In sticams subject to freshets and ice, the up-stream starlings should receive a greater projection than those down stream, and, moreover, be built in the form of an inclined plane (Fig. 135) to facilitate the breaking of the ice, and its passage through the arches.

583. Where the banks of a water-course spanned by a bridge are so steep and difficult of access that the roadway must be raised to the same level with their crests, security for the foundation, and economy in the construction demand that hollow or open piers be used instead of a solid mass of masonry. A construction of this kind requires great precaution. The facing courses of the piers must be of heavy blocks dressed with extreme accuracy. The starlings must be built solid. The faces must be connected by one or more cross tie-walls of heavy, well-bonded blocks; the tie-walls being connected from distance to distance vertically by strong tie-blocks; or, if the width of the pier be considerable, by a tie-wall along its centre line.

584. The foundations, the dimensions, and the form of the abutments of a bridge will be regulated upon the same principles as the like parts of other arched structures; a judicious conformity to the character of strength demanded by the

structure and to the requirements of the locality, being observed. The wails which at the extremities of the ridge form the continuation of the heads, and sustain the embankments of the approaches,--and which, from their widening out from the general line of the heads, so as to form a gradual contraction of the avenue by which the bridge is approached, are termed the wing-walls, serve as firm buttresses to the abutments. In some cases the back of the abutment is terminated by a cylindrical arch (Fig. 136) placed on end, or having its right-line elements vertical, which connects the

two wing-walls. In others (Fig. 137) a rectangular-shaped buttress is built back from the centre line of the abutment, and is connected with the wing-walls either by horizontal arches, or by a vertical cross tie-wall.

585. The wing-walls may be either plane surface walls (Fig. 138) arranged to make a given angle with the heads of the bridge, or they may be curved surface-walls presenting their concavity (Fig. 145) or their convexity to the exterior; or of any other shape, whether presenting a continuous or a broken surface, that the locality may demand.

586. The arches of bridges demand great care in propor

N

M

Fig. 138-Represents an elevation M and plan N of a portion of a single arch bridge with straight wingwalls sustaining an embankment across the valley of the water-course.

a, a', face of wing-wall.

b, b', side slope of embankment.

c, c', top of wing-wall.

o, o', fender or guard stones.

tioning the dimensions of the voussoirs, and procuring accuracy in their forms, as the strength of the structure, and the permanence of its figure, will chiefly depend upon the attention bestowed on these points. Peculiar care should be given in arranging the masonry above the piers which lies between the two adjacent arches. In some of the more recent bridges, (Fig. 139,) this part is built up solid but a short distance above the imposts, generally not higher than a fourth of the rise, and is finished with a reversed arch to give greater security against the effects of the pressure thrown upon it.

The backs of the arches should be covered with a watertight capping of beton, and a coating of asphaltumn.

587. The entire spandrel courses of the heads are usually not laid until the arches have been uncentred, and have settled, in order that the joints of these courses may not be subject to any other cause of displacement than what may arise from the effects of variations of temperature upon the arches. The thickness of the head-walls will depend upon the method adopted for supporting the roadway. If this be by a filling of earth between the head-walls, then their thickness must be calculated not only to resist the pressure of the earth which they sustain, but allowance must also be made for the

Fig. 139-Represents a longitudinal section of a portion of a pier and foundations, and of an arch and its centre of the new London bridge over the Thames. A, finish of solid spandrel with reversed arch.

effects of the shocks of floating bodies in weakening the bond, and separating the blocks from their mortar-bed. The more approved methods of supporting the roadway, except for very flat segment arches, are to lay the road materials either upon broad flagging stones (Figs. 139, 140,) which rest upon thin brick walls built parallel to the head-walls, and supported by the piers and arches; or by small arches, (Fig. 141) for which these walls serve as piers; or by a system of small groined arches supported by pillars resting upon the piers

and main arches. When either of these methods is used, the head-walls may receive a mean thickness of one fifth of their height above the solid spandrel.

588. Superstructure. The superstructure of a bridge consists of a cornice, the roadway and footpaths, &c., and a parapet.

The object of the cornice is to shelter the face of the headwalls from rain. To subserve this purpose, its projection beyond the surface to be sheltered should be the greater as the altitude of the sheltered part is the more considerable. This rule will require a cornice with supporting blocks, (Fig. 142,) termed modillions, below it, whenever the projecting part would be actually, or might seem, insecure from its weight. The height of the cornice, including its supports, should generally be equal to its projections; this will often require more or less of detail in the profile of the cornice, in order that it may not appear heavy. The top surface of the cornice should be a little above that of the footpath, or roadway, and be slightly sloped outward; the bottom should be arranged with a suitable larmier, or drip, to prevent the water from finding a passage along its under surface to the face of the wall.

589. The parapet surmounts the cornice, and should be high enough to secure vehicles and foot-passengers from accidents, without however intercepting the view from the bridge. The parapet is usually a plain low wall of cut stone, surmounted by a coping slightly rounded on its top surface. In bridges which have a character of lightness, like those with flat seg ment arches, the parapet may consist of alternate panels of