How the CN Tower was Built

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.

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