CN TOWER

The CN Tower (French: Tour CN) is a 553.33 m-high (1,815.4 ft) concrete communications and observation tower in Downtown Toronto, Ontario, Canada. Built on the former Railway Lands, it was completed in 1976, becoming the world's tallest free-standing structure and world's tallest tower at the time. It held both records for 34 years until the completion of Burj Khalifa and Canton Tower in 2010. It remains the tallest free-standing structure in the Western Hemisphere, a signature icon of Toronto's skyline, and a symbol of Canada, attracting more than two million international visitors annually.

Construction on the CN Tower began on February 6, 1973 with massive excavations at the tower base for the foundation. By the time the foundation was complete, 56,000 t (61,729 short tons; 55,116 long tons) of earth and shale were removed to a depth of 15 m (49.2 ft) in the centre, and a base incorporating 7,000 m³ (9,156 cu yd) of concrete with 450 t (496 short tons; 443 long tons) of rebar and 36 t (40 short tons; 35 long tons) of steel cable had been built to a thickness of 6.7 m (22.0 ft). This portion of the construction was fairly rapid, with only four months needed between the start and the foundation being ready for construction on top.
In August 1974, construction of the main level commenced. Using 45 hydraulic jacks attached to cables strung from a temporary steel crown anchored to the top of the tower, twelve giant steel and wooden bracket forms were slowly raised, ultimately taking about a week to crawl up to their final position. These forms were used to create the brackets that support the main level, as well as a base for the construction of the main level itself. The Space Deck (currently named SkyPod) was built of concrete poured into a wooden frame attached to rebar at the lower level deck, and then reinforced with a large steel compression band around the outside.

The main deck level is seven storeys, some of which are open to the public. Below the public areas — at 338 m (1,108.9 ft) — is a large white donut-shaped radomecontaining the structure's microwave receivers. The glass floor and outdoor observation deck are at 342 metres (1,122.0 ft). The glass floor has an area of 24 m² (258 sq ft) and can withstand a pressure of 4,100kilopascals (595 psi). The floor's thermal glass units are 64 mm (2.5 in) thick, consisting of a pane of 25-millimetre (1.0 in) laminated glass, 25 mm (1.0 in) airspace and a pane of 13 mm (0.5 in) laminated glass. Some people experience acrophobia when standing on the glass floor and looking down at the ground 342 m (1,122.0 ft) below. In 2008, one elevator was upgraded to add a glass floor panel, believed to have the highest vertical rise of any elevator equipped with this feature. The Horizons Cafe and the lookout level are at 346 metres (1,135.2 ft). The 360 Restaurant, a revolving restaurant that completes a full rotation once every 72 minutes, is at 351 m (1,151.6 ft). When the tower first opened, it also featured a disco named Sparkles, billed as the highest disco and dance floor in the world.

EdgeWalk

On August 1, 2011, the CN Tower opened the EdgeWalk, an amusement in which thrill-seekers can walk on and around the roof of the main pod of the tower at 356 m (1,168.0 ft), which is directly above the 360 Restaurant. It is the world's highest full-circle, hands-free walk. Visitors are tethered to an overhead rail system and walk around the edge of the CN Tower's main pod above the 360 Restaurant on a 1.5 m (4.9 ft) metal floor. The attraction is closed throughout the winter season and during periods of electrical storms and high winds.

Source From : http://en.wikipedia.org/wiki/CN_Tower