How the CN Tower was Built

If you've ever had the pleasure of seeing Toronto's skyline whether it's in person or through a photo, there's one structure that undoubtably makes an impression. The CN Tower stands at 553.3 m tall and was built from 1973 to 1976. It was the world's largest free-standing structure for 32 years until the Burj Khalifa was built in 2007. However, the design and construction of the CN Tower was incredibly advanced for its time, with Burj taking on a similar design decades later. The engineers and designers built a structure that is truly a testament to the amazing things that can be accomplished by humans - with the CN Tower still being the tallest tower in the western hemisphere.  

The tower was originally built to address communication problems, serve as a world class entertainment destination, and achieve international recognition as the world's tallest tower. For an overview of how it was built, check out this video below:

Y-SHAPED FOUNDATION AND PLAN  

Photo by Robert Taylor

Most buildings sit on top of a square or rectangular shaped foundation because this is the easiest shape to construct. However, the engineers who built the CN Tower were way ahead of their time in the 60s when they decided to design a Y-shaped foundation and corresponding building PLAN. The world's current tallest structure, the Burj Khalifa also shares a Y-shaped PLAN though it was built about 40 years after the CN Tower. Before we take a look into why they chose to go with a Y shape, let's start with the construction of the foundation itself.

After excavating about 56,000 metric tonnes of earth and shale from the construction site, the foundation was ready to be poured. It was made from a combined 500 tonnes of rebar and steel cables, and topped off with 7000 cubic meters of concrete,  These numbers probably don't mean very much to you so to give you some perspective, the amount of concrete they used is the equivalent of about three Olympic swimming pools - and that's just for the foundation! The entire CN Tower required 40,500 cubic metres of concrete, which is the equivalent of 16 Olympic swimming pools!

CONSTRUCTION METHODS 

SLIPFORM CONSTRUCTION 

If you ever really looked at the CN Tower, you may have noticed that it actually tapers from the bottom to top meaning it effectively gets smaller as you reach the top. Not only does this look great but improves safety and saves money. 

From a structural engineering perspective, it is generally thought that the safest building design keeps loads as close to the ground as possible. One reason is because the potential gravitational energy is directly proportional to the height of an object. For example, imagine you’re holding a ball over your head – it probably won’t do too much harm if it fell. Now bring that ball a bit higher and the potential danger increases as it gets higher and higher. However, if we place the ball on the ground, the gravitational potential is effectively zero. In structural design, we apply the same concept where we want to keep loads as low as possible. Therefore, a tapered tower improves the safety of the structure as the heavier loads are closer to the ground and saves money because less material is used overall. 

This is all great information, but one question does comes to mind: How did the construction crew achieve such a spectacular shape for such a tall structure?

The entirety of the tower was constructed with a slipform which in other words is a continuous pour of concrete into a moving form. You can think of a form as a mold shaped as the final product - for example, if you wanted to create a rectangular concrete box, you would generally make a form out of a material like wood then pour your concrete in and release the form once the concrete has set. 

In the case of the CN Tower, it used a large “Y” shaped box that was continuously pushed up by a series of hydraulic-powered climbing jacks. Workers worked around the clock 24 hours a day, five days a week for 40 months to pour the concrete, set the reinforcement, and adjust the slipform to become smaller as each layer was completed. The result was a beautifully tapered Y-shaped tower that was within 2.8 cm of plumb. 

OLGA THE HELICOPTER 

Photo by WikiPedant at Wikimedia Commons

After the slipforming was completed and the SkyPod was built, it was time for Olga to come in. Olga was the nick name of a giant Russian Sikorsky helicopter used to assemble the tower’s crowning jewel, it’s antenna. Piece by piece, Olga put together 36 pieces of the antenna precisely together while workers maneuvered and bolted the pieces in place at the top of the tower. With the antenna installed, the CN Tower claimed the title of the world’s tallest tower for over 30 years. 

The CN Tower continues to be one of the world’s tallest towers and represents the incredible feats of engineering that were achieved over 50 years ago. To learn more about and visit the CN Tower, check out their website here (http://cntower.ca).  

Thanks for reading! Let me know in the comments what you learned - did anything surprise you?

This article was written for Structures Simplified, an Online Media project creating simple and engaging infotainment content across multiple platforms. The content breaks the barriers to simplified knowledge on the construction processes of structures. The goal is to propel people to learn more about the structures around us and inspire new ideas in STEM. For more interesting content, check out its social media avenues here: 

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REFERENCES

[1] http://www.ieee.ca/millennium/cntower/cntower_overview.html

[2] https://www.cntower.ca/site_Files/Content/PDF/Press_Kit/CN_Tower_General_Information_2013_Revised.pdf