London's Tower Bridge is one of the most unusual bridges in the world. Its form arose from the unique conditions under which it was designed and constructed. Built in the 1880s, it had been described as a double-leaf bascule with suspended side-spans, incorporating two high-level footbridges. Construction of the Tower Bridge had to consider the historical, topographical and commercial constraints imposed by the location. The bridge had to either be a high-level crossing, so as not to obstruct shipping, or have a movable span. The option of a bridge with high clearance would have necessitated long approaches, which would have added to the expense, not to mention presenting an unwelcome structure beside the Tower of London. By incorporating a movable span in the center of the bridge, the side spans could be built close to the water and hence minimize the approach viaducts on the low banks.
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