BRIDGE. 488 BBIDGE. about by a vertical shaft running in bearings attached to the outside of the drum and liaving a gear-wheel at its bottom end which meslies into the gear-teeth on the outside of the fixed pier-track; the rotation of this shaft causes the drum to turn on the rollers and swing the span. In large spans two shafts are employed. The motive powers employed to operate the shafts are steam-engines, gas-engines, electric motors, and hydraulic motors. One modification of the cen- tre pier swing span is the form already men- tioned as having the pivot pier nearer to one end than the other ; another is where the pivot is at the extreme sliore end of the span while the other end is supported by a floating pontoon which swings with the span ; another consists of two unequal arm spans with their pivot piers in the opposite banks, and the ends of the long arms meeting and locking over the centre of the channel. During recent years there has been a notable development in the use of iascule spans, where the waterways to be crossed are so restricted in width as to prevent further restriction by placing a pivot pier in the centre or where the land along the banks is too valuable to permit space to be taken up by a swing space with a shore pivot pier. Notable bridges of this type have been built in Chicago, Jlilwaukee, Buffalo, and Bos- ton in the United States, and also in various foreign cities. The Tower Bridge, crossing the river Thames, near the Tower, in London, has a bascule span of 200 feet, formed of two leaves hinged at the opposite abutments, and meeting and locking at the centre. Lift bridges are seldom used. Perhaps the most important one ever constructed crosses the Chicago River at North Halsted Street, in Chicago. On each side of the river are erected two steel framework towers '2V^ feet high, be- tween which is suspended a truss span 130 feet long. When the bridge is closed this span rests on abutments like any simple-truss span. To open it, steel cables passing from each end of each span up and over sheaves at the tops of the towers, and thence to suitable winding drums at the surface, are wound up, raising the span to near the tops of the towers so that vessels may pass beneath. The clear lift of this bridge is 15.5 feet above low water. TRESTLE ADUCTS. Originating with the earliest forms of timber trestles, a type of bridge has been developed, chiefly in the United States, which is known as the trestle viaduct, a bridge of wood or metal in which the different spans are supported directly upon legs or towers composed of two or more trestle bents braced together in all directions. The first application of this system to a bridge of any great magnitude was in the Kinzua Viaduct, which is well known for the boldness of its design and its embodiment of the distinctive features of the system. This viaduct was luiilt in 1S82, and is located at the crossing of Kinzua Creek, which flows through a gorge some 300 feet below the level of the surroiuiding country, on the Erie Railroad, in McKean County, Pa. Each trestle tower consists of four legs braced to- gether in all directions; the longest bent in any tower is 270 feet between the to)) of the masonry and the toj) of the tower, in I!MK) this viaduct was entirely rebuilt to practically the original dimensions, but with steel instead of wrought iron and with heavier and stitier members throughout. Some years later the general plan of tlie Kinzua Viaduct was closely copied in the construction of the Loa Viaduct on the Anto- fagasta Railway in Chile. This viaduct spans the canyon of the River Loa in the Upper Andes at an elevation of 10,000 feet above the sea- level, and if counted from the surface of the water to the top of the rails, is the highest in the world, if we except the Santa Giustina Bridge in the Tyrol and the Carabit Viaduct in France, which are merely iron arches over deep gorges and have towers of no great height. The length of the longest bent of the Loa Viaduct is 314 feet 2 inches. In 1893 a viaduct, ranking in size with the Kinzua and Loa, was built to carry the Southern Pacific Railway over the Pecos River in Texas. To enable a comparison between these three most notable examples of the trestle via- duct, the following has been compiled: Kinzua feet inches Total length 2O.'i0 302 279 Height above water. Length of longest bent Width of towers c. to c. of bents Longest span W'idth overall "Width c. toe. of trusses Weight of metal work. 38 6 61 18 U 10 1400 tons feet inches 800 336 314 i 32 80 10 8 10 1115 tons feet inches 2180 320 lOVi 241 % 35 18.^> 16 10 1820 tons DESIGN, SllOPWORK, AND ERECTION. The scientific design of a modern metal bridge is a task calling for a high degree of engineering skill, and an attemiit to explain this share of bridge work without technicalities must he, to some extent, a failure. Knowing the purpose for which the bridge is to be used and the topog- raphy and geography of its location, the first task of the engineer is to decide upon the niun- ber and character of the spans. Where the stream is narrow, a single simple s|)an with its masonry abutments will suffice, but for wider crossings two or more such spans will be needed. The length, and consequently the number of these spans has to be determined, and for this problem the engineer has two possible solutions, viz. to make a large number of short spans, in which case the number of piers is a maximum and the cost of the substructure increased, or to make a few very long spans, in which case the number of piers is the minimum, but the cost of the superstructure Is increased. Evidently, other things Ijcing equal, the solution to be sought is at the point between the two extremes, where the total cost of the bridge, substructure and super- structure combined, is the least. As a nile of thumb, it is assumed that this point is where the cost of the superstructure and the cost of the substructure are equal. It will readily be understood that there are occasions when the engineer does not have a free choice in selecting the number and heights of spans. Such cases arise where the interests of navigation forbid the construction of a pier in midstream, an example of which is the lhids(m River at New York, or where the depth of the gorge or of the water in the .stream, or a rapid current, or unsuitable bot- toms prevent a midstream pier, an example of
