Ambitious terrorist plotters had better beware: Experts say the world's tallest free-standing structure is solid enough to foil any mastermind.
Given its one-of-a-kind reinforced concrete construction, engineers who designed and built the CN Tower say it would not suffer the fate of the World Trade Center towers if it were attacked in the same way.
"The most likely outcome is that the plane would shear apart and fall to the ground," said CN Tower president Bud Purves.
"The tower was designed to withstand the impact of a Boeing 707, earthquakes, fires, lightning strikes and more than 200 km/h winds -- it's a remarkable edifice," Mr. Purves said.
Concern over the famed Toronto landmark was raised again recently, with the arrest of 19 men living in the Toronto area who are being held on suspicion of terrorist activity. Antiterrorism agents have suggested that the group's pattern of activities resembles that of the 19 airplane hijackers who struck U.S. targets on Sept. 11, 2001. And Canadian government lawyers argue that the men may have wanted to "find out the measurements and schematics" of the CN Tower and other North American buildings.
Jutting into the sky at 553.33 metres, the CN Tower seems an obvious terrorist target.
But Jamil Mardukhi, who has worked on the tower's construction as a designer and site engineer for more than 30 years, offers several sound reasons for plotters to think again.
The World Trade Center towers comprised steel columns, floors and beams that ignited and melted when two aircraft slammed into them. The result was a progressive structural collapse.
The CN Tower, by contrast, is built of 40,538 cubic metres of reinforced concrete.
"During the design phase in the late 60s and early 70s, we looked at what would happen if a Boeing 707 [the largest passenger plane at the time]hit the tower accidentally," Mr. Mardukhi of N.C.K. Engineering Ltd. said.
"The modelled computer analysis showed there would be local damage -- a hole in the tower at the place of impact -- but that the structure would still stand." The CN Tower begins nearly seven metres underground, with a foundation of more than 450 metric tonnes of reinforcing steel embedded in thick concrete that rests on a base of hand- and machine-smoothed shale.
The reinforced concrete shaft with a hexagonal core and three curved support arms extends for another 335 metres above ground.
More than 1,500 people worked non-stop for 40 weeks starting in the spring of 1973, pouring the concrete continuously into a mould to prevent the formation of "cold joints." These deep fissures develop when concrete is allowed to dry, rendering the structure penetrable to water (which freezes and expands in the winter causing cracks in the concrete); or worse, to flammable fluids.
To maintain consistency, all the concrete had to come from the same source and workers mixed all of it on site, continuously testing and retesting it. Once the concrete dried, the steel post-tensioning cables embedded within it were stretched taut with jacks to keep the tower under constant compressive forces.
Perched at 338 metres is the seven-storey Skypod, which houses microwave receivers, observation decks, two restaurants and various technical rooms for broadcasters.
The Skypod sits like a doughnut around the hexagonal concrete core, its steel floors and columns covered with drywall, concrete and various sprays that make it fire-resistant. Finally, the antenna, a slim, stacked broadcasting receptor rising from the shaft above the pod, is enveloped by a thick fibreglass jacket.
Materials identical to those of the Skypod were subjected to furnace flames in rigorous lab tests, revealing that the pod would have to burn for two hours before any of its structural members failed, Mr. Mardukhi said.