For almost 50 years, the CN Tower has dominated
the skyline here in Toronto, Canada as one of the tallest communication towers ever built,
and as one of the greatest engineering achievements of the 20th century.
Standing at just over 553 m, the tower opened to the public in the summer of 1976 as the
tallest free-standing structure on Earth, and it held that title for more than 3 decades
before being surpassed by the Burj Khalifa in 2007.
Today, it remains as the tallest free-standing structure in the Western hemisphere, and also
among the top 10 tallest in the world, despite being more than 40 years old.
The CN Tower is an impressive feat of modern engineering, but it becomes even more impressive
when you realize that it was mostly designed by hand in the 1960’s and 70’s before
computers were available for structural design. The design and construction phases spanned
more than 9 years combined, requiring a number of innovative solutions to challenging problems
that had never been encountered before on a project of this scale.
In 1995, the tower’s engineers, architects, and construction crew were formally recognized
for their accomplishments when it was classified as one of the seven wonders of the modern
world by World by the American Society of Civil Engineers.
The CN Tower is Canada’s most celebrated icon, and in today’s video we are going
to take a look at its design and how it was constructed, starting from the foundation
and working our way to the top of the antenna more than half a kilometer above the ground. Today, the city of Toronto and the Greater Toronto Area make up one of the largest metropolitan
areas in the world, with a combined population of more than 6 million people.
The downtown core is positioned right on the shore of Lake Ontario, consisting of densely
packed skyscrapers and high-rise condos with the CN Tower serving as the centerpiece of
the skyline. This part of the city looked completely different
back in the 1960’s, however, which is when planning for the CN Tower first began.
It was during this time that Toronto was experiencing a large construction boom and the first skyscrapers
were starting to appear around the city. These tall buildings quickly became problematic
for local radio and television broadcasters because the existing transmission towers in
the area were too short to broadcast over them.
The solution to this problem was to build a massive communications tower that would
be able to transmit signals over any new skyscraper built within the foreseeable future.
To accomplish this, the design requirements called for UHF transmitters at a height of
338 m for television broadcasting, and VHF transmitters at heights ranging from about
460 m to 540 m primarily for FM radio broadcasting. These specifications governed much of the
tower’s design, from its size and geometry to the selected materials and construction
methods. The decision was made to exceed the minimum
height requirement of 540 m, however, in order to make the CN Tower the tallest free-standing
structure ever built and to demonstrate the strength of Canadian industry.
To help cover the cost of the $63 million project, which is about 350 million in today’s
dollars, several tourist attractions were also incorporated into the design to bring
in visitors from all around the world. These include a 360° revolving restaurant
more than 350 m in the air, as well as several observation decks offering incredible views
of Toronto and Lake Ontario. The tower consists of a 450 m tall concrete
shaft with a Y-shaped cross-section that tapers along its height, and this is topped off with
a 104 m steel antenna mast. The UHF transmitters are positioned at the
base of a 7-storey pod structure, which houses the revolving restaurant, 2 observation decks,
and other facilities. There is a second smaller pod known as the
skypod located at the top of the concrete shaft, which features a single observation
deck, and the VHF transmitters are positioned above along the height of the steel mast.
The tower was built by The Foundation Company of Canada for Canadian National, commonly
abbreviated as CN, in collaboration with architects John Andrews and Roger Du Toit, and structural
engineering firm Nicolet Carrier Dressel & Associates. Construction began in February of 1973, and
took 3 years and 4 months to complete, with crews working 24 hours a day and 5 days a
week, totaling some 32 million man-hours between the 1,537 workers.
Just like any vertical construction project, work started with the foundation before moving
upwards, and this is where we will begin taking a more detailed look at the tower’s design.
The reinforced concrete foundation is 5.5 m thick and Y-shaped, with three wings extending
just over 33 m from the center to accommodate the three legs of the tower.
Its primary purpose is to support the self-weight of the tower as well the overturning moments
and shear forces caused by wind, and to distribute these forces to the ground below.
More than 56 metric tonnes of earth and rock had to be excavated in order to construct
the foundation, thus allowing it to rest directly on solid bedrock which consists of horizontal
layers of shale. The self-weight of the entire structure is
just under 118,000 metric tonnes including the foundation, and the average normal stress
applied onto the bedrock under this load is about 575 kPa.
Most concrete foundations have rectangular cross-sections with straight vertical edges,
however this can lead to high shear stress concentrations in the soil due to sudden changes
in normal stress. This was not acceptable for the CN Tower because
the loads are so large, and the thin layers of shale bedrock would be susceptible to fracture.
Fracturing of the bedrock under the foundation could lead to progressive failure and instability
of the tower, and this was identified as one of the critical failure modes for the entire
structure. To mitigate this problem, the edges of the
foundation were simply tapered from a thickness of 5.5 m in the middle to 1.2 m around the
outside. The reduction in thickness corresponds to
a decrease in stiffness around the perimeter, thus reducing the stresses in the bedrock
underneath the tapered portions. This ensures that the layers of shale will
remain intact, providing a stable base for the foundation and the rest of the tower.
Although the self-weight of the tower acts vertically on the foundation and induces compressive
stress, the tapered shape of the concrete shaft generates spreading forces that also
induce tensile stress in the horizontal direction. Concrete is a material that is very strong
in compression, but weak in tension, and so reinforcement such as steel rebar is often
used to carry tensile forces within concrete structures.
Reinforced concrete is still prone to cracking under tension, however, which is undesirable
because water can enter the cracks and cause damage due to freeze-thaw cycles and corrosion
of steel rebar. This was a major concern for the CN Tower’s
foundation since it was going to be built below the water table, and so the concrete
was pre-stressed using a post-tensioning system in order to counteract the tensile forces
and reduce the probability of cracking. After the concrete was poured, 48 high-strength
steel cables were fed through cable ducts which were pre-installed inside the formwork
in a symmetric triangle pattern. Each cable is made up of many individual strands,
and each strand consists of 7 steel wires with a tensile strength of 1,862 MPa.
The 48 cables were tensioned using hydraulic jacks and then anchored to the exterior of
the foundation, thus providing equivalent compressive forces within the concrete.
The final load applied to each cable was just under 1,800 kN, and the average 2-dimensional
compressive stress within the foundation was about 6.9 kPa prior to long-term pre-stress
losses. A portion of the initial pre-stress was inevitably
lost over time due to concrete shrinkage and creep, as well as relaxation of the steel
cables, however this was accounted for in the design to ensure that the foundation will
always remain in compression. To prevent cracks from developing during the
curing process, a concrete mix with a modest dosage of cement was used so that shrinkage
and large temperature gradients could be minimized. This was particularly important due to the
foundation’s size, because large amounts of heat would have accumulated deep within
the concrete due to the chemical reactions that occur during curing.
Another design element that helped to alleviate the buildup of heat is the fact that the foundation
is not solid, but rather hollow on the inside with a series of caverns that extend from
the center and into each of the wings. The caverns are approximately 5.3 m wide by
2.3 m tall, and they contain the anchorage points that were used for post-tensioning
the concrete shaft later on in the construction process.
The workers were able to enter the caverns through four 1.8 m diameter access holes in
the top of the foundation, and they performed their post-tensioning operations from inside
while being protected from all the other work still going on overhead.
Despite being hollow, the ’s foundation still required more than 7,000 cubic meters
of concrete and 450 metric tonnes of reinforcing steel, and it took roughly 3 months to construct.
As soon as it was finished, work then began on the 450 m tall concrete shaft.
This is the primary structural component of the tower, providing support for the broadcasting
equipment and other facilities, while also allowing vertical access for people and services.
The shaft consists of a hollow hexagonal prism which forms the central core, and three elevator
shafts protrude off the prism with glass panels facing towards the outside.
Between the elevator shafts, three tapered support legs extend away from the central
core, and they gradually increase in size from the top of the tower to the bottom.
At the base, a circumscribed circle can be drawn around the supports with a diameter
of about 54 m. The tapered legs extend to a height of 342
m, which coincides with the base of the main pod structure, and the elevator shafts continue
above this point to a height of approximately 382 m.
The central core is the only component that spans the entire height of the shaft, extending
beyond the legs and exterior elevators to a height of 450 m.
The core houses all the necessary service lines for the tower, as well as a staircase
that was added in 1996 for emergency access and egress.
One of the three elevator shafts originally contained stairs when the tower first opened,
but they were re-located inside the core so that two additional elevators could be added.
The various walls that make up the concrete shaft range in thickness from just under ½
a meter to 1.5 m, and they are all vertical and straight with the only exception being
the tapered outer edges of the support legs. The curvature of the legs was carefully optimized
for structural efficiency, allowing the shaft to carry gravity and wind loads with a minimal
amount of material. The shape is similar to an inverted icicle
or a tall tree trunk, and an equation describing the optimal curvature can be derived mathematically
based on the material properties and applied loads.
This equation can be used for any free-standing tower that is both very tall and not supported
by guy wires, and it will always result in a design that is far more economical than
a straight vertical prism. In the case of the CN Tower, the tapered legs
are not only more economical, but also required for structural support because the shaft is
so tall that a straight concrete prism would be crushed under its own weight.
Ideally, the concrete shaft should have used a circular cross-section for optimal efficiency,
however the designers wanted the tower to have outward-facing glass elevators so that
visitors could see outside on their way up and down.
This ultimately led to the Y-shaped design with a prismatic core and three tapered legs,
and although it sacrificed efficiency, the shape also solved a number of problems involving
aerodynamics, local buckling, and post-tensioning. The shaft was constructed using a method known
as slip-forming, which involved a continuously moving form on hydraulic jacks.
First, a massive three-storey form structure with several work platforms was built on top
of the foundation using steel and timber. Concrete was then mixed on-site and poured
in layers, and the form was slowly lifted upwards as the bottom layers cured and gained
strength. The form had an open bottom so that it could
slide, or slip, up the constructed portion of the tower, which is why it is referred
to as a slip-form. As it continued to rise, one crew continuously
poured concrete from one work platform, while another crew placed steel reinforcement and
post-tensioning ducts from another platform. The walls of the slip-form were also adjusted
constantly to give the legs their tapered shape, effectively shrinking the form as it
climbed. In order to deliver materials and equipment
to the top of the shaft, a tower crane was constructed inside the central core, and it
was simultaneously raised alongside the form structure.
In total, nearly 29,000 cubic meters of concrete and 3,600 metric tonnes of reinforcing steel
were required to build the shaft, and the slip-forming process took 8 months with crews
working 24 hours a day. Extreme care was taken to keep the form perfectly
straight and level, and as a result, the top of the completed shaft was only out of plumb
by about 28 mm, or just over 1 inch. This corresponds to a vertical deviation of
less than 0.01%, which is quite impressive given the 450 m height and the fact that it
was built in 1973 without the use of electronic equipment.
The concrete shaft of the CN Tower can be thought of as a vertical cantilever that is
anchored to the foundation, and it responsible for carrying significant wind loads in addition
to vertical gravity loads. The tower is so tall that the distribution
of wind pressure is essentially linear over most its height, and it has been designed
to withstand wind gusts exceeding 400 km/hr, which is greater than the highest recorded
wind speed on Earth. Prior to construction, the wind action and
response were studied in detail for 6-years at the Boundary Layer Wind Tunnel Laboratory
located at the University of Western Ontario. Here, a 1:450 scale aeroelastic model was
used to determine the loads on the various components of the tower using displacement,
rotation, and acceleration data. The overall dynamic response of the structure
was also studied, and the results were used to design two passive tuned mass dampers which
are located on the steel antenna mast. Although the wind tunnel testing did not predict
any excessive movement under wind loading, the dampers were added anyway to improve the
reliability and performance of the antenna. Under a sustained wind speed of 200 km/h with
gusts reaching 320 km/h, the top of the concrete shaft will sway as much as 0.5 m from center,
and the top of the antenna will sway as much as 1 m.
When the tower is subjected to any lateral load, either from wind or from an earthquake,
shear and bending stresses are induced within the structure, with the highest bending moment
and shear force being located at the base. As the shaft bends under the load, one side
will experience compressive stress, while the other side will experience tensile stress.
It is undesirable to have concrete in tension due to the possibility of cracking as we have
already discussed for the foundation, and so the shaft was pre-stressed using a post-tensioning
system in order to counteract the tensile forces.
The system provides a uniform compressive stress over the cross-section so that the
shaft will remain fully pre-stressed and not experience any tension up to a 1 in 50-year
wind event. For wind speeds exceeding a 1 in 50-year event,
the shaft will become partially pre-stressed and cracking may occur on the windward face,
however this will not compromise the structural integrity of the tower and the cracks can
be patched to prevent further damage. The post-tensioning system consists of 144
high-strength steel cables, which were fed through the vertical ducts installed in the
concrete. Each cable contains 16 to 31 individual strands,
and just like the cables in the foundation, each strand is made up of 7 steel wires with
a tensile strength of 1,862 MPa. The cables that are located within the core
are 450 m long, extending from the top all the way down to the foundation, while the
ones in the tapered legs are shorter in length. The top ends of the cables are anchored into
the tops of the vertical concrete walls of the shaft, and the bottom ends are anchored
into the foundation inside the hollow caverns. Most of the cables were tensioned from the
bottom using hydraulic jacks, and an effective pre-stress force between 1,800 kN and 3,500
kN was applied to each one depending on the number strands.
Vertical post-tensioning was a relatively new process in the 1970’s when the CN Tower
was built, and thus it was difficult for the engineers to predict pre-stress losses and
constructability issues prior to actual construction, especially on a project of this scale.
In order to account for this, extra cable ducts and post-tensioning cables were installed
in the shaft, and the pre-stress levels were continuously monitored and adjusted as construction
progressed. Two spare ducts were provided for every group
of 10 cables, and the ducts were over-sized so that the number of strands in each cable
could be increased by up to 20%. Although the ducts were made from rigid helically-corrugated
steel pipes, a number of them became dented or blocked during the slip-forming process
due to physical damage or wet concrete falling inside.
Many individual steel strands were also lost due to the brake and stressing jack losing
grip during post-tensioning, and some strands snapped under the load before the full force
could be applied. By the time post-tensioning was complete,
most of the additional pre-stress capacity had been used, and two blocked ducts in the
core had to be cleared with a diamond drill so that cables could be installed.
In total, nearly 129 km of steel cable weighing 762 metric tonnes was used to pre-stress the
shaft, providing a combined compressive force of about 365 MN, or 82 million lbs.
Once all the cables were fully stressed, the structural elements of the concrete shaft
were complete, and work could proceed on the main pod.
Construction of the pod began in the summer of 1974, about a year and a half after ground
was first broken, and it was one of the last components to be completed before the tower
opened 2 years later in 1976. There are seven primary levels, as shown in
this cross-section through the center of the tower, and the internal structure consists
of concrete floor slabs supported by steel framing.
The first level is actually suspended below the pod, and this is where the UHF transmitters
are located. A Teflon-coated fiberglass fabric wraps all
the way around the base of the structure to form a radome, and it is inflated with air
to maintain its shape. The radome protects the broadcasting equipment
from strong winds and ice build-up without interfering with any of the signals, and it
also improves the overall aerodynamics of the pod.
Levels 2 and 3 feature dual observation decks that are open to visitors, offering incredible
views of the city with floor-to-ceiling windows, an outdoor terrace, and glass panels that
are installed in the concrete floor. I had the opportunity to visit the tower before
making this video, and although I was assured that the glass floor is 2.5” thick and can
support the weight of 35 moose, it was still terrifying to stand up there looking 342 m
straight down. Level 4 of the pod contains a revolving restaurant
with a ring-shaped turntable that rotates 360 degrees around the circumference.
The turntable sits on top of a staggered concrete slab, which allows the rotating floor to remain
flush with the stationary portion as it moves around the outside.
The dining area is positioned on the moving floor, and guests get to experience a full
360-degree view of Toronto and Lake Ontario as it makes one full rotation about every
72 minutes. Levels 5, 6, and 7 are off-limits to visitors,
with level 5 being used for UHF broadcasting equipment, level 6 for VHF broadcasting equipment,
and level 7 for general mechanical equipment for the tower.
There are several additional levels located above the pod inside the concrete core, and
these are used for elevator equipment, as well as for pumping and storage of domestic
water. The structural steel frame of the pod forms
a 12-sided polygon around the outside of the tower, and the entire structure is supported
by 12 reinforced concrete bracket walls which are mounted to the exterior of the shaft.
The bracket walls are triangular in shape, and they are arranged in a radial pattern
to align with the steel framing. They are also exposed on the outside of the
tower, and you can see them from the base if you look up inside the radome.
The walls were cast integrally with the floor slab at the second level of the pod, and a
post-tensioned concrete ring beam was built into the floor around the circumference.
The circular ring beam clamps the bracket walls against the shaft, which provides lateral
restraint much like a metal strap wrapped around the wooden planks of a barrel.
This eliminates tensile stresses in the concrete that would otherwise result from the weight
of the pod sitting on the cantilevered brackets. The post-tensioning system consists of 12
high-strength steel cables arranged concentrically around the center of the tower, each with
49 individual steel wires. Each cable spans 180 degrees around the beam,
and they are staggered every 30 degrees so that there are 6 cables present at every section.
All the cable ends are anchored towards the inside of the beam, and post-tensioning was
carried out sequentially in order to ensure symmetry of the resulting stresses.
An effective pre-stress force of about 890 kN was applied to each cable, which corresponds
to a total compressive force of 5,340 kN at any given section within the ring beam.
Construction of the pod began at ground level around the base of the shaft, where concrete
formwork was built for the bracket walls. Six individual assemblies were built from
steel and timber, and the internal steel reinforcement for the brackets was placed inside the forms.
A large steel crown was constructed at the top of shaft, and this was used to hoist the
formwork up the exterior of the tower using hydraulic jacks.
The lift spanned a total of 5 days, with the assemblies rising 68 m per day before reaching
their final position at a height of 342 m. Once in position, the forms were tied together
with a steel truss system, and concrete was poured to cast the bracket walls.
This was followed by the construction of the second level floor slab and ring beam on top
of the brackets, completing the base structure for the pod.
The entire formwork assembly was then lowered by roughly 15 m, and it served as a working
platform under the pod for the remainder of construction, before being disassembled and
lowered back to the ground. The steel framing for the upper 5 levels was
erected on top of the concrete base by a team of iron workers, which formed a skeleton for
the superstructure. All the iron work for the tower was completed
by Canron, and it was an extremely dangerous task for the workers, who were often perched
on cantilevered beams without any kind of fall protection.
Despite the lack of adequate safety standards, there were some safety measures in place,
and no deaths resulting from falls were reported during construction.
As work on the main pod continued, the upper SkyPod was constructed simultaneously just
below the top of the concrete shaft at a height of 447 m, about 80 m above the roof of the
main pod. It is supported by a circular concrete corbel
that wraps around the exterior of the shaft, and access is provided by an elevator located
inside the core. The SkyPod is significantly smaller than the
main pod, with only a single level observation deck enclosed by glass panels, but it is the
highest observation platform in the western hemisphere, and among the top 5 highest in
the world. On a clear day, guests can experience sight
lines of up to 160 km, all the way to Niagara Falls and New York State on the other side
of Lake Ontario. I also discovered that if you stand up there
in strong winds, you can actually feel the tower sway beneath your feet, which was a
pretty amazing thing to experience, but also slightly terrifying at the same time.
Above the skypod, the steel antenna extends for an additional 104 m beyond the concrete
shaft, reaching a height of just over 553 m at the very top of the tower.
It is supported by a massive steel base that is anchored into the shaft, and it is made
up of 39 high-strength steel segments referred to as cans, each measuring between 1 and 6
m in height. The cans are hollow with a pentagon-shaped
cross-section, and they decrease in size moving up the antenna to create a 95 m tall mast
measuring 3.7 m wide at the bottom and a 0.6 m wide at the tip.
This is topped off by a smaller 9 m segment with a hollow square cross-section, completing
the steel structure which weighs more than 272 metric tonnes all together.
VHF transmitters are installed all the way up the exterior of the antenna in groups of
5, which align with the cutouts in the pentagon-shaped cans, and they are secured in place with light
steel framing. The cables for the transmitters are fed through
the cutouts, where they run back down to the tower, taking up most of the space inside
the hollow mast. The antenna is also fitted with various warning
lights, 2 passive tuned mass dampers, and lightning rods at the very top, which guide
electrical discharge to the ground through copper strips that run the entire height of
the tower. In order to protect the broadcasting equipment
and prevent ice build-up on the outside of the steel structure, the antenna is fully
enclosed by a glass-reinforced plastic skin, which consists of many individual panels with
flexible joints. The skin is essentially a radome, similar
to the one installed around the UHF transmitters at the base of the main pod, and a plastic
material was chosen so that it would not interfere with radio signals.
At the time of construction, there was a tight schedule to erect the antenna on top of the
tower, and this ultimately led to the decision to use a helicopter to place the steel can
sections. A Sikorsky SkyCrane nicknamed Olga was leased
for a 30-day period in the spring of 1975 at a cost of $230,000 USD, which equates to
just over $1 million today, and it decreased the overall construction time by an estimated
5 months. Once it arrived on site, the helicopter was
first used to remove the tower crane from inside the central core, before completing
a total of 56 flights to lift the cans and other components of the antenna, the heaviest
of which weighed more than 7 metric tonnes. The steel cans were fabricated and erected
by Canron, and all the sections were pre-assembled on the ground several months prior to construction
in order to test-fit the connections and minimize alignment issues during the lifts.
Since the helicopter was able to place the cans faster than the iron workers could bolt
them together, several sections were often placed in quick succession and secured with
just a few temporary bolts, and another team of workers went in afterwards to finish the
installation. Roughly 30,000 1-inch diameter bolts were
installed and torqued from both inside and outside the antenna mast, with bolting crews
running 2 shifts 7 days a week from a work platform that was slowly winched up the tower.
On April 2nd, 1975, the CN Tower was topped off with the final steel section of the antenna,
and an official height measurement of 553.3 m was recorded, marking the tower as the tallest
free-standing structure in the world. This was followed by the installation of the
various electronic components and the plastic radome, which was attached to the exterior
of the antenna with the aid of an aluminum cage to protect the workers from falls.
With the antenna complete, work continued on the main pod below for about another year
until the structure was finally finished, and the tower went into service on June 26,
1976, almost a decade after the design process first began.
Since that day, the CN Tower has been celebrated as a Canadian icon, and it has been recognized
worldwide as an incredible feat of engineering and construction.
More than 1.5 million people continue to visit the landmark every year, and it still serves
its primary purpose as a communications tower, providing transmission services for more than
16 TV and radio stations. Additional infrastructure has also been installed
over the years, allowing the tower to support cell phone providers and digital audio broadcasting.
Notable renovations and upgrades have taken place on the attractions side as well, including
the addition of more than 1,300 programmable LED lights on the exterior;
The installation of glass floor panels in the elevators;
And the construction of an open-air EdgeWalk on top of the main pod, where guests can experience
unobstructed views of the city from a height of 356 m while tethered to an overhead safety
rail. The CN Tower is an awe-inspiring structure
that has dominated the Toronto skyline for almost 50 years, and with an expected design
life of 300 years, it will certainly continue to inspire many future generations as one
of the greatest engineering achievements of the 20th century. Hey guys, if you enjoyed today’s video about the CN Tower, then please consider subscribing
to see more engineering videos, and remember to hit the bell to get notified every time
a new one comes out. This was a challenging project that has taken
several months to put together, and your support really helps me out so I can continue making
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in the description. As always, thanks so much for watching, and
I’ll see you in the next one.
Great job on this OP.
Little side note-before the final ‘cans’ were flown up kids were given the opportunity to crawl inside and sign their names on the pieces.
Watching this was unsettling. I think my brain was simply ignoring the fact that the tower is not indestructible(obviously it isn't, but buildings feel as though they are as solid as the earth they sit on), each time I've been in it. I don't have a better way of explaining that. I guess it's kind of like if you were to be standing on solid ground, and someone told you there's a 10m layer of rock you're standing on, over a 500m deep cavern. Suddenly it doesn't really seem like a good idea to stand there, despite it being solid lol.
Great job though OP.
Cant believe iron workers had no safety up there!! Just casually running around on these beams, one slip away from certain death. Id love to see more footage of that and get an explanation to how these workers stayed alive
Awesome thanks!
Nice.
Nice work. I was half-expecting to see a rehash of what's on wikipedia, but you went deep.
Great video! Loved seeing some of that classic footage.
great work, I actually learned a lot watching this. How long did this project take for you to put together? It is quite extensive.
Great work OP.
If this was for an assignment, your instructor should consider this as an A+ and as a future example to their students.
Well done