Soldier Field job relied on 3-D model to fit new structure
(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.
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."
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
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,"
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
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
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"
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,
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
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,"
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."