Engineering Inc - July/August 2017 - 38

Two-Tower Project
Leverages Seismic Design
PROJECT: LINCOLN SQUARE
EXPANSION
BELLEVUE, WASHINGTON
FIRM: HART CROWSER, INC.,
SEATTLE, WASHINGTON, AND
CKC STRUCTURAL ENGINEERS
BELLEVUE, WASHINGTON

he Lincoln Square Expansion (LSE)
includes two 450-foot-tall towers-currently among the tallest in
the area-connected by a three-level
retail podium building with six levels of
subterranean parking for 2,200 cars over
a two-block site in Bellevue, Washington.
The 2.6-million-square-foot mixed-use
project includes a 41-story tower featuring

an upscale hotel, luxury apartments and a
31-story office tower providing 700,000
square feet of Class A office space. Building
height, excavation depth, regional seismicity
and other factors dictated a peer-reviewed,
performance-based design that required
numerous innovations. These included an
optimized shoring system, record-breaking
mat foundations, state-of-the-art seismic
design, subterranean post-tensioned slabs
and shrinkage control as well as steel fiber
reinforced concrete coupling beams.
All structures on the site were demolished before construction with the exception of the Bellevue Arts Museum (BAM)
on the corner. Constructing the LSE's six
levels of subterranean parking required an
80-foot-deep excavation that would not
undermine the BAM's foundation or adjaThe office tower and
the hotel/residential
tower in the Lincoln
Square Expansion
are two of Bellevue's
tallest buildings.

Doug Lindquist

Mark Whiteley

cent streets and utilities. The design team
developed an optimized shoring system
using a combination of soil nails, tensioned
soil nails, soil nails with vertical elements,
and soldier piles with tiebacks to accommodate the garage-level vehicle tunnel.
The design team used performancebased seismic design (PBSD) procedures
for the two towers. This is the first project
in Bellevue to consider amplification effects
of the geologic depression known as the
Seattle Basin for seismic design. In the
PBSD process, the geotechnical engineer
evaluates the seismic hazard, then selects
and scales ground motions representative of
the earthquake faults that contribute most
to the site hazard.
Doug Lindquist, principal geotechnical
engineer with Hart Crowser, was part of a
team of engineers that went
to Japan to gather data after
the Tōhoku earthquake and
tsunami. "We have that same
hazard off our coast, and
we were able to incorporate
some of the Tōhoku earthquake ground motions into
our design," Lindquist said.
Above grade steel fiber
reinforced concrete in 87
percent of the 392 coupling
beams eliminated the need
for diagonal reinforcement
and made building the towers' core systems much easier.
"With a steel fiber dosage of 200 pounds per
cubic yard, using a selfconsolidating concrete produced workability similar
to a traditional high-slump
mix," said Mark Whiteley,
principal with CKC Structural Engineers,
the project's structural engineer.
Tenant build-outs in the Lincoln Square
Expansion were underway in 2017, the
hotel was expected to come online in May
and residences in the fall of 2017. n
Tom Klemens is a freelance writer based
near Chicago and is a registered Professional
Engineer in Illinois.

ENGINEERING INC.

JULY / AUGUST 2017

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