Fabric arena shaped by past domes

Advances in design, techniques keep Minneapolis stadium’s cost
down

Engineering News Record, October 1, 1981

Refinements in the design and construction of stadiums with
air-supported fabric roofs continue to fulfill fabric’s promise of
simple, economical cover for large spans.

Many advances, some of them subtle, some significant, are evident in the
construction of the Hubert H. Humphrey Memorial dome in downtown
Minneapolis, scheduled for inflation this week. Changes in the fabric,
edge ring, support system and seating give the stadium’s owners a
flexible covered building, on schedule for a cost of about $52 million.

The Metrodome, which began construction in December, 1979, is scheduled
to be ready for the opening of the Minnesota Twins baseball season next
April. It will also be the home of the Minnesota Vikings and University
of Minnesota football teams and the Minnesota Kicks soccer team.

Metrodome took on rounded shape
for baseball, never before played beneath an air dome.

Apollo moonshot

At first glance the Metrodome will look much like the four air-dome
stadiums that have preceded it, a low-rising, white bubble crisscrossed
by diagonal restraining cables. But there are differences.

“This is a descendant of the Pontiac Silverdome,” says Donald Poss,
executive director of the Metropolitan Sports Facilities Commission,
owner of the stadium. “But it’s the Apollo moon shot compared to the
Mercury space program.”

The most obvious advance is in the scope of events the stadium can
support. This is the first time a baseball field has been put under an
air-supported fabric roof, observes David H. Geiger. Geiger’s firm,
Geiger Berger Associates, P C., New York City, which has designed the
roofs of all the air-supported domes built so far, added mechanical
design to its duties for the Minneapolis stadium.

The addition of baseball, with its large, nonrectangular playing field,
affected the shape of the stadium, says Stanley Korista, senior
structural engineer with Skidmore, Owings and Merrill, Chicago. SOM,
along with its associate, Setter, Leach & Lindstrom, Inc, Minneapolis,
were the A-Es for the Metrodome. Baseball prompted a stadium shape that
was “more toward a circular oval than a rectilinear oval,” Korista says.

In addition, baseball required a higher roof rise, Geiger says, 75 ft
versus the 50-ft rise of the Pontiac, Mich, Silverdome, which has
roughly the same span. One benefit of the higher rise is that it
allowed the engineers to design for the high snow loads of Minneapolis
by merely increasing the cables from the 3 in. used at Pontiac to3 3/8s
in. in diameter and spacing the cables slightly closer together than the
Silverdome’s spacing, Geiger says.

Sloped ring

Another advance is in the compression ring that runs around the upper
rim of the stadium to distribute the stress from the restraining cables.
Unlike the air-dome stadium built to date, the Metrodome has a ring that
slopes up to follow the angle of the rising roof fabric. The angled
design allows the compression ring to take the loads of the fabric roof
cables without many of the extra stresses induced in a flat-lying
compression ring.

Cables are fastened to compression
ring that angles upward to match fabric roof’s slope.

The sloping ring beam has a built-in gutter for catching rain and melted
snow runoff from the roof, and an air diffuser cavity underneath. The
stadium’s air is exhausted through ducts at the bottom of the enclosed
space to pull cold air off the floor and make the interior more
comfortable in the winter. The system grew out of observations of the
performance of the Pontiac Silverdome’s system, which left the stadium
chilly before it was changed, Bell says.

The Metrodome’s ring also has a simple support more advanced than
earlier ones, Geiger says. Traditionally, the fabric roof compression
rings have been supported by an earth berm or a precast wall that was
structurally independent of the stadium’s seating. This was necessary
because the expansion joints in the seating would have interfered with
the integrity of the compression ring.

Tiedowns and rocking supports
allow compression ring to move.

To avoid building two structures instead of one in the Metrodome, Geiger
Berger supported the compression ring directly from the seating. To give
the compression ring the necessary independence, the engineer used pairs
of pin-ended steel supports that form movable bents and allow the ring
to float above the stands.

Each of the bents is braced by a 12-in.dia. steel tiedown that helps
pass the roof cable loads into the stadium’s foundation. The tiedowns
also act as lateral springs that keep the ring in equilibrium, says
Geiger Berger’s Paul A. Gossen. As the compression ring moves in
response to stress, as much as 3 in., the tiedown bends in and out and
forces the ring back into its equilibrium position. “The spring can take
tension or compression,” Gossen says. “lt’s a self-adjusting mechanism.”

Double skin

The 9 1/2 acres of fabric, manufactured and installed by Birdair
Structures, Inc., Buffalo, N.Y., is actually two layers, the largest
expanse ever done in this way. The outer layer is Teflon-coated
fiberglass and the inner is a proprietary acoustical fabric. The dead
air space between the layers insulates the roof, Birdair says, and warm
air will be blown between the layers in the winter to melt snow that has
accumulated on the roof.

Another advantage to the acoustical inner lining, Geiger says, is that
it allowed the Metrodome’s owners to avoid acoustical baffles hung from
the ceiling. Baffles give the illusion of dark beams against the
translucent fabric, Geiger says, and this interferes with a baseball
player’s ability to follow the progress of a fly ball. The Metrodome’s
lights are hung from the roof’s cables by winches so that they can be
raised and lowered for different events.

The stadium will hold 63,000 for football, and soccer, and 55,360 for
baseball. Those numbers include 8,000 seats that move hydraulically on
tracks to adjust to the requirements of different events. The seats
shift in a matter of minutes, says sports commission administrative
assistant Jerry Bell, and the whole stadium can be converted in a matter
of hours. Baseball and football can be scheduled for the same day.

Stadiums in Denver and other cities may have more movable seats than the
Metrodome, Poss says, but they take much longer to adjust than the
Minneapolis stadium’s 8,000.

Checks and balances

There is more to the Humphrey Metrodome Stadium than its engineering
design, according to the project team. “There are two significant things
about the stadium,” Poss says, “the technology and the technical
management of it.”

Early in the schematic phase of the project, the sports commission hired
architect SOM and the joint-venture construction managers Barton Malow,
Detroit, and Construction Management Services, Inc, Minneapolis. Poss
says the commission gave the architect and the construction manager
equal power in a checks-and-balances system, the sports commission
acting as final arbiter. “We didn’t hire an architect, step back and
watch the building go up,” he says.

The construction management joint venture is now overseeing about 80
separate prime construction contracts for the fast-track construction
process, says Barton Malow’s John R. Gockel. These include a
$6.2-million contract for the major concrete superstructure held by the
Minneapolis-based Knutson Construction Co.

The team whittled away at the design and construction schemes for the
stadium until it had reached the basic stadium cost of $825 per seat
“There has been more financial fat wrung out of this building than any
building in this country of comparable size,” Poss claims.

The team, which had already decided that the stadium would have to be
roofed, chose fabric because it would be able to handle Minnesota’s
harsh weather with fewer leakage problems, would be cheaper to build and
repair and wouldn’t raise the specter of long-span roof collapse.

The team also decided on a playing field sunk 47 ft below grade so that
the contractors could build the lower-deck seats on sloping grade and
the temperatures of the ground would keep the stadium cool in summer and
warm in winter. The sports commission sited the stadium on a 20-acre lot
in the downtown district to attract bigger crowds and to take advantage
of existing parking left vacant at night and on weekends, Poss says.

According to SOM’s Korista, it is this gradual evolution of the
Metrodome that makes the stadium noteworthy, not any big breakthrough.
“There’s not something that’s totally different, just refinements,
keeping things simple,” he says.