This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
261. When there is sufficient height above a window, door, or any other opening in a brick wall, a brick arch, either circular or segmental, is used to span the opening, and forms a very durable and easily constructed support for the wall above. Whenever brick arches are built, great care should be taken in their construction, and they should be laid with full mortar joints. If the span is more than 10 feet, the arch should be laid in cement mortar; in fact, it is the best and safest to lay all brick arches in cement mortar.
262. When semicircular arches are constructed of common brick, the bricks are laid close together on the inner edge, or intrados, with wedge-shaped joints on the outer edge, or extrados; that is to say, the mortar joints are wider at the upper surface of the brick ring than at the lower surface, so that there is more mortar at the top of the joint than at the bottom. The bed surfaces of the brick are therefore not on radial lines, as they are in a gauged brick arch, but the radial lines are assumed to pass through the center of each mortar joint.
Fig. 112 shows a semicircular arch consisting of four rowlock courses of brick. These arch brick are all laid as headers, and show an 8-inch reveal on the under side or soffit of the arch. Arches built in this way, of a series of rowlocks or concentric rings, have no connection between the rings other than that afforded by the adhesion of the mortar.
Rowlock arches are frequently bonded back into the rear wall with hoop iron let in at right angles to the joints.
263. In order to obtain a better bond, the arch shown in Fig. 113 is often used. This arch is bonded in several places, with stretcher brick set on end, serving the same purpose as voussoirs in stone construction. The header brick are shown at a, and the stretcher brick forming the voussoirs at b. An arch of this kind can be bonded back into the rear wall by the use of headers where the voussoir stretchers occur, and is known as a block-in-course arch.
264. In arches of large span built of common brick, especially in the brick lining of tunnels and vaults, the bond is often effected by building in headers, which will unite the concentric rings where the joints of two of the rings come together. An example of this is given in Fig. 114, which shows an arch of four rowlocks, two being header and two stretcher courses, the header and stretcher courses being bonded by headers, as shown at a.
Skewbacks. When brick arches of large span are to be built, they should in all cases have a solid bearing for the arch to spring from; such a bearing is called a skewback, or springing stone, and is shown at b in Figs. 114 and 116. The stone should be cut so as to bond into the brickwork of the pier, and the surface c, that the arch springs from, should be cut to a true radial plane.
266. Fig. 115 shows a semicircular arch constructed of gauged, or shaped, brick. The gauging, or shaping, may be accomplished by laying out the arch ring on a floor, and cutting, rubbing, or grinding the brick to a certain gauge, or pattern, so that each brick will fit exactly in the place chosen for it, and all the mortar or radial joints will be of the same thickness throughout.
When the reveal, or space between the window frame and the outside of the wall, is only 4 inches, gauged-brick arches do not usually have any bond in the body of the wall, and the brick in the arch must be laid with great care and accuracy.
Gauged, or shaped, brick are supplied by most of the extensive pressed-brick manufacturers, who prepare the brick so that each one will fit accurately in its position in the arch. When these brick are ordered from the manufacturers, either full-sized or large-scale drawings should be furnished, giving the span of the opening, the radius of the arch, and the depth of the reveal.
267. Figs. 116 and 117 give examples of brick segmental arches. Fig. 116 shows a three-rowlock arch constructed entirely of stretcher brick. This form of arch, unless bonded back into the rear wall with strap iron, is not a strong construction.
Fig. 117 shows a 12-inch segmental brick arch over a window opening, with the arch constructed of gauged brick, and the window frame fitted into the head.
Fig. 118 shows a flat arch, bonded into the rear wall by the headers a, a. It is best to give the soffit of such arches a slight camber, or curve, as shown at b, because when they are made level they are almost sure to settle and sag a little, and crack the glass in the sash. In the plan of the window opening and sill shown at (b), Fig. 118, a shows the reveal of the brickwork; b, the 2-inch offset for the box frame of the window; and c, the window sill. The section (c), Fig-. 118, shows the under side, or soffit, of the arch at a, and the wood lintel at b. This wood lintel is placed behind the brick arch, and may also be used as a support for the floor joists. The lintel may have from 4 to 6 inches bearing at each window jamb.