks and struts get replaced, crews will replace brakes and equalizers on the lift span.

The Rall-type bascule bridge uses a system of counterweights, gears and motors to raise the “leaves” of the bridge up and down. On the Broadway Bridge, two 75-horsepower motors turn the reduction gears connected to shafts and gears. These gears pull the operating struts connected to the bascule span and counterweights. The bascule span is supported and guided on the rear corners by steel Rall wheels, named for their inventor, Theador Rall. Two concrete counterweights located above the roadway of the bridge, each approximately 1,250 tons, balance the weight of the lift span.

To lower the lift span back into place the process simply happens in reverse.

At over 100 years old, the 1,742-ft-long bridge needs new wheels. Since opening, the bridge has undergone a series of repairs, including changing the bridge deck from concrete to steel grating in the 1940s and then to fiber-reinforced polymer composite in 2005. This summer’s latest fix will signify one of the most intense repairs of the Broadway Bridge in over 100 years.

Follow Tim Newcomb on Twitter at @tdnewcomb.