September/October 2023 - 15

Designed by architect Paul Thiry and constructed for the
1962 Seattle World's Fair, KeyArena was built at grade at the
plaza and street level and could accommodate up to 13,200
people. In 2017, the Seattle Landmarks Preservation Board
classified the venue as a local landmark. This distinction
required that the roof, curtain wall and exterior concrete
elements be preserved as part of the renovation, with little to
no impact to their aesthetics. This meant that any expansion
of the arena had to extend below grade, and to meet the
project's space requirements, it would have to extend about
65 ft (20 m) below the existing ground level. This required a
good deal of collaboration from architectural design firm
Populous; the project's structural, geostructural, façade and
construction engineer, Thornton Tomasetti; and the rest of the
project team.
Roof and Other Support Requirements
The original steel and concrete roof was supported vertically
by 20 concrete Y columns on shallow foundations spaced
approximately 60 ft (18 m) apart around its perimeter. The
lateral loads were resisted by pylons, consisting of two
chevron legs parallel with the perimeter concrete ring beam
and one buttress leg in line with the roof's steel arch truss.
Since the foundations below the chevron and Y columns
would be undermined by the excavation for the new event
level, a temporary roof support system would be needed to
support most of the roof's gravity load
and resist wind and seismic lateral
forces during construction.
To support the gravity loads, a
shoring frame was installed at each Y
column. A pair of W36 wide flange
beams sandwiched the Y columns with
a bearing pad placed under the concrete
roof structure. At each end of the beams,
a 36 in (914 mm) diameter shoring pipe
was socketed into the earth from the
existing ground level to the new arena
floor, some 85 ft (26 m) below. The weight
of the existing roof was transferred to the
pipe shores with jacks and shims prior to
undermining the existing footings. This
preloading resulted in nearly zero movement
of the rigid roof upon load transfer.
On the north, east and west sides of
the building, the buttress footing was
untouched by the excavation, except for
the removal of the support below the
chevron footings. The new parking
garage at the south side, however,
required the removal and temporary
support of the buttress footing. Since the
buttresses resist both thrust from the
steel roof arch truss and wind and seismic forces, the
approximately 3,300 kip (14,680 kN) force was resisted by a
" kickstand " using a cone-shaped steel plate sleeve around the
concrete buttress leg and large diameter pipe struts flying
above the garage construction. This assembly transferred the
load to the new foundation wall at the south property line 160 ft
(49 m) away, and trussed moment frames temporarily replaced
the chevrons at all four sides to resist vertical and lateral forces.
Wall System
Sports arenas come with a unique set of design challenges,
especially with regard to below grade foundation walls. With
the event space being an open bowl, there are no continuous
floor plates, which usually provide support to below grade
walls. In addition, the perimeter walls had to be constructed
around the existing roof and the large pylons and their
foundations, which extended from each side of the roof
structure. The foundation walls had to be built while other
construction operations, like underpinning to support the
existing roof, were also occurring. To meet the tight schedule,
excavation would have to start even before the entire
perimeter wall was in place. Finally, because the design of the
new arena was still evolving in the early phases of
construction, it became clear that some flexibility in the wall
system would be needed to accommodate changes as well as
the construction sequence.
The weight of the existing roof was transferred to the
pipe shores with jacks and shims prior to undermining
the existing footings. This preloading resulted in nearly
zero movement of the rigid roof upon load transfer.
Model of the shoring, temporary roof support and structure
DEEP FOUNDATIONS * SEPT/OCT 2023 * 15
September/October 2023

Table of Contents for the Digital Edition of September/October 2023
