C/assification.—Stone bridges are arched structures formed of a single span when the width to be spanned is small Co/. 39, figs. 12, 13), or of a number of vaulted arches springing, from piers of masonry (pi. 4o, fig. 7) when the width is considerable. The roadway is supported on the arches. According to the form of curvature exhibited by the arch, several varieties mav be distinguished—for example, the semicircular arch (pi. 39, fig. r2), forming a half circle from its springing lines; the low-circular or seg mental arch (fie,-. 13), in which the centre of curvature lies below the line of the springing-s; the stilted arch, in which the centre of curvature lies above the springing lines; the low, flattened, or elliptical arch (pi. 40, fig. 7; pi. 41, figs. 6, 7); and the pointed or Gothic arch (fie,. 13).
Arcb-construction.—The form of the arch has a certain static relation to the manner of distribution of the load it sustains. The feet of the arch rest upon substantial masonry supports or abutments. The space behind the arch along thc flanks is filled in with masonry (pi. 39, fig. 12) or with earth, or partly with masonry and partly with earth, to give the necessary stiffness to the structure. In arches of large size a series of parallel walls of masonry, called spandrel-walls, are built astride the arch; these are sometimes streng-,thened laterally bv vertical cross-wall connections. The spaces left are then covered with flat stones or small arches carried from wall to wall, which thus serve as the support for the material of the road way. Figure I (pi. 4o) exhibits a half elevation aud a section of the details of arch construction. To prevent injury to the arches by the penetration of moisture, the entire backs of important structures should be covered with a layer of cement-mortar, and the same material should be used liberally in the masonry, above them. Special provision for drainage by means of suit able channels left for the purpose is advantageous in protecting the struc ture from deterioration caused by rain-soakage. The depth of the arch stones (voussoirs) either may be left uniform, or the arch stones may be made to increase in ciepth uniformly from the centre- or key-stone to the spring-ers forming the two feet of the arch, the increase corresponding to the increasing pressure upon the members from the crown to the feet. This practice is not necessary in arches of small span. With arches made
of hewn stone, particular attention should be given to the shaping of each member. Where broken stone or bricks are used for the arch, the selection of a proper cement becomes important; this should be quick-setting and have good binding qualities.
Coares.—For supporting the arch while it is building, temporary wooden structures—called " centres "—are used. For arches of moderate span and weight, a centre of the general form of construction shown on Plate 39 (fig. 12) is employed, in which a number of wooden frames or trusses placed at suitable distances apart are covered with a flooring of planks laid on edge and trimmed so as to conform to the curvature of the arch on which the arch-stones are laid. These frames are supported at their ends upon posts whose tops are united by cap-pieces and whose feet rest on string-pieces, the entire series being firmly secured together by diagonal braces. The string-pieces, in turn, according to the nature of the ground, may rest on wooden blocks, which may be adjusted to conform to any irreg-ularity of the surface; where the earth is not sufficiently firm for this, or where the arch is very heavy, they should be supported on blocks of brickwork, or, as is frequently the case, upon projecting offsets provided for the purpose in the piers or abutments. If the arch is to be erected over water, centre supports of crib-work or piling may be neces sary. In this form of centre it will be observed that the space beneath the framework is left free from obstruction, and there will be no interference to free communication while the work is progressing.
This form of centre, however, is adapted only for arches of moderate size (3o to 6o feet span). Beyond this it will be found necessary, in order to secure the requisite support during construction, to use two or three rows of posts, or to modify the construction as shown in Figure 13, in which, in addition to the braced posts at the ends, the weight of the arch is supported upon three intermediate braced posts resting in turn upon piles. In the case of the Grosvenor Bridge, at Chester, England, across the river Dee, the centre was supported by four sets of vertical and inclined posts, each set spreading out like a fan from the top of a temporary pier of masonry on which it rested. The span of this arch is zoo feet.