In brickwork such openings are generally covered by a gauged (or sometimes axed) arch, which shows on the face of the wall for ornament, having a relieving arch on the inside to support the weight of the wall above (see Figs. 524, 549).
The external arch may be "flat," "camber," "segmental," "semicircular," or struck to any curve, such as the semi-ellipse or parabola.
Fig. 97. Window Opening with Segmental Arched Head,
It will be seen that the gauged arch extends into the wall only the depth of the reveal (in this case 4 1/2 inches). As this arch has no connection whatever with the rest of the wall, it should be built with the greatest care.
A rough brick arch in two half-brick rings is turned over the opening in the remainder of the thickness of the wall, and contains wooden bricks, wb, or, in good work, plugs, to which the sash frame may be secured.
Semicircular Arches are arranged in exactly the same way as those just described, the only difference being in the curve of the soffit.
If the head of the opening is to be flat on the soffit, or nearly so, a straight gauged arch may be adopted, as shown in Fig. 98.
Fig. 98. Straight Arch Window Head.
Fig. 99. Showing Believing Arch.
This gauged arch extends into the wall for a thickness of only half a brick. Behind it, the opening is spanned by a wood lintel, to which the frame of the door or window is fixed.
In order to protect the lintel from the weight of the wall above, a rough relieving arch is turned over it, as shown in the back elevation, Fig. 99. The portion between the top of the lintel and the soffit of the relieving arch is called the core.
Care must be taken that this relieving arch abuts on the wall clear of the ends of the lintel, otherwise, when the timber shrinks, rots, or is destroyed by fire, the arch would lose its supports and fall in.
Examples of wood lintels with relieving arches are given in Figs. 543, 549.
Another plan is to do away with the lintel altogether, and to substitute for it a flat or slightly cambered rough-axed arch like that in Fig. 101. In this wood plugs are inserted, to which the frame may be attached. An example of this form of construction is given in Figs. 524, 525, and others.
Arches of this kind for wide spans may be supported by flat wrought-iron tension bars, extending along the soffit, and held up at short intervals by iron bolts passing through the depth of the arch, each secured by a plate and nut on the extrados.
French or Dutch Arches (see Fig. 100) are sometimes adopted in walls that are to be stuccoed or plastered. They may also be used as flat relieving arches, but the construction is not theoretically a good one, and should never be adopted.
Fig. 100. French or Dutch Arch.