THINKING BIG : Spanning the Waters : Tower Bridge

Location: River Thames, London

Date: 1886-1894

Builders: Horace Jones, John Wolfe Barry

Description and dimensions: Two 205-foot main towers connect this iron and granite bridge with the shore; it is able to open electrically in 1 1/2 minutes for river traffic and is also acessable to pedestrians.

Construction: Originally the Tower Bridge opened by haudraulic power, taking up to six minutes to open. The wonder of this structure lies in the moving arms, or bascules , which lift to create a passageway for incoming ships. Each of these arms is 162.5 feet long and attached to a huge forged steel axis 21 feet in diameter and 48 feet wide on which the basucles turn. The total weight involved in lifting the arms of the bridge is an astounding 2,400 tons. Contrary to popular belief, the arms are not lifted completely upright but to an 82 degree angle.

The Biggest Bridges

* Suspension: Humber

Location: Hull, Britain

Size: 4,626 feet

Year completed: 1981

* Cantilever: Quebec Railway

Location: St. Lawrence River at Quebec, Canada

Size: 1,800 feet

Year completed: 1917

* Steel Arch: New River Gorge

Location: Fayetteville, West Virginia

Size: 1,700 feet

Year completed: 1977

* Cable-Stayed: Ikuchi

Location: Honshu-Shikoku, Japan

Size: 1,608 feet

Year completed: 1991

* Continuous Truss: Cooper River

Location: Charleston, South Carolina

Length: 1,600 feet

Year completed: 1992