DESCRIPTIONS OF SOME IMPORTANT BRIDGES London bridge was designed by Sir John Rennie and built during the period 1824-31. Its arches, of which there are five, vary in size from i 3of t., in the case of the side arch, to 15 2 f t. for the centre. It was originally 54f t. wide, but was widened (1902-04) to 65ft. by carrying the footpaths on granite corbels. The bridge is of granite, and its total cost was approximately £1,500,00o, of which £692,000 was for the purchase of land.
The Forth bridge has a total length of 5,33oft., made up of two cantilever spans of 1, 71 of t. each, two shore arms of 68oft. each, the remaining distance being occupied by steel towers. It cost £3,200,000, and is probably the most impressive bridge in existence. It was designed by Sir Benjamin Baker, and was con structed during the period 1882-89. The clear headway above high water is i 50f t., and the towers are 361 ft. high. The cantilevers project 68oft. and support suspended spans of 35oft. in length.
The Camden bridge, Philadelphia, built over the Delaware, 1921-26, at completion the largest suspension bridge in the world, has a span of 1,75oft. It is 125ft. in width, and designed to carry a central roadway 57ft. wide with four railway tracks and two footpaths in addition. It is supported by two main cables, 3oin. in diameter, there being 18,666 wires laid parallel to each other in each main cable. The clearance above high water is 135ft. The bridge cost, with its approaches but without land, about $25,000,000, the total cost being nearly $35,000,000. It was de signed by Ralph Modjeski and erected by a board of engineers under his direction.
The Hudson river bridge, New York city, was begun in 1927 and is by far the largest span bridge ever built, having a span of and a width of 12oft. It carries four roadway lines and two pedestrian sidewalks. The roadway is 2 5 of t. above the river and the clearance 213ft. The steel towers stand 600ft. above water. The total cost was about $6o,000,000. The bridge may be enlarged to eight vehicular lines on an upper deck and four rapid transit tracks on a lower deck.
The Tower bridge, London, was built 1886-94 at a total cost of £830,000. It has an opening span of goof t. clear between the two main towers. The side spans are suspended from braced chains, whose pull is taken up by heavy masonry anchorages at the shores and by a steel tie concealed in the horizontal girder that unites the towers at a height of 141f t. above high water. The towers are of steel, faced with granite and Portland stone.
The opening span consists of two leaves or bascules, pivoted at the faces of the towers and counterweighted at their shorter end; they are operated by hydraulic power and take about one and a half minutes to open fully. The bridge was designed by Sir John Wolfe Barry and his partners.
The Newcastle-on-Tyne bridge was begun in 1925 and was opened in the autumn of 1928; it is the largest arched bridge in Great Britain. It has a span of 531 ft. and a width of 64ft. between parapets with a headway for navigation of 84ft. The main arch springs from the quay levels on each side of the river, and is supported by concrete abutments carried down to the sandstone rock. The abutment towers have been designed to contain lifts for giving access from the bridge roadway to the quays. The approximate cost was £750,000 excluding land and property, which brought the total amount up to about £1,200,000—little short of half as much again as the Tower Bridge.
Alexander III. bridge, Paris, built during the period 1896— 1 is remarkable on account of the very flat rise given to its arch in order to provide easy gradients from the quays on each side of the river. The rise of 2oft. Tin. is only one-seventeenth of the span of 353ft. The ribs are of cast steel, and they have pins both at the abutments and at the centre.