An engineering "tour-de-force"

Journal staff

In the Rainier Tower offices of the engineering firm Skilling Ward Magnusson Barkshire, the walls are lined with photos of some of the city's most prominent structures: the Kingdome, Key Arena, Husky Stadium, One and Two Union Square. All of them rest upon the engineering expertise SWMB contributed to their construction.

But soon they will be joined by a new trophy, and it will be perhaps the most striking of all. It is the city's new major league ballpark now under construction south of the Kingdome.

That building will be impressive not just because of its dramatic appearance. It will, in fact, be an engineering tour de force from the ground up.

This is not immediately apparent if you happen to be talking to SWMB Vice President Kurt Nordquist, a soft-spoken, patient man who heads a core team of some 35 engineers who are working on the project. He can matter-of-factly describe the design and function of every inch of the stadium. Whether it's the 1,432 piles that support the building, the cantilevered seating decks, the roof-moving mechanism or the roof itself, Nordquist can lay it out so simply and clearly that it all seems absurdly obvious.

The bowl will be seven separate structures, each with a unique shape.

The truth is, however, that nothing about the stadium is obvious, save the for the fact that baseball will be played there.

For starters, consider the site. Except for the buildings at one end and the railroad tracks at the other, it's clear and level. Little excavation will be required. But the geotechnical consulting firm Shannon & Wilson found that the loose fill soil under the surface isn't sufficient to support something like a ballpark, so down go the piles -- 950 of them 24 inches in diameter and filled with concrete, plus another 482 of 18-inch-diameter piles. Each one will descend 60 to 80 feet to glacial till.

The seating bowl and roof supports are to be built on top of the piles. But even the bowl, which looks like a traditional ballpark, will be a highly irregular building. Nordquist said the design calls for seven separate structures, arrayed in a U shape around the field, with seismic and expansion gaps between them. Each segment has a unique shape and is joined to the others with flexible earthquake and expansion joints.

Starting from the field level, each section of the stands will be a concrete structure up to the main concourse. Above that it will be structural steel with poured concrete floors and precast concrete seating risers. Nordquist said a big challenge was to quell vibration in the cantilevered seating decks; they had the potential of causing uneasiness among fans as they felt tremors in the structure resulting from the movement of people.

"A lot of engineering went into ensuring that they are stiff enough to make sure that does not happen," he said.

Outside of the seating bowl, the stadium structure is a conventional braced frame of steel. But the canopy, the steeply angled sunshade around the rim of the bowl, was another tricky problem to solve. The reason is that, from its base to the upper edge, it is a long span. The structure must nonetheless resist snow and wind loads without adding excess weight.

"It becomes a significant system to design," Nordquist said.

Not to discount that effort, but above the canopy will be the piece de resistance, a spider web of steel trusses that make up the stadium's unique rolling roof.

What it rolls on, of course, is just as important as the roof itself. That means tracks. Specifically crane rails, just like the ones found a block away at the Port of Seattle's container docks. Those tracks will be supported by their own steel structures, independent -- except at one point -- of the stadium bowl. The south structure, behind the bowl, will support the track some 112 feet above the street. The north track will be exactly half that high, to permit views out of the stadium toward downtown.

Each track structure must also be designed to resist seismic and other loads while being free to expand and contract with temperature changes. Nordquist said the necessary rigidity will be built into the heavily braced center section of each track support, leaving the ends to expand and contract by up to three inches overall. On the south side, this center section will be physically connected to the stadium structure, mainly because there is no room on the site to separate them.

Upon the 780-foot tracks will roll 16 eight-wheel travel trucks, each one supporting one roof leg. Each will consist of four two-wheel bogeys. Half of the 36-inch diameter steel wheels will be powered by electric motors via geared transmissions.

As a safety measure, each truck will incorporate steel angle plates that will reach under the crane rails, preventing the wheels from lifting off -- should the roof ever encounter a force strong enough to do that. The drive system is designed to be able to move the roof sections against winds up to 30 mph, and to automatically shut down in higher winds.

The roof will move on crane rails like those found at the nearby docks.

In solving these problems, Skilling has been working closely with Ederer Services, Inc., the Seattle manufacturer that is designing and building the trucks and their related controls.

"We had Ederer go through extensive testing to make sure the whole assembly would take the seismic and wind loads," Nordquist said.

Some other failsafe features have been included in the design. Should the roof controls somehow fail and be unable to stop a moving section, each track will terminate in a barrier equipped with hydraulic snubbers, or rams, to take the impact. And each truck will carry a 10-inch-diameter locking pin, which will engage fittings on the track to secure the roof when it is extended or stowed.

Attached to the trucks, of course, will be the legs of the roof. Because the south track is higher, the roof legs there will be only 57 feet tall. But on the north side the lanky legs will stretch 100 feet.

The roof structures they carry will be, in essence, three bridges. The telescoping roof sections will be built of trichord trusses that arch all the way across the field: 655 feet for the larger central section, and 630 feet for the smaller two that fit underneath it. The highest point of the center section will soar 230 feet above the field, or about as high as the summit of the Kingdome.

Needless to say, such an immense structure, with such a large roof so high in the air, would be subject to enormous wind, snow and seismic loads.

"One of the unique features is, it was designed using state-of-the-art earthquake engineering," Nordquist said. "It uses viscous dampers in the support of the roof structure -- like automobile shock absorbers."

The eight dampers, each one 18 inches in diameter and rated for 800,000 pounds, consist of cylinders containing pistons in hydraulic fluid. They are placed at the tops of the longer north legs, where the legs are joined to the roof trusses with hinge pins. As the roof deflects under load, the hinges flex and the energy is absorbed by the dampers.

"This allows us to dissipate a great deal of the dynamic wind and earthquake forces that would be put into this roof system," Nordquist said. "The amount of dynamic energy increases until a structure yields or you dissipate the energy; the dampers in the roof start absorbing energy from the beginning, reducing the force for the duration."

On the south side, the legs have rigid connections to the roof, although as the roof deflects they may rock slightly on their rails.

In designing such a system it is essential to know what the loads are going to be. One big question is snow. After the unusually heavy snowfall of the past winter, the SWMB team asked RWDI, their wind-tunnel testing consultant in Guelph, Ontario, to collect the data on snow loads from that period.

"Our design snow loads went up 15 percent because of that," Nordquist said.

Partly to accommodate such loads, the roof design allows two feet of clearance between the underside of the upper roof section and the top of the lower ones. But there are possible uplift forces as well. Nordquist said the average weight of the roof is 50 pounds per square foot, but a strong wind can generate lift of 25 pounds per square foot.

That's not enough to cause concern.

"There isn't a tendency for these things to lift up and blow away," Nordquist said.

Someone, however, is going to have to lift the roof into place. Although no erection contractor has been selected yet, SWMB and the stadium's general contractor/construction manager Hunt/Kiewit have a tentative plan for that delicate operation.

The key is the staging area to the east of the Burlington Northern Santa Fe railroad tracks, where the trusses will be assembled. Nordquist said the first step will be to erect a pair of roof legs on their trucks, one per track, and stabilize them vertically in position at the ends of the tracks over the staging area. Then a truss will be lifted with jacking towers until its ends can be connected to the legs.

Each leg-and-truss combination will be done in the same way, with the intervening roof structure assembled in the air over the stadium.

"We can do panels one and three (the lower, smaller roof segments) first, then they can be a working platform for the upper truss," Nordquist said.

Then, presumably, the job will be finished. But if the roof works as well as its designers expect, it may generate similar projects for SWMB. Nordquist said there is a lot of interest these days in stadiums with retractable roofs, both here and abroad.

"From what we can tell, retractable-roof stadiums are the wave of the future," he said.

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