The first important steel bridge in the world was completed in 1874. It is the arch bridge which in three spans crosses the Mississippi River at Saint Louis. Its arches are without hinges and their ends are firmly fixed to the piers. This is one of the most famous bridges in existence. It is the longest span in America for any arch bridge without hinges.
The longest span of any metallic arch bridge with two hinges is that of the Hell Gate Bridge. Its length is feet between the centres of end hinges. It is on the line of the New York Connecting Railroad, which connects the Penn sylvania with the New York, New Haven and Hartford at New York It is a four-track bridge with ordinary ballasted tracks laid on a solid floor of metal and concrete. It con tains a number of unusual features in its de sign and is notable for its beauty of form as well as its magnitude. The curved bottom chord has a rise of 220 feet. It is 7 feet wide and varies in depth from to feet, and thus compares closely in size with the bottom chord of the Quebec Bridge. It was completed in 1917. Next in length of span is the upper arch over the Niagara River, built in 1898. Its span is 840 feet.
The largest arch with three hinges is in the highway bridge over the Colorado River at Needles, Cal., the span being 592 feet and the rise 100 feet. The roadway is suspended from the arch near the centre and supported by posts near the ends. It was opened to traffic in 1916. The steel arch span in the Detroit-Superior Viaduct in Cleveland, built in 1916, has a span only one foot shorter. See BRIDGE.
Since 1900 an unusual development has taken place in the construction of reinforced-concrete arches. Not only has a very large number of such bridges been built in this country, but the greatest span now exceeds that of any stone arch bridge in the world. The largest clear span in America is 300 feet 47A inches, that of the Larimer Avenue Bridge in Pittsburgh, Pa., completed in 1912. The floor system of slabs and beams is supported by columns resting upon the two arch ribs, which are braced to gether by horizontal struts, all constructed of reinforced concrete. The largest span in which plain concrete is used in the arch rings is 281 feet and is in the highway bridge at Monroe street, Spokane, Wash., completed in 1911.
The largest concrete arch viaduct in the world is located on the Delaware, Lackawanna and Western Railroad at Nicholson, Pa., and is known as the Tunkhannock Viaduct on ac count of the name of the creek which it crosses. It contains 10 spans of 180 feet and 2 spans of 100 feet, the total length being 2,230 feet. It is a double-track structure and was com pleted in 1915. It is not the longest but the largest in total mass. It contains approxi mately 4,509,000 cubic feet of concrete and 2, 280,000 pounds of reinforcing steel. Its height is 242 feet above the bed of the stream.
It is the smaller steel structures which are destined more and more to be replaced by arches of this material. The steel bridges require re painting at frequent intervals, constant inspec tion, occasional repairs and finally replacing by a new structure after a relatively short life, on account of rust and wear, unless it is required even sooner on account of a considerable in crease in the live load. The concrete arch re quires practically no attention except at very long intervals.
The safety of operating the traffic makes it desirable to have as few breaks as possible in the regular track construction of a railroad, and this constitutes an additional reason why con crete or stone arches are being substituted for the smaller openings. The decreasing cost of concrete tends to an extension of this practice to openings of increasing size. In 1901, how ever, a bridge was completed which marks a decided departure from previous practice. The Pennsylvania Railroad built a stone bridge, consisting of 48 segmental arches of 70 feet span, at the crossing of the Susquehanna River at Rockville, Pa. It is 52 feet wide, accom modates four tracks and cost $1,000,000. This bridge has not only the advantage of almost en tirely eliminating the cost of maintenance, but it also has sufficient mass to withstand the floods which occasionally wreck the other bridges on that river. In 1903 the same rail road built a similar bridge over the Raritan River at New Brunswick, N. J.