TRESTLE VIADUCTS.
Originating with the earliest forms of timber trestles, a type of bridge has been developed, chietly in the United States, which is known as the trestle viaduct, a bridge of wood or metal in which the different spans arc 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 built, in ISS2, and is located at the crossing of Kinzua Creek, which flows through a gorge some 300 feet below the level of the surrounding 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 279 feet between the top of the masonry and the top of the tower. In 1900 this viaduct was entirely rebuilt to practically the original dimensions, but with steel instead of wrought. iron and with heavier and stiffer members throughout. Sonic years later the general plan of the 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 Oa•abit 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: The scientific design of a modern metal bridge is a task calling for a high degree of engineering skill, and an attempt to explain this share of bridge work without techniealities must be, 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 num ber and character of the spans. Where the stream is narrow, a single simple span with its masonry abutments will suflice, 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 ease the number of piers is the minimum, but the cost of the superstructure 'is increased. Evidently, other things being equal, the solution to he 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 rube 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 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 Hudson 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 which is the gorge of the Niagara River. In these cases the engineer must use a single long span, and is, therefore, restricted to the types of bridge which will permit great length of clear span, such as the metal arch, the cantilever truss, or the suspension types. In other instances the handsome appearance of the bridge is paramount, and some design which will fulfill this require ment is necessary. If a navigable stream is to be crossed at low level, a drawbridge is necessary. Thus it will be seen that the local conditions and requirements have to be most carefully stud ied, and the available designs in each case care fully integrated, to secure the one which will best meet the controlling conditions at the low est cost. The design once selected, the engineer, knowing the loads which the bridge is to carry, proceeds to calculate the character and intensity of the strain in each principal member. This being done and the strength of the material known, it is possible to determine the size re quired of each member to resist the strain coming upon it. The next process is to determine the form and dimensions of each member and to pre pare drawing, showing closely the form and these dimensions.