At a construction site at Ninth and Pine crews are finishing up a unique job: bracing for a six-story, 74-unit apartment building on a site where the downtown transit tunnel is only five to eight feet below ground.
As part of the project being developed by Teutsch Partners, the project team installed trusses — similar to those seen in bridge construction — just above ground level to hold the weight of the building.
The five steel and concrete transfer trusses span the length of the building, and crews put the final one in place last week.
“We’re transferring the entire building weight around the tunnel that is right beneath our footprint,” said Dane Egusa of KPFF Consulting Engineers, the engineer on the project.
The 1.3-mile tunnel moves closer to the surface between Westlake station and Convention Place station, which is across the street from Teutsch Partners’ project.
Trusses are not often used in mid-rise apartment projects but are more common in mid-rise and high-rise hotels with a lot of conference space because of the weight of equipment needed to put on events, said Amie Sullivan, an associate at KPFF.
The trusses range from eight to 16 feet deep and the longest is nearly 97 feet. The heaviest truss is 65,000 pounds and was welded on site. One truss had to be installed in pieces and then bolted together with 240 bolts. That 59,000-pound truss required a special police escort because it was deemed a “super load,” said Matt DeFranco of Sierra Construction Co., the project’s general contractor.
Zoning on the site allowed for up to 22 stories, but Teutsch Partners elected to go smaller because of the complexity of the site. More trusses, or perhaps much larger ones, would be needed in a 22-story building, Sullivan said.
The trusses added about $800,000 to the $20 million project, said John Teutsch of Teutsch Partners.
Crews drilled pilings 75 to 105 feet deep on the north and south edges of the building before installing the trusses.
The building at 1601 Ninth Ave. is scheduled to open in November. Installing the trusses and pilings was the most unpredictable part of the job, DeFranco said.
Not only did the team have to account for the shallow tunnel, but much of Runberg Architecture Group’s design had to be built around a transformer room required by Seattle City Light requirements.
“It’s a little building, but it’s pretty complicated,” Teutsch said.
The trusses separate the apartments from the ground floor, which has parking on one side and a 3,800-square-foot restaurant space on the other. Teutsch said he is close to an agreement with a well-known restaurateur to lease the space, which faces Pine Street.
Runberg used the trusses as a design feature throughout the ground floor. A “false truss” was added along Ninth Avenue for continuity with the rest of the building.
The developer will use a semi-automated stacking system in lieu of traditional parking in the building. Going below grade is not feasible because of the tunnel, and surface parking would reduce the building area given the small footprint of the site.
Sullivan of KPFF said the small site and challenges related to the shallow tunnel made this one of the more complicated apartment projects KPFF has done.
“It’s like a Tetris piece,” she said of designing and engineering the building. “Everything has to fit in exactly the right place. Every square inch is accounted for.”