Wind

Every day, the CN Tower must withstand an invisible force, pushing and pulling from all sides. That force of nature is wind.

When it comes to wind and tall buildings, it is particularly difficult to prepare for. Compare the force of wind to that of gravity. Gravity is constant and only acts downward. Wind can come from east, west, north or south, can increase and decrease in an intensity or stop just as quickly, and change directions in an instant. To counter act this, the CN Tower needed to be designed with particular ingenuity.

There are two things that contribute to the CN Tower’s wind resistance: shape and structure.

Shape

The CN Tower’s triangular base provides a strong foundation. Each leg follows a parabolic curve. If you look at the diagram to the left, you can see the legs slowly tapering and narrowing as they approach the Observation pod. This tapered design reduces the surface area that the wind can push on, and lowers the centre of gravity, making the foundation that much sturdier.

                  

The triangular cross-section helps to balance the force on the Tower. If the wind is coming edge-on, it is split and deflected along either face, like the prow of a boat passing through water, which you can see in the diagram above, in green. If the wind approaches face on, the surface area decreases the further up the wind goes (orange wind in the diagram above), reducing the force and the leg of the side opposite can provide a brace. However, shape alone is not enough to counter the wind blowing on the Tower. The materials and internal structure of the CN Tower also make the difference.

Structure

Within the concrete of the Tower are hundreds of bundles of steel cables, which run the full height of the Tower’s legs and core. In a process called post-tensioning, these bundles were tightened, creating a strong, flexible “skeleton”.

Concrete is very good at supporting heavy loads, but it is not flexible. When wind pushes the Tower, the Tower needs to bend. Consider a tree in high winds. The trunk of the tree is made of rings of long plant fibres. As wind pushes on the tree, the side of the trunk being pushed is stretched, but the fibres in the trunk resist and pull the trunk back straight. As the wind pushes on the Tower, the concrete on the back face is forced together, but because it is not very compressible and pushes back. If the Tower did not have the post-tensions cables, the front face would stretch, which concrete is not good at, and could crack. With post-tensioned cables, the face under pressure is actively pulling itself back into the wind, preventing cracks or fractures.

Flexibility is the key to the success of the CN Tower’s structure. A tree does not withstand a storm by remaining immobile; it bends and flexes. And flex the Tower does! In high winds, the Observation deck, at 346 m (1,136 ft) can deflect (or sway) as much as 22.9 cm (9 inches), and in SkyPod, at 447 m (1,465 ft) that sway could be almost half a meter! There is another factor besides concrete and post-tension cables that help the Tower resist the forces of wind. In the CN Tower’s antenna, starting just above SkyPod, there are two heavily weight rings called tuned mass-dampers. Their purpose is to counter act the sway of the Tower. Think of them like lead-weighted hula-hoops. When wind pushes one way, these rings resist that motion. With two rings, wind from multiple directions can be resisted.

You can however rest assured that these wind conditions are rare. The designers and engineers of the CN Tower were planning for extreme weather conditions and, if they do arise, the CN Tower is more than capable of weathering the storm.

Want to learn more? Check out these source links:

http://www.dsicanada.ca/uploads/media/DSI-USA_What_is_Post-Tensioning_us_02.pdf

http://joa.isa-arbor.com/request.asp?JournalID=1&ArticleID=2715&Type=2