Structures: Put a building right on top of a tunnel? We can do that!

Across from the Paramount Theatre in downtown Seattle, an unusual six-story apartment building is rising.

Its wood-framed upper floors look normal enough. But at ground level, a bridge appears to have crashed into the site and woven its massive, rusty steel framework into the design.

It’s not a bridge, in fact, but the transit tunnel that the workers on the apartment project must accommodate. Because the tunnel sits as little as six feet below the apartment house at Ninth Avenue and Pine Street, five giant steel trusses — weighing up to 65,000 pounds each — are needed to “span” the building’s weight across it.

“You could almost think of it as a bridge,” developer John Teutsch said. “A bridge with a building on top of it.”

A bridge that required precision engineering and fitting — and added $800,000 to the project’s $20 million cost.

The costly apartment project highlights how the Seattle area’s construction boom is pushing developers into more challenging locations, some times requiring them to deploy the kind of pricey engineering tactics that used to be confined to crowded world capitals.

At the same time, high-density features such as stacked parking and breakthroughs in computerized design are enabling developers to squeeze their projects into sites that once would have been rejected as uneconomical.

But why build apartments in such a tricky Seattle location?

“Are you kidding?” asked Teutsch, who immediately launched into a list of the site’s attractions: It’s two blocks from Nordstrom, five blocks from Whole Foods, a couple of blocks from Target, close to light rail and, of course, right next to the bus tunnel.

Plus, it’s a hop, skip and a jump from South Lake Union, where Amazon is importing armies of young workers, all of whom will need a place to live. These advantages, he said, make the site well worth the extra money and design headaches.

Low-rise appeal
When the 74-unit project — as yet unnamed — opens in November, it will be a low-rise in a forest of giants. It will have only five levels of one- and two-bedroom apartments atop a street-level restaurant that intentionally exposes some of the trusses, one of which is not even weight-bearing but was added for purely aesthetic reasons.

Designing the building posed serious challenges.

“It required some pretty creative structures,” said Constanza Marcheselli, project manager and architect at Seattle-based Runberg Architecture Group.

Such as parking. Since nothing can go underground, a standard parking garage was out. Instead, the firm designed Seattle’s first “car stacker,” a mechanical device that will lift each car into an assigned a slot in a tri-level structure just above the lobby. To retrieve their car, tenants will punch in a code that tells the machine to lift the vehicle out of its stack and bring it down again.
Architects are working closely with engineering firm KPFF on the stacker and other elements of the building.

“In a standard project, you would have the design, then bring in the structural engineer,” Marcheselli said. “For this one, the structural engineer was already on board — they had to be.”

Sixty-five percent of the apartment site lies over the tunnel. The tunnel’s shape is an irregular trapezoid, and though workers had drawings and a scan, “that doesn’t necessarily have a high level of accuracy,” said Dane Egusa, project engineer at KPFF.

To avoid an embarrassing encounter like the pipe struck by Bertha, State Route 99’s stuck tunnel machine, workers carefully drilled small “potholes” in sensitive areas until they hit the top of the tunnel, so that they’d know exactly where it was.

And then there were all the little things, stuff builders would never have to think about for ordinary, concrete garden apartments. To comply with building code, one of the trusses had to be placed on bearing pads allowing it to slide north and south if there’s an earthquake. Some trusses had to be specially designed so that people could walk through them. They will all need to be coated with “intumescent” fireproof paint, adding an extra $110,000.

Tense moments
But everyone involved with the project agrees that the riskiest part, the tensest moments, came the day the trusses were installed.
Sierra Construction brought in a 550-ton crane, which required a special permit, to do the uncommonly heavy lifting. The combined weight of structural steel exceeds that of a 43-story building going up across the street, according to Teutsch.
“We had to do some engineering on where the crane would sit,” said Matt deFranco, Sierra’s project manager for the site.
With a bus tunnel on one side and an underground parking garage on another, placing the crane in a safe location required tight coordination.

The biggest worry: Everything had to fit together perfectly. If it didn’t, materials couldn’t be fixed on-site and retooling would cause expensive delays. The long-est truss arrived in pieces and couldn’t be welded into place like the others. Assembly required fastening more than 360 bolts.
“When you’re constructing a structural member in a shop and it comes out to the site with a police escort,” Egusa said, “you always worry if some dimension is incorrect.”
Fortunately, everything went smoothly.
“We’re out of the difficult part,” said John Walker, a partner at Teutsch.
That may be true for the construction. But the financial pieces still need to fall into place.

Making it pencil
Teutsch has owned the site since 2009, when it bought the property from Wynd-ham Hotels for around $2 million. The price reflected the fact that the parcel was “impaired” by the tunnel, Teutsch said.
Because Teutsch has a track record — it built the Chandler’s Cove shopping complex in South Lake Union and other high-visibility projects — the firm didn’t have trouble borrowing $14 million from Banner Bank, and used cash for the rest. Company officials say they’re not worried about renting the 74 apartments when they’re complete.
Average rents are projected at $1,750 for a one-bedroom apartment and $2,900 for two.
Still, an awful lot of apartments are being built in Seattle.
“We’re bringing on several thousand units this year and next,” said John Gardner, principal at Gardner Economics, a Seattle real estate and economic advisory firm. “We’re getting towards the upper threshold for rents. It’s going to become a far more competitive market, and developers are going to be offering fairly substantial concessions.”
But the other buildings under construction are high-rises. Code allowed Teutsch to go 22 stories, but the cost of extending trusses to support that height would have been prohibitive. Now the developers hope that that tunnel-induced intimate size will prove an advantage.
“We provide a smaller-scale alternative for folks that want to know their neighbors,”Teutsch’s Walker said.
Gardner said Walker may be onto something. “It’s a unique product type with a greater relation to the street than the 30- or 40-foot towers,” Gardner said. “It could occupy a niche for renters.”