|GRANDSTANDING Soldier Field job relied on 3-D model to fit new structure into old. |
(Image courtesy of Thornton-Tomasetti Engineers)
Shoehorning a football field and seating bowl, complete with suites, into the horseshoe-shaped perimeter bay of a narrow venue built 80 years ago for track and field was enough to drive many toward distraction. Then, pile on the charge to gut and reconstruct the landmark sports facility in four to six months less than is comfortable and customary for National Football League stadiums. And top that with an unprecedented foray, at least on this scale in the U.S., into the largely alien world of three-dimensional computer modeling for design and fabrication of the job's 13,000-ton structural steel frame. It's not surprising that the push toward a "paperless" project at the $365-million makeover of Chicago's Soldier Field caused minor shock waves along the shores of Lake Michigan.
The structural engineer got the green light to create a
3-D model and share it with the steel fabricatorfor connection detailing and to drive its computer numerically controlled (CNC) fabrication equipmentbecause the steel structure and its complex radial geometry was on the critical path of the job's 20-month, fast-track schedule. The need to fit the new 61,500-seat bowl snugly into the historic colonnade structure, like a jumbo egg in a small egg cup, also drove the decision to use a 3-D approach. "If you can model it in 3-D, you can build it," says Joseph G. Burns, principal in charge for project structural engineer, the Chicago office of Thornton-Tomasetti Engineers. Click here to view diagram
Smart digital models provide exact representations of structural components and recognize members such as columns and beams. They hold the promise for increased construction speed and quality.
If created correctly, 3-D models are more accurate than 2-D drawings because they rely on exact dimensions and geometries. A 3-D model, shared between designer and fabricator, wipes out the need to create shop drawings, a process vulnerable to human error. Instead, the steel detailer simply enhances the engineer's design model by adding all the elements, such as bolt holes, bolts, angles and plates required for fabrication and erection.
The process minimizes requests for information and reduces the time to answer them. The engineer then approves the detailed model. The fabricator uses the approved model to produce the steel members and the erector to build the structure.
In theory, everybody, from the owner to the steel erector, wins. But at Soldier Field, the process was rife with tension. To work, 3-D modeling requires a shift in the work flow. The structural engineer is dependent on the architect, during design development, for much of the precise dimensions required for the model. And the architect is not accustomed to setting the details in stone at such an early stage.
"For this kind of large, complex project, the 3-D model is the wave of the future," says Joseph Caprile, principal of Lohan Caprile Goettsch, part of project architect, the LW&Z Joint Venture, which also includes Wood & Zapata, Boston. "But the way we develop drawings is going to have to change," says Caprile. "The architect and engineer are going to have to work differently."
On Soldier Field, which could be studied as a guinea pig for 3-D modeling on a large, fast-tracked, design-bid-build job, there was "definitely a high learning curve," says Caprile. That mostly emanated from the inexperience of many team members with Xsteel, the project's modeling software produced by Tekla Corp., Espoo, Finland. "We spent a lot of time" up front "in coordination of the steel and architecture," Caprile says. It was "intense," he adds, but once done, the pieces fit together.
Says Sid Dickerson, senior vice president in the Irving, Texas, office of project steel contractor Hirschfeld Steel Co. Inc.: "We managed to muddle through and we are literally making the goals, but along the line, we had a lot of argument."
|CHICAGO LANDMARK New seating bowl fits snugly into Soldier Field's historic colonnade. (Photo courtesy of LW&Z Joint Venture/Anthony Montalto)|
Subsequent to the Soldier Field experience, Dickerson says he had been told by at least two contractors, which he declines to name, not to use Xsteel on other jobs.
The steel erector says the frame went together better than expected, but says the success of the job hinged on the ironworkers' dedication and six to eight months of planning. That is at least twice the usual amount. "It wasn't a simple process," says OB O'Brien, owner of erector Danny's Construction Co. Inc., Shakopee, Minn. "We planned, plotted and schemed in coordination" with the construction team, he says.
The subject of 3-D modeling is a hot button for the job's local construction manager-general contractor. The Turner/Barton Malow/ Kenny Joint Venture declines to comment on the use of Xsteel or on the project in general, under orders from the developer, the Chicago Bears football team. A project spokesman, Barnaby Dinges, attributes the silence to pending "issues relating to construction and engineering." The developer's representative, Alice Hoffman of the local Hoffman Management Partners, will say no more than Xsteel had "pluses and minuses."
In a partial postmortem, it is tough to separate out the causes of the job's "issues." Sources say the complexity combined with the fast-track schedule would have resulted in disagreements even if the job had been done traditionally.
Thornton-Tomasetti gets an "A" for its effort to use Xsteel even though its vision was only partially realized. "Did Soldier Field reap all the benefits of object-based modeling? No," says Dickerson. "But did they get benefit? Yes," he adds.
In the end, the steel portion of the job was conducted in a hybrid manner. The contract documents were issued as traditional 2-D drawings with an Xsteel model. "The 3-D model was used as a means of communication between the design and construction team from design development onward," says Burns.
The rebuilding of the city-owned stadium is part of a $606-million civic project. The development also includes two underground parking garages and 17 acres of parkland on the west side of Lake Michigan. Lohan is lead architect on the lakefront redevelopment and Wood & Zapata is the lead on the stadium.
The budget includes $200 million from the Chicago Bearssaid to be the largest contribution ever to a publicly owned stadiumand $406 million from the sale of Illinois Sports Facilities Authority bonds, backed by revenue from an existing city hotel-motel tax. Though the Chicago Park District will own the stadium and expects to net at least $10 million annually from it, the developer is responsible for any construction cost overruns.
The saga of Soldier Field is a long one. The Bears moved there "temporarily" more than 30 years ago and looked long and hard for another location. Eventually, the city and the Bears agreed on a partial preservation plan for the stadiumgutting the center and keeping the colonnade. It was a compromise, for the building is much narrower than modern- day football venues. The distance inside the horseshoe is just under 600 ft at the 50 yard line, rather than a typical 700 to 750 ft. That made it a challenge to fit the 61,500 seats within the colonnade.
Soldier Field is Wood & Zapata's first stadium. "We had no preconceived notions," says Anthony O. Montalto, a W&Z associate. That freed the firm to develop an asymmetrical seating bowl to reap the spatial rewards of separating the grandstand from the suites building.
To provide enough seats, keep the seating angle to 33° and to avoid imposing loads on the historic structure, the designers cantilevered the upper portion of the grandstand 40 ft out and over the west colonnade. The engineer specified tuned mass dampers to improve creature comfort by controlling vibrations of the 55-ft backspan.
The frame has other cantilevers. The north scoreboard frame cantilevers 120 ft to its side and the south scoreboard cantilevers 90 ft. And balcony seating, close to the field, also cantilevers to bring fans closer to the field.
A braced steel frame was picked for the grandstand and suites, given steel's speed of erection, says David P. McLean, Thornton-Tomasetti's senior associate.
To create a manageable 3-D model, the engineer split the structure into four pieces. The four sections were still too cumbersome for the fabricator, so it split the file into more than 60 mini models, which were detailed, approved by the design team and fabricated in the order of steel erection.
After Hirschfeld came on board, Thornton-Tomasetti gathered representatives from the architect, the contractor, the fabricator and its detailer, Steelplan, Perth, Australia, to determine the best method to review the model and get comments back. In the approval process, the designers also had 2-D erection drawings generated from the Xsteel model. People are reluctant to give up on the traditional medium, says Burns.
Demolition began on Jan. 21, 2002, at 12:01 a.m. The approach to construction was to overlap phases, working both sides of the stadium south to north, "almost like two jobs," says Hoffman, the developer's representative. The idea was to accelerate the first 100 workers rather than the last 1,000. At times, work went on around the clock. A major goal, which was achieved, was to enclose the suites by January 2003 so finish work could begin. To keep the project moving ahead, Hoffman had the job's key decision-makers full-time at the site.
The team did other things to advance the schedule, including starting infrastructure early and performing $2 million worth of asbestos and lead abatement inside the old stadium during the last home season. The team also did some early buying of long lead items, such as the steel.
Substantial steel erection finished in September after five months, two weeks ahead of schedule. To have any chance of getting the work done on time, Danny's insisted on having the field level clear for its two crawler cranes. The erector worked both sides of the seating bowl, in layers, generally moving south to north. To avoid interfering with steel erection, the precast seating risers followed the steel at night, using the same crawler cranes.
O'Brien says the west side grandstand raker trusses were difficult to erect because of the radial geometry. But the scoreboards were even trickier in terms of fit-up. Some elements had to be field welded instead of bolted, as planned. Luckily, the scoreboards were not on the critical path, says O'Brien. Though some changes are still being negotiated, O'Brien says "it was a good job" for Danny's.
There were some problems coordinating with the suite building's skin. "The 3-D model didn't take into consideration certain interfaces between the structural steel and the exterior wall" of the suites, says O'Brien.
The bulk of the $20-million curtain wall package consists of 100,000 sq ft of conventional high-rise building units facing the lake and 45,000 sq ft of suite windows. These are a highly customized, mullion-free system consisting of sloped units that cantilever from moment connections at the edge of the slab. There is also an operable portion of the window, above the fixed panel, that opens out like an awning to allow people to hear the game. Of 1,100 curtain wall units, 450 are unique.
The architect designed the skin in two dimensions. The supplier then created a 3-D model using CATIA software and used the model to drive fabrication equipment. "When there were problems with the 2-D version, we sent 3-D models to the architect," which were then coordinated with the 2-D version, says Michael Budd, a vice president of the skin supplier, Permasteelisa/Gartner, Mendota Heights, Minn.
Permasteelisa/Gartner was able to pull the Xsteel structural wire frame models into its 3-D model via a neutral file format and perform document coordination. "The Xsteel model was helpful," says Budd, as long as it was kept up to date.
Budd says his portion of the job got started late because of difficulties getting the 3-D geometry from the architect. He doesn't think the delay will affect the end-date.
|COMPLEX GEOMETRY |
Scoreboard cantilevers (top),
curved walls (center),
radial geometry and tight schedule
led to use of 3-D models. grandstand.
(Photos courtesy of Thornton-Tomasetti Engineers
/ David P. Mclean)
To date, project-wide, there are 48 change orders and more than 4,064 RFIs. The first football game is set for Sept. 29, and, with the project roughly 75% complete, the big push is on. To make the schedule, Hoffman says the developer expects to spend $25 million each month until the end. "We're going to be right on budget," she says.
At Soldier Field, the awkward but giant steps toward a paperless project were driven by the structural engineer. Sources agree that to get the most value from 3-D modeling, the effort must be driven from the top down. And to make the process work best, Budd and Dickerson agree that the steel and skin suppliers should be brought into the design process earlier than is customary. "Even if the bid package had been awarded two to three months earlier, it would have made a big difference," says Budd, in terms of avoiding coordination problems.
Budd thinks it is going to be a long time before 3-D models completely replace 2-D drawings. Others disagree, predicting anywhere from five to 10 years until smart models are so pervasive that 2-D drawings will be a dim memory.
"Model-based programs, especially linked to computer numerically controlled fabrication equipment, have the potential to change the delivery systems" for commercial building projects, predicts Dickerson. "If I'm right, all the heat [the engineer] might be taking now on Soldier Field is going to come back in rewards."