Drainage. The drainage of arch bridges of more than one span is generally effected by giving the top surface of the backing a slight inclination from each side toward the center of the width of the bridge and also from the center toward the end of the span. The water is thus collected over the piers, from whence it is discharged through pipes laid in the masonry.
To prevent leakage through the backing and through the arch sheeting, the top of the former should be covered with a layer of puddle, or plastered with a coat of cement mortar, or painted with coal tar or asphaltum.
Brick Arches. The only matter requiring special mention in connection with brick arches is the bond to be employed. When the thickness of the arch exceeds a brick and a half, the bond from the soffit outward is a very important matter. There are three principal methods employed in bonding brick arches: (1) The arch may be built in concentric rings; i.e., all the brick may be laid as stretchers, with only the tenacity of the mortar to unite the several rings. This method is called rowlock bond: (2) Part of the brick may be laid as stretchers and part as headers, by thickening the outer ends of the joints—either by using more mortar or by driving in thin pieces of slate, so that there shall ba the same number of brick in each ring. This form of construction is called header and stretcher
bond: (3) Block in course bond is formed by dividing the arch into sections similar in shape to the voussoirs of stone arches, and laying the brick in each section with any desired bond.
Skewback. In brick arches of large span a stone skewback is used for the arch to spring from. The stone should be cut so as to bond into the abutment, and the springing surface should be cut to a true plane, radiating from the center from which the arch is struck.
Flat Arches are often built over door or window openings; they are always liable to settle and should be supported by an angle bar, the vertical flange of which may be concealed behind the arch.
Relieving Arches. This term is applied to arches turned over openings in walls to support the wall above; beams called lintels are usually used in connection with this type of arch, the lintel should not have a bearing on the wall of more than 4 inches, and the arch should spring from beyond the ends of the lintel as shown in A, Fig. 31, and not as at B.