As Sound Transit extends its East Link light rail project in Bellevue, the transit agency showed off its specialized crane construction method to build a new 860-ft guideway over a section of Interstate 90.

The balanced cantilever construction method requires two movable forms for concrete, known as “travelers,” that will move outward as crews use them to construct bridge segments. The travelers will move at an equal pace, keeping the structure balanced as crews add concrete segments to each side. Such specialized cranes allow workers to build the bridge over the freeway with minimal disruption to traffic.

After the bridge extends partially over the eastbound lanes of I-90, the travelers will move to the north side of the freeway and repeat the process. The two structures will meet above the freeway, with crews using surveying equipment throughout the process for exacting alignment.

The traveler method was chosen as the safest and most cost-effective option, along with the least disruptive to interstate traffic. The length and curve of the bridge and the narrow width of the westbound freeway wouldn't allow workers any space to place temporary columns to support framing, a method similar to the one used to build the Interstate 405 overcrossing in downtown Bellevue. The cranes and traveler method became the next-best option.

Work will take place overnight and require some lane and ramp closures in order to have construction complete by early 2019. Contractor Shimmick/Parsons Joint Venture is handling the $359-million project that includes more than two miles of guideway, the South Bellevue station, a 1,500-stall parking garage and the I-90 overcrossing.

In addition to the guideway plans, the project includes girders for the guideway along Bellevue Way Southeast between I-90 and the future South Bellevue Station. Farther east, crews are excavating the trench in which light-rail vehicles will travel in front of the Winters House. Work has also started for the undercrossing on 112^th Avenue Southeast.

The entire East Link project will extend light rail 14 miles from downtown Seattle to downtown Bellevue and the Overlake area of Redmond via Interstate 90, with 10 stations. Construction of the East Link extension is more than 40 percent complete and the entire line will be operational in 2023. Sound Transit is simultaneously working to extend light rail north, south, east and west, opening new stations every few years to form a 116-mile regional system by 2041.

Starting in 2021 the Northgate project will open, followed by the 2023 service to Mercer island, Bellevue and Redmond’s Overlake community. Further extensions will open in 2024 and then light rail should extend to West Seattle and then south to Fife and Tacoma in 2030, Ballard in 2035 and Paine Field and Everett in 2036. South Kirkland and Issaquah expect service in 2041.

Follow Tim Newcomb on Twitter at @tdnewcomb.