7 Modern Wonders
of the World
The CN Tower, located
in Downtown Toronto, Ontario, Canada,
is a communications and observation
tower standing 553.33 metres (1,815.4
ft) tall. It surpassed the height
of the Ostankino Tower while still
under construction in 1975, becoming
the tallest free-standing structure
on land in the world. On September
12, 2007, after holding the record
for 31 years, the CN Tower was surpassed
in height by the still-under-construction
Burj Dubai. It remains the tallest
free-standing structure in the Americas,
the signature icon of Toronto's skyline,
and a symbol of Canada, attracting
more than two million international
visitors annually.
CN originally referred
to Canadian National, the railway
company that built the tower. Following
the railway's decision to divest non-core
freight railway assets, prior to the
company's privatization in 1995 it
transferred the tower to the Canada
Lands Company, a federal Crown corporation
responsible for real estate development.
Since local residents wished to retain
the name CN Tower, the abbreviation
is now said to expand to Canada's
National Tower rather than the original
Canadian National Tower; however,
neither of these names is commonly
used.
In 1995, the CN Tower
was declared one of the modern Seven
Wonders of the World by the American
Society of Civil Engineers. It also
belongs to the World Federation of
Great Towers.
Construction
View of Downtown Toronto from the
CN TowerConstruction on the CN Tower
began on February 6, 1973 with massive
excavations at the tower base for
the foundation. By the time the foundation
was complete, 56,000 t (61,729 ST;
55,116 LT) of dirt and shale were
removed to a depth of 15 metres (49.2
ft) in the centre, and a base incorporating
7,000 cubic metres (9,156 cu yd) of
concrete with 450 tonnes (496 ST;
443 LT) of rebar and 36 tonnes (40
ST; 35 LT) of steel cable had been
built to a thickness of 6.7 metres
(22.0 ft). This portion of the construction
was fairly rapid, with only four months
needed between the start and the foundation
being ready for construction on top.
To build the main support
pillar, a hydraulically-raised slipform
was built at the base. This was a
fairly impressive engineering feat
on its own, consisting of a large
metal platform that raised itself
on jacks at about 6 metres (19.7 ft)
per day as the concrete below set.
Concrete was poured continuously by
a team of 1,532 people until February
22, 1974, during which it had already
become the tallest structure in Canada,
surpassing the recently built Inco
Superstack, which was built using
similar methods. In total, the tower
contains 40,500 cubic metres (52,972
cu yd) of concrete, all of which was
mixed on-site in order to ensure batch
consistency. Through the pour, the
vertical accuracy of the tower was
maintained by comparing the slip form's
location to massive plumb-bobs hanging
from it, observed by small telescopes
from the ground. Over the height of
the tower, it varies from true vertical
accuracy by only 29 millimetres (1.1
in).
Brackets being raised, August 1974
The CN Tower as seen from its baseIn
August 1974, construction of the main
level commenced. Using 45 hydraulic
jacks attached to cables strung from
a temporary steel crown anchored to
the top of the tower, twelve giant
steel and wooden bracket forms were
slowly raised, ultimately taking about
a week to crawl up to their final
position. These forms were used to
create the brackets that support the
main level, as well as a base for
the construction of the main level
itself. The Sky Pod was built of concrete
poured into a wooden frame attached
to rebar at the lower level deck,
and then reinforced with a large steel
compression band around the outside.
The antenna was originally
to be raised by crane as well, but
during construction the Sikorsky S-64
Skycrane helicopter became available
when the United States Army sold off
theirs to civilian operators. The
helicopter, named "Olga",
was first used to remove the crane,
and then flew the antenna up in 36
sections. The flights of the antenna
pieces were a minor tourist attraction
of their own, and the schedule was
printed in the local newspapers. Use
of the helicopter saved months of
construction time, with this phase
taking only three and a half weeks
instead of the planned six months.
The tower was topped off on April
2, 1975 after 26 months of construction,
officially capturing the height record
from Moscow's Ostankino Tower, and
bringing the total mass to 118,000
tonnes (130,073 ST; 116,136 LT).
Two years into the construction,
plans for Metro Centre were scrapped,
leaving the tower isolated on the
Railway Lands in what was then a largely
abandoned light-industrial space.
This caused serious problems for tourists
to access the tower. Ned Baldwin,
project architect with John Andrews,
wrote at the time that "All of
the logic which dictated the design
of the lower accommodation has been
upset," and that "Under
such ludicrous circumstances Canadian
National would hardly have chosen
this location to build."
Opening
The CN Tower opened to the public
on June 26, 1976, although the official
opening date was October 1. The construction
costs of approximately CDN$63 million
($330 million in 2005) were repaid
in fifteen years. Canadian National
Railway sold the tower prior to taking
the company public in 1995, when they
decided to divest themselves of all
operations not directly related to
their core freight shipping businesses.
As the area around the
tower was developed, particularly
with the introduction of the Metro
Toronto Convention Centre and Rogers
Centre (known as the SkyDome before
2005), the former railway "wasteland"
disappeared and the tower became the
centre of a newly developing entertainment
area. Access was greatly improved
with the construction of the SkyWalk
in 1989, which connected the tower
and SkyDome to the nearby railway
and subway station, Union Station.
By the mid-1990s, it was the centre
of a thriving tourist district. The
entire area continues to be an area
of intense building, notably a boom
in condominium construction in the
2000s.
From 1997 to January
2004, TrizecHahn Corporation managed
the building and instituted several
expansion projects including a $26
million entertainment expansion and
revitalization that included the addition
of two new elevators (to a total of
six) and the relocation of the staircase
from the north side leg to inside
the core of the building, a conversion
that also added nine stairs to the
climb.
Structure
A bolt of lightning strikes the CN
Tower. The CN Tower is struck by lightning
at least 40 to 50 times annually compared
to other places in Toronto which are
struck, on average, 2 times per km2
(5 times per sq mi) every year.The
CN Tower consists of several substructures.
The main portion of the tower is a
hollow concrete hexagonal pillar containing
the six elevators, stairwells, and
power and plumbing connections. On
top of this is a 102-metre (334.6
ft) tall metal broadcast antenna,
carrying TV and radio signals. There
are two visitor areas: the main deck
level (formerly known as SkyPod) located
at 346 metres (1,135 ft), and the
higher Sky Pod (formerly known as
"Space Deck") at 446.5 metres
(1,465 ft),[9] just below the metal
antenna. The hexagonal shape can be
seen between the two areas; however,
below the main deck, three large supporting
legs give the tower the appearance
of a large tripod.
The main level is seven
stories, some of which are open to
the public. Below the public areas
— at 338 metres (1,108.9 ft)
— is a large white donut-shaped
radome containing the structure's
microwave receivers. The glass floor
and outdoor observation deck are at
342 metres (1,122.0 ft). The glass
floor has an area of 24 square metres
(258 sq ft) and can withstand a pressure
of 4,100 kilopascals (595 psi). The
floor's thermal glass units are 64
millimetres (2.5 in) thick, consisting
of a pane of 25-millimetre (1.0 in)
laminated glass, 25 millimetres (1.0
in) airspace and a pane of 13-millimetre
(0.5 in) laminated glass. Some people
experience acrophobia when standing
on the glass floor and looking down
at the ground 342 metres (1,122.0
ft) below. In 2008, one elevator was
upgraded to add a glass floor panel,
believed to have the highest vertical
rise of any elevator equipped with
this feature. The Horizons Cafe and
the lookout level are at 346 metres
(1,135.2 ft). The 360 Restaurant,
a revolving restaurant that completes
a full rotation once every 72 minutes,
is at 351 metres (1,151.6 ft). When
the tower first opened, it also featured
a disco named Sparkles, billed as
the highest disco and dance floor
in the world.
The Sky Pod is the second-highest
public observation deck in the world,
surpassed only by the Shanghai World
Financial Center. On a clear day,
it is possible to see 100 to 120 kilometres
(62–75 mi) away, to the city
of Rochester across Lake Ontario in
the United States, the mist rising
from Niagara Falls, or the shores
of Lake Simcoe.
A metal staircase reaches
the main deck level after 1,776 steps,
and the Sky Pod 100 metres above after
2,579 steps; it is the tallest metal
staircase on Earth. These stairs are
intended for emergency use only and
are not open to the public, except
for three times per year for charity
stair-climb events. The average climber
takes approximately 30 minutes to
climb to the base of the radome, but
the fastest climb on record is 7 minutes
and 52 seconds in 1989 by Brendan
Keenoy, an Ontario Provincial Police
Officer. In 2002, Canadian Olympian
and Paralympic champion Jeff Adams
climbed the stairs of the tower in
a specially designed wheelchair.
[2]
A trip up and view from
the CN Tower in Toronto Canada from
July 2002.
