Here is some information on Tower Bridge.
Tower Bridge was completed in 1894, after eight years of construction. However, many people don't realise why it was even built in the first place, or why it is so different from London's other bridges. Originally, London Bridge was the only crossing over the Thames. As London grew, so more bridges were added, but these were all to the west of London Bridge, since the area east of London Bridge had become a busy port. In the 19th century, the east end of London became so densely populated that public pressure mounted for a bridge to the east of London Bridge, as journeys for pedestrians and vehicles were being delayed literally by hours.
The bridge consists of two towers which are tied together at the upper level by means of two horizontal walkways which are designed to withstand the horizontal forces exerted by the suspended sections of the bridge to the left and the right. The vertical component of the forces in the suspended sections and the vertical reactions of the two walkways are carried by the two robust towers. The bascule pivots and operating machinery are housed in the base of each tower. Its present colour dates from 1977 when it was painted red, white and blue for the Queen's Silver Jubilee. Before this, it was painted a chocolate brown colour.
The problem of building a bridge over a busy river with low banks so that shipping is not obstructed is one that taxes the resource and ingenuity of the engineer. He surmounts the difficulty by resorting to the opening type of bridge, of which the main types are the drawbridge or bascule bridge, turning about a horizontal axis ; the swing bridge, turning about a vertical axis ; the rolling lift bridge and the vertical lift bridge. One of the most famous examples of the bascule type is the Tower Bridge, which spans the River Thames just below London Bridge. It is the most distinctive of London's bridges and its construction was a masterly engineering achievement. The building of the Tower Bridge came about because the development of cross-Thames traffic had far outstripped the capacity of the existing bridges. By the year 1870 the position had become serious, and between 1874 and 1885 some thirty petitions from various public bodies were brought before the authorities urging either the widening of London Bridge or the building of a new bridge.
A two days' census taken during August 1882 showed that the average traffic for twenty-four hours over London Bridge -which at that time was only 54 feet wide-was 22,242 vehicles and 110,525 pedestrians. A committee was appointed to consider the matter and to report upon the different plans that had been proposed. In building the bridge there were used about 235,000 cubic feet of Cornish granite and Portland stone, 20,000 tons of cement, 70,000 cubic yards of concrete, 31,000,000 bricks and 14,000 tons of iron and steel.
The bridge is a combination of the suspension and bascule type. The width of the river between the abutments of the bridge on the north and south sides is 880 feet. This is crossed by three spans. The two side spans, each 270 feet long, are of the suspension type. They are carried on stout chains that pass at their landward ends over abutment towers of moderate height to anchorages in the shore. At their river ends the chains pass over lofty towers which are themselves connected at an elevation of 143 feet above high water. Heavy tie bars, at the level of the connecting girders, unite the two pairs of chains so that one acts as anchorage for the other at the centre.
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