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In 1996, Wall Financial Corporation proposed
to build the highest tower in Vancouver. They brought in a
team of consultants to design a building that would mesh with
both their multi-faceted objectives and the City of Vancouver's
requirement to maintain its protected view corridors. Utilizing
these criteria, architectural firm Busby+Associates
designed a slender glass tower, with a 70-foot wide elliptical
footprint and a 7:1 height to width ratio that has proven
to be one of the world's most unique tall towers. One Wall
Centre was awarded the 2001 skyscraper of the year by an independent
vote through skyscrapers.com.
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| Project Objectives, Solutions
and Achievements |
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This project was extremely complex.
Not only was it to be the first high-rise in Canada
to combine both hotel and residential floors, but its
phenomenal height-to-width ratio ensured that this landmark
building would not only garner a world-class reputation,
but that it would also pose world-class structural difficulties.
The original analysis of the structure
showed it to be far too flexible to be constructed as
it was first conceived. Also, wind tunnel tests indicated
that the storm winds coming across adjacent buildings
would buffet the tower. Vortices trailing its leeward
corner would suddenly move from one side to the other,
causing a sudden lateral shift load that would start
the tower vibrating. Once every 10 years, in an extreme
windstorm, that movement could sway the building enough
to cause discomfort for some occupants, perhaps even
motion sickness.
In addition, although the mix of hotel
and residential facilities created a wonderful opportunity
for the residential suite owners to utilize the hotel's
amenities and services, this combination of private
and commercial uses raised a myriad of security issues
that affected everything from the acoustics to the energy
control and mechanical systems to the development of
actual the elevator core. Separate lobbies and elevator
shafts were designed for each user-group to ensure privacy
and security for both guest and resident alike.
As well, to accommodate the 11,000 square foot ballroom,
five floors of parking and a 15-foot deep elevator pit,
the foundations had to be excavated to 75 feet below
grade into sandstone.
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Level of Difficulty
The structural engineer was presented with
the difficult challenge of a landmark 450-foot tall, 48-storey
building that was designed to be extremely thin to lessen
its impact on Vancouver's skyline, minimizing infringement
on any of the cities protected view cones. The resulting 7:1
height-to-width ratio created unique structural engineering
challenges that required innovative solutions to satisfy wind
and seismic requirements. Extensive advanced analysis using
several state-of-the-art software programs and intensive modeling
and simulation regimes were utilized to facilitate the design
process.
As well, a unique design challenge
was created by combining hotel and residential floor plans
in a single tower and marketed as the pinnacle of luxury and
convenience. Safety, security, economy and were all of utmost
importance when considering the needs of both hotel client
and residential occupant alike.
Technical Excellence and Innovation
To improve the buildings' stiffness, outriggers
were added to the design. Outriggers are beams that connect
the elevator core to the large columns at the buildings edge.
The system resists the forces of wind and earthquakes by spreading
the axial load of the building out to the columns. The outrigger
beams were heavily reinforced with large rebar and this project
showcases the first use of 55M with Lenton Terminators known
in Vancouver.
The idea was to design a large moment frame
to work with a central shear wall. The building contains a
concrete core whose walls are up to 3 feet thick. Two stairways
and six elevators were placed within the core whose floor
slabs are seven inches thick. Now the tower could be built,
but there was still the question of whether the occupants
would feel the motion of a strong windstorm.
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To solve the vibration problems shown
in the wind tunnel tests Glotman Simpson, with the help
of Toronto's RWDI, explored, designed and eventually
adopted an innovative and original damping system. Two
tuned liquid column dampers (TLCDs), each consisting
of a four-storey high, 50,000-gallon water tank, extending
nearly the full width of the tower, were placed at the
top of the 48-storey building. This amazing new system
not only solved the structural challenges presented
but also saved an estimated 2 million dollars in construction
costs compared to other conventional damping systems
like tuned mass dampers.
Each TLCD has a broad horizontal
chamber at the bottom with a column of water at each
end, thus resembling a cup within a cup. The dampers
work by allowing the water to move back and forth along
the bottom chamber of the tank and up into the columns.
When the building moves under wind loading, the water
moves back and forth in the opposing direction, transferring
its momentum to the building and counteracting the effects
of the wind vibration.
The liquid column dampers had
to be tuned exactly because once the tank walls had
been constructed only the water volume and a gate through
which the water flows could be changed to adjust the
vibration characteristics.
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| To confirm our analysis,
actual physical models were made of the tank, computer
models were made of the building, and field measurements
were taken of the tower when construction had reached
the 41st floor. Research showed that the natural frequency
of the building and TLCDs must match within 10% to be
effective. Our analysis and field measurements taken at
level 41 were within 3.2% agreement! |
To address the vertical (gravity)
load, the tower relies on just fourteen columns. Transfer
beams twenty-one feet deep located at level four transfer
this weight to just eight columns that pass through the lobby
level, thus creating a clearer, more open span for the lobby
of this prestigious hotel. The remaining eight columns make
a strong architectural statement using Agilia, a self-compacting
concrete recently introduced by LaFarge. Wall Centre was the
first commercial structure to utilize Agilia in Canada. The
Agilia finish is of such high quality that is could be mistaken
for a painted drywall wall. The steel-faced wood formwork
was laser guided while being raised ahead of the casting of
each floor.
Self climbing formwork was proposed
and supplied by Canada Scaffold Supply Ltd. and was incorporated
into the buildings design to save construction time.
The strength of a tower depends crucially upon its foundation.
Like the outrigger beams, the core footing extends the full
width of the tower and at 75-feet below-grade, is said to
be the deepest in Vancouver to date. The ground under the
tower was dense sandstone resulting in a tedious excavation.
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