It is impossible here to trace in detail the progress of the art. For a long time after the decay of the Roman empire, it made no progress. It revived in the 11th c., but again languished to the beginning of the 18th, when the formation of the corps of the Ponts et Chaussees in France favored its further growth. Henceforth, many splendid bridges were erected both in Britain and the continent. In 1775, Mr. Pritchard of Shrewsbury, introducing the use of cast iron in the erection of bridges, originated a valuable style of construction. The genius and works of Telford bring us to the present time. Within half a century, the use of steam, the development of the canal system, and the necessity especially for railway-bridges, with the immense amount of capital at the disposal of engineers for purposes of bridge-building, have caused a rapid evolution of all the principles and possible modes of the art. Among the new forms called forth within the century by the increasing demand for facilities of communication, are the suspension B., the wrought-iron girder and tubular bridges, and the lattice-bridges. Sev eral of the new bridges over the Thames are models of engineering skill and taste. The Menai and Britannia bridges were regarded when erected as perfect marvels of the art, and yet they have since been surpassed. In America, the B. of Trenton, over the Dela ware, the great Portage viaduct, and the Niagara suspension B., are equal to any similar works in the world. The Tay B., opened in May, 1878, is the longest (2 m. in length) and perhaps the greatest achievement of modern engineering skill. The variety of complex structures of wood and of iron that now span streams and hollows is endless. For some of the more important forms, see FRAME, LATTICE, TUBULAR, and SUSPEN SION BRIDGES. What follows here, relates chiefly to arched or masonic bridges, and is confined to the more general and obvious conditions which such bridges must fulfill, avoiding the mechanical theory of their stability as too abstruse for popular exposition.
An arched B. rests between masses of masonry on opposite sides of a river, called its abutments (q.v.). The intermediate points of support of the arches are the piers (q.v.), which are rarely built so strong as to be able of themselves to resist the lateral thrust of the arches resting on them, if the thrust of one arch did not counteract that of another. The arch itself is the curved construction between adjacent piers. The chief terms used in speaking of the arch itself are explained under ARCH. In addition, may be noticed the spandril, the name given to the filling in above the extrados to the road way. The chord or span is the distance between the piers; while the rise of the arch is the perpendicular distance between the level of the springing and the horizontal through the key.
When a B. has to be erected, the question of what form it should be, falls to be set tled by a variety of considerations. Regard to appearance affects the question, but the material points are its sufficiency for the purposes for which it is intended, and its security and durability The nature of the embankments and of the soil iu the water-bed, together with the nature of the water-shed, or country drained by the stream, may make it necessary that the 13. should not be an arched bridge at all, but a suspension or tubu lar bridge. But if it is to be an arched 13., then the most important questions respect the number of its piers and the form of its arches. If vessels must be free to pass under it, the arches must be lofty, and the abutments high; so also must they be if the river is exposed to sudden elevations of its level by floods. Formerly, a prejudice existed against laving a B. across a stream at any other angle than at right angles to its course. The reason was, that, the theory of the skewed arch (q.v.) being unknown, the obliquity of the B. to the water-course involved a corresponding obliquity of its piers to the water, which greatly increased the risk of the B. suffering from floods. But the skewed arch
allows a B. to be thrown at any angle across a river, with its piers all parallel to the stream; and many an awkward turn in our public roads would have been spared us, had the skewed arch only been earlier known.
After making allowance for the requirements of position and traffic, the form next must be considered, more particularly in relation to the stream. The stream principally affects the form, through prescribing the number of piers. Each pier takes up so much of the water-course, and thus narrows the effective passage of the water. The imme diate consequence of narrowing the channel is to increase the velocity of the stream. As the velocity of the stream increases, it tends more and more to carry off the soil in the neighborhood of the piers, and finally, by deepening its course, to undermine them. From this consideration, the effect of too many piers will be obvious; but indeed this is not matter of speculation. for many bridges—among others, a B. of Smeaton's at Hex ham—have, been destroyed from this cause, thus falling from the very overabundance of support! To know bow many piers may with safety be used, the volume of water that flows through the channel, both ordinarily- and in win-ter-floods, must be ascertained, which can be done very nearly by calculating the mean of many soundings taken at dif ferent states of the river, and at a succession of points across its bed. There is another way in which the stream affects the form. If it is liable to floods, care must be taken to make the piers so high as to elevate the spring of the arches above the highest level attainable by the water. In connection with this part of the subject, it must' be remem bered, too, that floods are apt to carry down trees and other floating masses, which, if the arches do not afford them passage, become powerful levers for the destruction of the bridge.
The form of the B. on, the remaining questions relate to its stabil ity. This depends on the strength of the abutments and piers, and the balanced equi librium of the arches. The importance of securing proper foundations for the abut ments and piers cannot be over-estimated, and very frequently their foundations, owing to the nature of the soil, have to be artificially constructed. See Pli,Es, COFFERDAM, and CONCRETE. In considering the stability of the B., the first thing is to ascertain the forces which will act to destroy it. This is ascertained by calculating the extreme pass ing load, and also the weight of the structure above the arches, and of the arches them selves. A scientific and skilled engineer is then able to judge what amount. of strain or destructive pressure will be exercised by these weights on the several parts of the struc ture, and thus to adapt the strength at every point to the strain. As to the passing load, it is usual to calculate ou 240 lbs. per foot, superficial, of the whole area in ordinary bridges, and on 960 lbs. in railway bridges. The weight of the superstructure and arches is a question for practical measurement. As to the remaining pressure—viz., that of the stream—it must be ascertained for the highest floods. It is calculated from knowing the mean velocity of t„he stream, and the amount of surface exposed to it. The surface is readily observed by means of floats: and when this is under 10 ft. per second, the mean velocity is found to be about one fifth less. The stress of the stream on the bridge is diminished by the expedient known as a cut-water, which is an angular projection from the pier. The best form for a cut-water has practically been ascertained to be an equilateral prism, presenting an angle of 60' to the water-course. Iu all bridges, these are to be found on the sides of the piers presented to the stream; and in tidal rivers, they are built on the lower side as well.