In the meantime, the principle of the truss, first enunciated by Palladio, the famous Italian architect, about 1560, and subsequently de veloped still further, and without doubt inde pendently, by such geniuses as Ulric Gruber mann, Timothy Palmer, Lewis Wernway, Theodore Burr, William Howe, Wendall Boll man, and others of equal merit, during the period extending from 1754 to 1854, was di verted from its application to wooden bridges, and used in the construction of metal struc tures. The combining of the truss and the girder was the next logical step in the evolu tionary process, and to Roebling belongs the credit of demonstrating the practicability of building long span braced girder or truss bridges with iron and steel as the material of construction. As the cantilever bridges are a class of structures actually composed of hinged continuous braced girders, their intimate rela tion to the girder and truss bridges is quite obvious. They were, however, not much used until the last Quarter of the 19th century. The construction of the Kentucky Viaduct in 1876, and the Niagara cantilever in 1883 demon strated the ease with which they could be built without the use of costly false work, and soon afterward many bridges were built both in Europe and America. The most important of these are: the bridge across the Hudson at Poughkeepsie, N. Y., built in 1889, which consists of five river spans ranging from 525 to 548 feet in length, and a total length of 6,767 feet; the Red Rock Cantilever Bridge across the Red River in California, built in 1890 with a centre suspended span of 30 feet, the shore and river cantilever arms being each 165 feet, and a total length of 990 feet; the Memphis Bridge across the Mississippi River at Memphis, Tenn., built in 1892, with a truss span of 621 feet, and two cantilever spans of 700 feet each, which are the longest of the kind in the United States; the Saint John River cantilever in New Brunswick, built in 1895, with a main span of 477 feet, and a total length of 813 feet; the great Forth Bridge across the Firth of Forth, in Scotland, which was com pleted in 1890, and consists of two shore spans of 680 feet each, and two main spans of 1,710 feet each, which are exceeded only by the Quebec cantilever across the Saint Lawrence, the new construction of which began in 1911, and has a clear span of 1,800 feet.
All bridge structures, regardless of the ma terials of construction, may be classified as to type: arch, suspension, cantilever, truss or girder, or a combination of these types. In de signing them the problem of first importance is to make them strong enough to withstand the forces brought to bear upon them. These in clude the stresses of *dead load,* the weight of the bridge itself ; the live, or moving load, of the traffic over the bridge; impact and vibration from the moving load; wind pressure; the mo mentum of a stopping train; and, if the bridge he built on a curve, centrifugal force. The wind pressure is not one of the least of a bridge's burdens. It is figured at 30 pounds per square foot of one girder in girder bridges, or on one truss of through bridges, or on two trusses in deck bridges, and the floor in truss bridges; and in addition, 300 pounds per lineal foot of any train which may pass over the bridge.
For spans of less than 25 feet, rolled beams are commonly used; from 25 feet up to 100 feet, plate girders; from 100 feet up to 150 feet, riveted trusses; above 150 feet, pin-con nected trusses. The depth of a plate girder is usually from one-twelfth to one-eighth of the span; of a truss, one-sixth of the span.
Outside of the economic purposes of the bridge itself, modern taste demands attention to the environment, and it is expected that the engineer will contrive a structure which shall be pleasing to the eye as well as useful.
Bridges are also classified as to their eco nomic uses, as: Railway Bridges and Bridges; and as to their structural peculiari ties, as Movable Bridges, Pontoon Bridges, Military Bridges, etc.
Modern railway bridges are almost exclu sively constructed of steel and consist of trusses or plate girders designed to sustain uniform loads ranging from 3,000 to 4,800 pounds per linear foot of track, according to the length of span and the service required. There are about 80,000 of such bridges in the United States ag gregating 1,400 miles in length and represent ing a cost of $800,000,000. All steel bridges, however, must be regarded as temporary, with a life limited to the crystallization of their metal. Many of them are being replaced by reinforced concrete.
Highway bridges include all bridges used for roadway purposes alone. They may be constructed of wood, metal, masonry or con crete and are usually designed to sustain uni form loads ranging from 1,000 to 1,800 pounds per linear foot of track, according to the length of spate and the service required.
Movable bridges or drawbridges include the various types of structures over rivers, that can be moved in order to allow a clear passage way for vessels.
The modern structures of this type may be divided into the following classes: (1) Swing Bridges; (2) Rolling Bridges; and (3) Lift Bridges; of which the first are the most com monly used.
A swing bridge consists of a wooden or metal truss supported at the centre by a pier located in the middle of the stream, so that when the bridge is closed the ends of the truss rest upon abutments on the shores on either side. It is operated by a turntable, upon which it rests, which is revolved by a rack and pinion arrangement worked by hand power, steam or electricity.
A rolling bridge consists of a single truss mounted on rollers, and which is pushed out from one side across the span, or of two trusses, on each side of the span which are pushed out and connected at the centre of the span where the water ends are locked together when the bridge is closed. It is operated by the rope and drum method. This type is not used to any great extent.
Lift bridges are of various kinds — the *vertical lift* the *hinged lift* or ((bascule,* and the *rolling lift* bridges. The simplest is the • vertical lift bridge consisting of a truss which is raised vertically to the desired height, both ends rising in arranged on towers. A typical example is the highway bridge over the Louisville and Portland Canal at Louisville, Ky. The lift span is a Pratt truss 210 feet long. It lifts 40 feet, giving 55 feet clearance. It is operated by crane-like trusses counter weighted with concrete masses, pivoted on the main posts of the approach spans. The lift span is raised by electric power in one minute. Another notable lift bridge with several unique features is that at Pine Bluff, Ark., across the L' Arkansas River. The river at this point is normally 2,000 feet wide and the channel is liable to shift to almost any section of that width. Six of the channel spans are 239 feet long and one of them is arranged to lift by towers attached to the ends of the two adjoin ing spans. In case of a shift of the channel, the towers can be moved and any other of the six spans converted to lift. The counter weights are of cast iron of the expanding type to balance the heavy sprocket chains used to operate the lift. The motive power is elec tricity. The hinged lift bridge is raised by being revolved in a vertical plane around hinges at one end. The rolling lift bridge is also lifted in a vertical plane, but has in addition a limited rolling motion. All lift bridges usually have a counterweight to assist the lifting effort, and are generally designed to move quickly—one , minute being frequently specified as the time limit for opening or closing at points where land and water traffic is heavy.