(Photo courtesy of Michael Dickter/Skilling Ward Magnusson Berkshire)
A megatruss redesign slashed $4 million from the cost of the grandstand roofs. An inventive use of seismic isolators sliced another $2.5 million off the budget. A tucking and reconfiguring exercise squeezed 250,000 sq ft out of the original 1.75-million-sq-ft program. At a $430-million football-soccer stadium in quake-prone Seattle, budget necessities were the mother of invention.
"We guaranteed the public would have to spend no more than $300 million," says Bob Collier, vice president of First & Goal Inc., the affiliate company of Allen's Vulcan Inc., set up to develop and operate the facility. "In return, we needed to operate with more autonomy and control," adds Collier, also Vulcan's director of construction.
F&G picked the design team. And state legislation written for the development exempts the project from state public works laws, except for prevailing wages. That meant F&G could negotiate with its contractor, the local office of Turner Corp. In turn, Turner can negotiate rather than hard-bid all construction contracts.
There are limits. "It's not like we took the ball and ran," says Collier. "There is oversight every step of the way."
|GETTING IT TOGETHER Workers wrangle with spans of 720-ft-long trichord truss. (Photo © by Joe Crux)|
The Washington State Public Stadium Authority, set up for the project, checks every detaildown to clearances in toilet stallsto make sure conditions conform to plans and specifications. PSA also approves payments to designers and contractors. "This is a unique process, a true public-private partnership," says Jake Jundt, PSA board member. "It is the only public project in the state, and maybe the country, ever executed this way."
The process is apparently working. Change orders are reportedly at 2% and the 91%-complete stadium is on schedule and budget.
Allen wanted to stick to the price tag, a difficult task for a stadium that had a $10-million budget bulge from day one. As a result, it's all been about getting more bang for the buck at the open-air Washington State Football/Soccer Stadium, set to open in August, and the two-year-old adjacent exhibition center. "Thrown into these situations, you come up with more creative solutions," says Kelly D. Kerns, senior project manager for architect Ellerbe Becket, Kansas City, Mo.
Allen's pact grew out of a drive to steer clear of mistakes at the Seattle Mariners ballpark, open since 1999. The cost of Safeco Field, owned by King County, skyrocketed $100 million. There, the county had control of budget and schedule but the Mariners were responsible for extras. The project was fast-tracked, leaving no time for cost accounting. "The Mariners suffered from that," says Jon D. Magnusson, chairman-CEO of structural engineer Skilling Ward Magnusson Barkshire Inc., Seattle, and the only major consultant on both jobs.
The Mariners and the Seahawks formerly shared the Kingdome. For the Seahawks, being second in the construction lineup has advantages. Voters approved the Allen deal in 1997. F&G then rushed the exhibition center to completion, so the Kingdome, a venue for flat-floor shows, could be razed, clearing space for the stadium. By the time stadium construction started in late 1999, design was complete. "This situation is extremely rare these days," says Kerns.
Going second also has downsides. Safeco's errors blackened the image of sports construction and dunked the football stadium team into a fishbowl.
The first elements to be value engineered were the mirror-image grandstand roofs, each 720x200 ft. Initially, each roof was to have a three-dimensional truss, a giant bowtie in plan, supported on four massive pylons. Instead, the engineer offered up a more axial expressionan inner-edge megatruss, spanning 720 ft between reinforced concrete pylons. A leading-edge truss would only have to take half the weight of the roof. The other half would go into extensions of grandstand supports along the bowl's outer edge.
Sharing the roof load between two support systems is analogous to carrying two small suitcases, one on wheels and the other not, instead of one large one. Not only did the megatruss weigh less than the bowtie, it only needed two pylons. The improvements sliced $3 million from the cost of steel and $1 million from the pylons, says the engineer.
Everyone embraced the idea, including the architect. It is a "simpler and more elegant scheme," says Kerns.
|FRAME UP Arch, above roof plane, sits in tops of A-frames. (Photo © by Joe Crux)|
The result is a box-girder arch with A-frame webs, stiffened by a trichord truss with triangulated webs. The arch's 4-ft-wide box girder is 6 ft deep at mid-span and 20 ft deep at the ends. The trichord has two, 14-in.-deep top chords, 34 ft apart at midspan tapering to 8 ft apart at the arch intersection, and a 3-ft-deep bottom chord. The roof deck sits on east-west secondary trusses that slope down from the trichord to the bowl's perimeter columns, leaving only the A-frame arch above the roof plane. Infill members span between roof trusses.
Trichord depth varies from 26 ft at midspan to 12 ft at the ends. Intersections of A-frame legs, 16 to 80 ft tall, and trichord webs occur every 44 ft along trichord top chords. Trichord bracing takes all shear forces in the megatruss. "The architect didn't want diagonals above the roof plane," says Magnusson.
The top arch radius, at 700 ft, is greater than the bottom arch radius, nearly 614 ft. "We made them different, so that like a truss, all the forces come together over the bearing point," says Brian H. Glover, SWMB project manager. This eliminates any eccentricities.
The arch is post-tensioned to keep it from flattening. In a quake, the tendons behave like a rubber band, stretching and returning to the original condition. A rigid tie would deform.
"I don't think anything that big or with a span of 720 ft has been post-tensioned before," says Glover. The post-tensioning, in combination with the isolation bearings, made the structural system work," he adds, "and I can sleep at night."
Glover is referring to four friction pendulum seismic-isolation bearings, each sitting atop a pylon at the arch bearing point. The bearings, more common under a structure than at the roof, decouple the reinforced concrete pylonsthe shear wallsfrom the roof. This arrests movement and keeps the roof from generating forces back into the pylons, says the engineer.
Above the bearings, the steel arch box is filled with concrete. It serves as the focal point of the centroids of the post-tensioning ducts, the arch and the bearing point.
The stadium was designed for a magnitude 7.0 quake on the Seattle fault, which runs under the site, and a magnitude 9.0 quake on a subduction zone off the nearby coast. The fill site is only a few feet above the water line and the soil is subject to liquefaction. "It's like a giant bowl of jello," says Glover. Pile foundations are 60 to 90 ft deep.
Under such conditions, spanning 720 ft is a problem, he adds. The bearings solve it. Pylons are designed to move independently of the roof as much as 24 in. north to south. It's like the table cloth trick, says Glover. In a quake, the bearings can "pull the tablecloth out and even put it back in really fast, repeatedly, and the dishes stay put."
Friction and dishing of the bearing transmit forces from the pylon to roof. In a quake, they are very small. Under wind, they are large enough to keep the roof from sliding.
The bearings, added during the construction document stage, saved $1 million on pylon rebar, $500,000 on foundations and $1 million on roof steel, says Glover.
A minor downside is that any building systems that are continuous from the bowl to the roof have to accommodate the motion. For instance, back-of-bowl columns had to be free to swivel and expandable loops are needed for conduits.
The system has already been tested. The east roof was largely up but still in a "high state of vulnerability" because it was without final resisting elements, when a quake hit last February. The temporary restraint broke, the pylon shook and the megatruss hovered as intended.
Construction of the split-bowl stadium was done a half at a time, beginning with the east side. The Erection Co., Arlington, Wash., split megatruss erection into two stagesthe entire trichord followed by piecemeal erection of the A-frames and arch. Workers preassembled trichord truss sections with post-tensioning ducts. Beginning from one end, last February, the trichord was essentially erected in four, 180-ft spans between three 100-ft-tall shoring towers. Each span was set and bolted 3 to 4 in. higher than its final theoretical position. During erection, SWMB, under a separate contract with TEC, calculated erection workpoint geometry, giving the field crew continuous updates on the elevation for each subsequent span based on the previous one. Click here to view graph1
For stability and to resist overturning introduced into the trichord by the top-heavy arch, roof trusses were loaded into the system before arch erection, says Adam Jones, TEC president.Click here to view graph2
The erector then used the trichord's upper chords to support a work platform for the arch. The trichord was not designed to support loads of the arch until the keystone was in, so the engineer had to "beef up" some primary members, says Jones. Workers installed and welded 44-ft-long arch segments from each pylon toward the keystone, in a back and forth pattern.
SWMB's on-the-spot deflection calculations for the arch were complicated because the platform and trichord deflected up and down, typically 4 to 5 in., as they were loaded, says Jones.
The post-tensioning system consists of five tendons per truss, each 725 ft long and with 26 one-half-in.-dia strands with an ultimate capacity of 1.5 million lb. The staged post-tensioning of the megatruss lifted the trichord off the shores. Concurrently, TEC released the A-frame connections to the trichord, reducing the load. The trichord rose as expected as much as 3 in. in the center span, says Jones. TEC then jacked up the arch, through the A-frames, to pull up the trichord another 4 in. at the center span.
For both roofs, the final arch position was "well within tolerances," says Glover.
Because the tendon anchorage is designed to adjust prestressing force during the life of the structure and because standard grout weighed too much, grease was selected as corrosion protection for the tendons, says Rene Friedrich, project manager for post-tensioning contractor avar Construction Systems Inc., Campbell, Calif.
The timing of the operation was critical, for grease temperature had to be at least 135°F or it would not be fluid enough to pump. The nearest location for reheating the grease, originally trucked from Pennsylvania, was at least three hours away. Above-normal temperatures allowed the operations to go smoothly, says Friedrich.
While roof work was ongoing, other trades kept moving ahead on the bowl. Turner, actively involved from the start even with design, had sequenced and scheduled the job to minimize work force peaks and allow punch list work to begin in certain areas by October. "We built the entire stadium with 23 masons," says Gus Sestrap, Turner's project executive, by getting them started early and providing an even flow of work.
To get the best prices in a hot market, Turner put the documents out on the street during preconstruction. It then engaged a detailer to address certain contractors' concerns. The engineer incorporated the details into the bid set. "We went out to bid with a highly developed work plan," says Sestrap. As a result, "we got great pricing." The exercise also allowed Turner to confirm budgets.
The only surprise during construction was the quake, which stopped work for three days for reinspection. There was little damage and no net loss of time, says Sestrap.
|UNDER COVER Roof over heads of Seastrap (left), Magnusson Glover.|
Sestrap hopes to avoid overtime, keeping the five-day-per-week pace going until the end, in June. After all, it wouldn't be good for the budget.
He credits F&G with setting the tone for the job. "The success of a project is established in preconstruction" and in the contractor's early involvement with the design team, he says. Turner did several budget reviews and two constructibility reviews.
Architect Kerns agrees. "Allen built upon his experience with the Rose Garden in Portland, where he used the same architect and contractor, and he learned from the mistakes of the Safeco Field organization to set up this team structure and delivery process, and to work out the arrangement with the state," he says.
But F&G is a demanding client: "We've asked Turner and Ellerbe Becket to step forward and take a little more responsibility," says Collier. "If there is a problem, the contractor, owner and architect work to come up with the best resolution."
F&G does expect the team to do more with less, says Kerns. But for him, the experience "reinforced the benefits of a client-supported, team-based approach." From a process standpoint, he calls the job a success.