Brick arches are generally used for spanning the openings in brick walls, and where there is sufficient height for the arch they form the most durable support for the wall above. The arches should be laid with great care with full joints, and all arches having a span of over 10 feet should be laid in strong cement mortar, and it is much safer to lay all brick arches in cement.
Gauged Arches. - When arches are built of common brick the bricks are laid close together on the inner edge, with wedge-shaped joints, as shown in Fig. 161, but when built of face bricks the arch rim is laid out on a floor and each brick is cut and rubbed to fit exactly the place chosen for it, so that the radial joints are of the same thickness throughout. Such work is called gauged work.
Bond. - The only point requiring especial mention in connection with brick arches is the bond. When gauged arches are used the bricks are generally bonded on the face of the arch to correspond with the face of the wall, as shown in Fig. 158. Such an arch is called a bonded arch. Bonded gauged work makes the neatest and strongest job, but it is too expensive for common brick arches.
Arches of common brick are generally built in concentric rings, either without connection with each other, except by the tenacity of the mortar, or else bonded every few feet with bonding courses built in at intervals like voussoirs, as shown by the heavy lines at A, Fig. 160. When the concentric rings are all headers, as in Fig. 159, the arch is designated as a rowlock arch, or bond, and when built with bonding courses, as in Fig. 160, it is known as block in course bond. Segmental arches are often built with concentric rings of stretchers (Fig. 161), which may be bonded at right angles to the face by hoop iron. When the radius is over 15 feet this should be stronger than the rowlock bond.
Common brick arches are sometimes bonded by introducing headers so as to unite two half brick rings wherever the joints of two such rings happen to coincide. Fig. 162 shows the bonding employed in arching the Vosburg tunnel on the Lehigh Valley Railroad, the span being 28 feet. Building an arch in concentric rings has the objection that each ring acts nearly or quite independent of the other, and the least settlement in the outer rings throws the entire pressure on the inner ring, which may not be able to resist it. When bonding courses are used, however, they serve to tie the rings together and to distribute the pressure between them, so that the above objection is overcome. For arches of wide span, or when heavily loaded, some form of block in course bond should be used. Hoop iron is often built into arch rings parallel to the soffit, and is also often worked in the radial joints to unite the different rings. The stability of an arch may be greatly increased by its use.
Skewback. - In building brick arches of large span it is important to have a solid bearing for the arch to spring from. Such a bearing may be best obtained by using a stone skewback, as shown in Figs. 161 and 162. The stone should be cut so as to bond into the brickwork of the pier, and the springing surface should be cut to a true plane, radiating to the centre from which the arch is struck. For large arches the skewbacks should be bedded in cement.
Flat Arches. - Flat arches are often built over door or window openings in external walls for architectural effect. Such arches, if built with a perfectly level soffit, almost always settle a little, and it is better to give a slight curve to the soffit, as in Fig. 163, or else support the soffit of the arch on an angle bar, the vertical flange of the bar being concealed behind the arch.
Relieving Arches. - The portion of a wall back of the face brick arch, or stone lintel over door or window openings, should be supported by a rough brick arch, as shown in Fig. 164. A wooden lintel is first put across the opening, and on this a brick core or centre is built for turning the arch. Sometimes arched wooden lintels are used and the arch turned on them. When the walls are plastered without furring the method shown in the figure is the best, as there will be less woodwork. The lintel should not have a bearing on the wall of more than 4 inches, and the arch should spring from beyond the end of the lintel as at A, and not as at B, as in the latter method the arch is affected by the shrinkage of the lintel.