BRIDGES

Elevated structures erected acro a gap between other structures or between points of land separated by water or depre ions in the earth. Many of these nece ary erections raised acro rivers for pedestrian, wheel and railroad traffic are remarkable for the engineering skill developed in their construction and the architectural beauty of their designs. Bridges are great landmarks of cities and sections. New York, London, Paris and Berlin have river structures so characterful in their commanding strength and design that they loom as outstanding facts of each city. The same may be said of all towns whose means of communication are augmented by the genius of the builder of bridges. The modern bridge is constructed of stone, concrete or steel arches, or embody types of steel structures known as simple tru , continuous, cantilever, suspension, and high viaduct bridges. Among those of steel arches, a representative erection is that at Hell Gate, N. Y. Metropolis, lll., has a typical simple tru bridge. At Sciotsville, Ohio, is a notable example of the continuous bridge, which cro es the Ohio river over two spans each 775 feet long. Suspension bridges have impre ive representatives in two erected over the East River, at New York City, namely, the Williams burg and Brooklyn bridges. Lethbridge, Canada, has a high viaduct bridge rising 5,327 feet on a span of 314 feet.: Bridge construction clung to old methods until some 160 years ago, about the time when industrial development began to provide better and cheaper materials and stimulate progre in engineering science and _ technology. The growth of railroads also stood out as a leading influence in the building of bridges as they are today. Of all present types the tru bridge is wholly new and its use has largely affected other types. The tru displaced the beam; it is a mainstay in arched bridges and also figures as supports of suspension bridges built to bear modern trafiic. The tru structure of a bridge comprises a network of more or le slender bars linked up in a triangular cro work. They were first built of wrought iron (about 1850). Previously the tru features frequently formed a combina-~ tion of both iron and timber. There would be timber struts and iron tension rods; then came iron rolled beams and plate girders for short spans, and tru bridges with cast iron struts and wrought iron tie rods for longer spans. Wrought iron bridge construction only developed, both as to tru es and plate girders, largely affecting the use of time-honored wood and stone arches, except for minor bridge work. Soft steel, nickel steel and silicon steel in turn displaced wrought iron in bridge building. The production of Portland cement provided another serviceable material, but for a long time stone arches remained in favor despite its growing use. Cement concrete finally (about 1890) came into use for arches. It was followed by reinforced concrete, that is, concrete in which steel rods are embedded in the required directions to provide tensile strength—for beam and arch construction, in which this material is now extensively used, especially for city bridges and those on leading country roads, as well as for railways. A remarkable example of concrete bridge work is the Tunkhannock Viaduct of the Delaware, Lack a wanna and Western Railroad. in Pennsylvania, believed to contain a greater volume of concrete than any other bridge of its kind. Bridge engineers have achieved great feats with various types of suspension and cantilever bridges calling for long spans. Their enterprise and ingenuity have been largely due to improved proce es of metal manufacture, which greatly increased the tensile strength of wrought iron and steel. As bridge materials, wrought iron and soft steel are of high strength in both tension and compre ion (50,000 to 70,000 pounds to the square inch). Being also malleable and tough, holes can be punched in them and pieces bent. Safe stre es are 15,000 to 20,000 pounds, Other steel of high strength, like nickel steel, be stre ed to 20,000 to 30,000 to the square inch. Cast iron, brittle and liable to cracks and defects, is not now used in bridgework except for such parts as bearing In tension is it only about twice as as wood, which can be stre ed to 10,000 pounds to the square inch, cast iron is very strong in compre ion— about 100,000 pounds to the inch, Bridges must be made enough to withstand the forces to bear on them. They must bear own weight, or the stre es of its load; then the traffic moving over or the live or moving load, and the ay and vibration produced therefrom. ind pre ure is quite a serious not to mention the momentum stopping train. Railroads have produced most bridges. On American lines they ber some 80,000. Most of them’are of steel, with tru es or plate designed to sustain uniform loads ing from 3,000 to 4,800 pounds per foot of track, according to the length span and the service required. Highway bridges bear loads of 1,000 to 1,800 pounds per linear foot track and are constructed of metal, masonry or concrete. Over in some parts of the country are able bridges or drawbridges to of a clear pa age way for ve els. are known as swing, rolling or bridges, swing bridges being those commonly used. Here and there pontoon bridges, a development of ancient bridge of boats. The one is a railroad structure running the Mi i ippi river at Prairie du Wis. Bridges of enduring material long when well built and maintained. Steel bridges, however, must be regarded as temporary, with a life limited to crystallization of their metal. bridges, despite being subject to deterioration of mortar joints, have lasted hundreds and even you sands of given good foundations. Modern bridges tend to deteriorate from lescence due to increase of traffic or other causes before their limit of has been indicated.