Arena Update: Digging Deep

The New Arena at Seattle Center construction unearths innovations from cutting concrete with water to building a giant ‘kickstand’ for seismic support

By: Bob Condor

As the nightly excavation of 6,000 cubic yards of dirt continues on the New Arena at Seattle Center construction site, by day there are several signs of progress – and innovation.

One marker of progress is the depth of the site. Let’s just agree it is already a gigantic hole and on its way to cavernous or, well, get out the thesaurus to find synonyms for “really, really, really deep and really, really wide.”

Plus, the temporary steel columns supporting the roof are now more visible down below as dirt is removed around them. Those fast-appearing downward lengths are a brighter shade of orange more convenient for the eyes to see and the brain to measure.

Another tipoff is deeper shoring walls on the site’s rectangular perimeter. The walls look even longer when workers at the subterranean base appear tinier by the day.

“On the north end of the site we are almost down to the floor level [of the now demolished KeyArena] and going wider in every direction,” says Ken Johnson, construction executive for Oak View Group and NHL Seattle.

All directions will be at least 53 feet below ground when the excavation is complete, with the perimeter area reaching 70 feet or approximately seven subterranean stories to lay a foundation for the brand-new arena.

For now, those temporary steel columns are a major part of supporting the iconic roof from the 1962 World’s Fair, substituting for the concrete y-columns and buttresses that have now all been severed from the foundation. The columns and buttresses adjoined to the roof will be systematically reattached to the permanent foundation during 2020.

Detaching those concrete columns and buttresses required a highly innovation construction method using, of all things, water.

The first job was to remove thick encasements of concrete to expose steel rebar or steel rods that reinforce concrete. The rebar that remains, jutting out of the concrete, will help create a secure connection with the poured concrete of the new foundation in the coming months.

Johnsen explained why hydro-power was the innovative choice.

“To take the concrete off, you couldn’t shake it or pound it because we would be moving the [columns and buttresses) into the roof,” says Johnsen. “So we are using extremely high-pressured water.”

The pressurized water stream at 25,000 psi (pounds of force per square inch) cuts through the concrete without shaking or pounding to separate the concrete from the rebar”: “Look at the base of the columns and buttresses,” says Johnsen, “the concrete is gone.”

One of the ongoing challenges of the New Arena construction is to maintain the required seismic codes for the roof throughout the building phase along with the final structure. The same group of structural engineers, designers and construction executives that solved the temporary steel-column support system needed to devise additional support to keep the roof, the site and its workers safe in the case of an earthquake event.

“The group decided to create a south-buttress shoring to provide lateral and gravity support [to hold the roof in place],” says Steve Hofstetter, managing principal at Thorton Tomasetti, an engineering consulting firm that is part of the New Arena team of innovators. “We call it a kickstand. We are transferring the weight of the roof ring beam to the steel.”

The kickstand is a lever of sorts that runs a steel column angled horizontally (kickstand-like slant) some 230 feet from the southern buttress to the nearest stable load, which is Thomas Street. It helps the New Arena project attain earthquake code during all phases of construction and temporary support of the roof.

“It’s another part of the formula from these really smart engineers and contractors,” says Johnsen. “They asked how we hold up the roof, including during an earthquake. The kickstand is meant to hold it in place any lateral movements on the roof … we have a very, very secure roof.”