The CN Tower could one day be a very expensive demolition project, but luckily that day is centuries away.

One of the most critical components of the structure — anchors that hold steel cables that compress the concrete and give it its strength — can never be replaced.

“If you x-ray the anchorages and found corrosion, and you couldn’t engineer a solution, you’d have to take it down,” says Ned Baldwin, one of the architects who worked on the tower, which was completed in 1976.

“Without it, the tower would be thrown into tension under high winds and would fall down,” says Baldwin.

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But the anchors were built to last at least 300 years and could last much longer, says the 78-year-old architect, who is on the phone from New York where he lives and works.

The CN Tower also has strength from its shape: Try turning a golf tee on its head and blowing it down.

“It takes a fair amount of breath to blow it over,” says the architect, who estimates it would take a 1,600 km/H wind to topple the 553-metre tower, “a wind that would never exist.”

Other components in the structure have already been replaced, such as the radome, the donut shaped structure underneath the observation deck which hides all the microwave dishes.

The radome’s fabric was originally guaranteed for 20 years, but lasted until 2002, when it was replaced.

“It was a fantastic engineering achievement that they replaced it so smoothly and no one even noticed,” said Baldwin. “When I learned that it had had happened, I thought it was an amazing achievement.”

Baldwin climbed to the top of the CN Tower many times during construction and remembers having to tightly grip a ladder that was moving around on the antenna mass.

In fact, his colleagues went up so many times that one of the contractors complained “we were building a jungle gym for the architects,” says Baldwin.

The architect lived in Canada for 40 years before moving to the U.S.

After building the CN Tower, he worked on the Stratosphere tower in Las Vegas, as well as towers elsewhere in the world.

Many of them were never built.

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“Towers are very hard to get built because they cost a lot,” says Baldwin. “They only work as tourist attractions and politically, they are hard to get approved.”

The same was true in Toronto, despite the fact that skyscrapers were beginning to block out aerial reception for televisions and the city needed a new transmission antenna.

The CN Tower was part of a much larger project called the Metro Centre, a massive office and entertainment development planned for the Toronto railway yards.

But politicians were against the project because it would involve the destruction of Union Station.

Despite the opposition, the tower was built in part because the head of CN, Norman MacMillan, was determined to go ahead, with or without local approval and build the tower on the crown corporation’s land.

“It was pretty amazing that it got approved and CN happened to have a chairman that said ‘we’re going to build it’,” says Baldwin.

What’s keeping it up

The CN tower will turn 40 in two years, but the structure was built to last. The most critical component — metal anchors that secure the post-tension cables, and which can’t be replaced — were designed to last 300 years, says Ned Baldwin, an architect who worked on the project.

DAMPERS: Twenty tonne lead weights supported on pendulums in the antennae mast move independently to slow down vibration.

MOVEMENT: The tower can accelerate and move in an ellipse pattern if the wind is strong enough, but the restaurant level was designed to keep acceleration so low that most people wouldn’t sense movement, says Baldwin. Some experts think the tower could move as much as 4.3 metres, but there is no tool to measure the distance — only an accelerometer to measure acceleration — and Baldwin says the estimate is “poor speculation.” The tower may only move as little as 1.8 metres, he says.

POST-TENSION CABLES: Bundles of steel cables are anchored into concrete throughout the tower. The cables run through steel ducts 40.6 centimetres apart in the tower’s hexagonal concrete core, as well as in ducts in the wings. A pneumatic jack was used at the base of the foundation to pull the cables tight, moving them up to 2.4 metres. The tension compresses the concrete, making it stronger and allowing the structure to maintain its slender profile.

CONCRETE: At the time, it was the strongest in the world.

FOUNDATION: The base of the tower floats on bedrock more than 12 metres below ground. The tower is stable because it is so massive and heavy and its centre of gravity is low — less than 61 metres from the ground.