When the CN Tower(1) was completed in 1975, it held the title of the world's tallest freestanding structure. Back then, two taller structures have been built, but neither of them are freestanding.
A freestanding structure is one that can support its own weight without external support. This means it can withstand loads like wind, earthquakes, and the weight of people, all while remaining stable. These structures typically consist of two main parts:
- Substructure: This is the foundation, usually buried underground, that provides stability and supports the weight of the structure.
- Superstructure: This is the visible part of the structure, rising above the ground.
The substructure must be designed to resist forces like groundwater pressure and soil movement to ensure the overall stability of the building.
On the other hand, a supported structure requires external elements for stability. While this type of structure is more cost-effective to build, it is generally not suitable for habitation. Its inherent instability, caused by its reliance on external support, makes it susceptible to movement and external factors. Additionally, supported structures often require a larger footprint, further limiting their practicality.
The two tallest supported structures in the world are the two towers in North Dakota.
Ironically, the CN Tower and these North Dakota towers were built for the same purpose: to broadcast television signals. In the era of analog television, when signals were transmitted directly from antenna to antenna, these towering structures were essential for covering vast geographic areas.
With their impressive height and line-of-sight capabilities, the CN Tower and the North Dakota towers could reach an area of approximately 5 million square kilometers. That's an area nine times larger than France or roughly half the size of the entire European continent!
The CN Tower held the title of the world's tallest freestanding structure until it was surpassed by the Burj Khalifa(2) in 2009. It will soon be the tenth tallest building in the world when the Jeddah Tower(3) is completed. Though currently on hold, the Jeddah Tower, once finished, will be the first structure of any kind to exceed 1 kilometer in height.
The CN Tower showcased a highly advanced design for its time. Its distinctive feature is a variable cross-section, meaning the structure widens at the base and gradually narrows as it ascends. This innovative design presented a significant technical challenge for the construction team, as traditional concrete formwork was not suitable. Each section of the tower required a unique form, and creating numerous custom forms would have been prohibitively expensive.
To overcome this challenge, the engineers employed a sliding formwork system and adapted it to accommodate the variable dimensions. As the formwork ascended, it progressively decreased in size, mirroring the narrowing profile of the tower. Further complicating the process, the top of the formwork was also smaller than the bottom, and the ratio between the two varied along the height of the structure.
Adding to the complexity, the reinforcement bars (rebars) within the concrete also varied throughout the construction process. This included changes in the number, size, and spacing of the rebars. While it's common practice to use different rebar configurations between floors or sections of a building, the CN Tower's design required a gradual transition in reinforcement.
To delve deeper into the intricacies of the construction, one would need to explore the post-tensioning cables and their sleeves. These elements also varied in number and spacing along the height of the tower, further contributing to the engineering challenges.
Once the structure was complete, the final step was to install a massive antenna at the very top. In the 1970s, reaching such heights with traditional cranes was impossible, and helicopters struggled to lift the heavy antenna load. The strong, consistent winds at that altitude further complicated the task, causing the helicopter to sway and the antenna to move erratically.
To overcome these challenges, workers had to manually secure the antenna to the tower before the helicopter's fuel ran out. This delicate and dangerous operation required precision and courage, as a single mistake could have led to a catastrophic accident.
Once the CN Tower was completed, it had to maintain very precise tolerances for side-to-side movement to ensure the proper functioning of mechanical systems like elevators. The tragic fire at the Ostankino Tower(4) in Moscow serves as a stark reminder of the dangers of elevator malfunctions during emergencies. Some individuals lost their lives as elevators became trapped in the tower's shaft.
Beyond its impressive engineering and technological feats, the CN Tower's sleek and elegant design enhances the city's skyline. It has become a major tourist attraction, drawing millions of visitors each year.
The tower offers two main sections for visitors. Just above the telecommunications floor, visitors can access the observation deck and the rotating restaurant, both offering panoramic 360-degree views of the city. From there, an elevator takes visitors to an even higher observation deck for a more elevated perspective.
Inside, thrill-seekers can test their nerves on the glass floor, offering a bird's-eye view of the city below. For the truly adventurous, there are even glass-floored elevators that provide an exhilarating ride to the top.
Sometimes, the value of something cannot be measured solely in economic terms. Life and the world are not merely about money. The CN Tower is a testament to human ingenuity and a symbol of overcoming challenges. While it may no longer serve as a primary telecommunications antenna, it continues to leave a lasting impression on those who visit.
(c) I personally took these photos and own the rights to them.
Please feel free to use them as you wish.
(2) https://en.wikipedia.org/wiki/Burj_Khalifa
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