The CN Tower is a tower in Toronto, Canada. It has been the highest tower ever built for 30
years and it is one of the icons of the city.
Probably the key feature of the building is the post-tension cables, post-tension cable on the
CN tower will last 300 years, because they are the only components that can’t be replaced so if
corrosion happens, the tower must be demolished.
“The tower was designed to withstand the impact of a Boeing 707, earthquakes, fires, lightning
strikes and more than 200 km/h winds”
Bud Purves (CN tower president) said. Experts say that if a plane would collapse against the
tower, it would just shred apart because the construction has over 450 tonnes of reinforcing
steel in the concrete. Since that the Cn tower has a large base and a small top, the centre of
mass is low. This is because more of the weight is at the bottom. Having a low centre of mass
is important because if lower the centre of gravity, the structure is more stable.
Engineers who planned the Cn tower had to considerer several forces and risks, they were
internal (compression and tension) and external (wind, weight, and gravity).
The Cn tower is a concrete structure with a cavity in the middle, so it can be flexible against
powerful winds. Also, the glass-floored deck near the top moves, this movement press the
specially shaped foundation more firmly into the ground.
If the CN Tower would fall it would do it basically in place. Wouldn’t topple over like a stick
because concrete simply wouldn’t be able to shear forces for that.
Although the top of the CN Tower, from the Sky Pod on up, is all steel, not concrete, that’s the
only 75m of the total height. It has a significant purpose that, without it, the tower would be
thrown into tension under high winds and would fall down. The base of the tower is more than
12 meters below ground and since that the tower is so massive and heavy and its centre of
gravity is low (61 from the ground) it has a great stability.
Like the other tall, narrow buildings, the CN Tower swings in extreme wind conditions but it can
resist to winds blowing up to 260 mph and two ten-ton swinging counterparts, mounted on the
antenna, ensure that it never exceeds acceptable conditions. The armour-plated windows on
the observation levels and restaurant were carefully designed for extreme wind tolerance with
outside panes of 95 cm thickness and inside panes of 64 cm thickness.