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Roof is 'Floating,' Dirt is Leaving

The daily excavation work at the New Arena at Seattle Center is exceeding expectations, while the landmark roof amazes

by Bob Condor / @NHLSeattle_ /

Arena Excavation is Well Underway

Excavation Phase of New Arena Yields Stunning Results

Construction at the New Arena at Seattle Center continues the excavating phase while yielding stunning views of the iconic floating roof.

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Ten months ago, a group of engineers and construction leaders sat down in a conference room amid whiteboards and laptops loaded with three-dimensional software programs.

They were on hand to solve a problem and meet a challenge never done at this scale: Figure out how to temporarily support the 44-million-pound landmark roof atop the former arena at Seattle Center while building a brand-new 800,000-square-foot structure underneath it. 

The engineers gathered at the whiteboards working out mathematical formulations. Some of the construction executives tapped on keyboards to visually simulate outcomes. But ultimately the group came together to apply never-out-of-date human collaboration.

They hatched a plan to drill and secure more than 70 temporary steel columns to hold up the roof while the former supporting concrete y-columns and buttresses were detached with a water jet system that cut away the concrete without harming the inner "re-bar" or steel reinforcement rods. This allowed the construction crew to leave enough re-bar extending out so this calendar year new concrete foundation pilings to be sunk 70 feet below ground and ultimately reach up to re-attach to the y-columns and buttresses.


And that's the summary version. The plan worked and the dreams of brothers Tim Leiweke (Oak View Group CEO) and Tod Leiweke (NHL Seattle CEO) are materializing. The roof is currently fully supported by the temporary steel columns, plus a lateral-support steel "kickstand" that spans from the east buttress all the way out to Thomas Street. Y-columns and buttresses are hanging in mid-air. The roof and temporary support columns meet and exceed seismic activity codes. The effect is magical, especially at night, as it appears the iconic roof designed for the primary building of the 1962 World's Fair is now floating.

Oak View Group and NHL Seattle construction executive Ken Johnson has his own technique of gazing at roof-designed by original architect Paul Thiry in honor of the cedar-woven rain hat worn by First Nation people. Johnsen at the roof intently for a few seconds and all other arena elements "blank out."

Video: Ken Johnsen Discusses NHL Seattle's Iconic Arena Roof

"Then it is floating like the 'Star Wars' hovercraft just sitting up there waiting to land back on the permanent building," says Johnsen, smiling at the thought. 

The collaborative plan developed by the engineering/construction group, like any successful plan, has required some adjustments and changes of course. For example: The temporary steel columns, attached in pairs with a steel cross-beam, required additional steel bracketing to support the 70-foot length going subterranean. But if those brackets were a solid web all the way down, then the construction crew could not build floors into the New Arena. It was decided the brackets would be staged in segments so that concourses, landings, the hockey rink/concert floor/basketball court and more can be created in between. 

"One of the fun things [on this project] is every day an issue comes up," says Johnsen. "It is a unique project, people get together to figure how to solve it."

One more example: Keeping the roof in place while planning a brand-new arena double the size requires digging and removing more than 600,000 cubic yards of dirt. The usual methods for digging that hole were challenged by those 70-plus steel columns (painted bright orange for safety). The excavator operators must maneuver their bulldozers and other equipment in and around those poles.

Work on the inner side of the roof (electrical, acoustic panels, etc.) calls for scaffolding to be built and attached to every square foot of the roof so workers can accomplish intricate tasks. Hauling out the dirt is unlike any other project (more on that challenge in a future story); the original plan to use conveyor belts at each truck station has been adjusted. A series of trucks now circle the New Arena bowl every night, entering for load-up and exiting via a temporary ramp. 

"We set a target of 6,000 cubic yards a night," says Johnsen. "But with some smart thinking, how to choreograph [the dirt loading and hauling], how the equipments come together and the fact that Seattle Center lets us haul through the campus at night, we are doing 7,000 yards a night [with some 60 trucks night]. We are really proud of that."

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