The principal kind of arch, most commonly used, is the ordinary brick relieving or discharging arch, made of rough bricks, over the inside openings of doors and windows and internal openings. It springs from skewbacks outside the ends of lintels, and is bedded on a brick core, over the lintel, as shown in fig. 135, or on a circular extrados cut on the lintel, as fig. 136. These relieving arches rise in the centre about one inch for every foot of span.
Such arches, of a very small span and of a considerable rise, are built up in half-brick rings, as shown in fig. 137, in order to obviate the excessively large joints, which would result from the use of whole bricks, at the extrados, as shown in fig. 138.
French or Dutch arches are flat or straight, as shown in fig. 139, differing from fig. 129.
Other kinds of arches are built on temporary wooden framings or centres of the required dimensions and curves, to support the brickwork until it has settled into its proper position, and become duly bound together and solidified by the setting and hardening of the cement or mortar.
The commonest arch of this class, which requires centres, is the French or Dutch arch. This is a rough flat or cambered arch, but distinct from the ordinary flat arch (fig. 129), by a different mode of construction in regard to the arrangement of the bricks, as appears from fig.139; but, from its unsightliness, it is only suitable for walls which are to be plastered or otherwise covered up.
All the different kinds of arches can be built, either rough for plastering, with the ordinary common bricks, or they can be axed or rubbed and gauged for facework, as hereinafter explained; and all such arches should be built in cement, for the best work.
An axed arch is one .formed of hard bricks, which are cut, either roughly or finely, by the bricklayer's axe, to the wedge-shaped size and shape required.
Rubbed and gauged arches are built of "rubber" or other soft bricks, which have been cut and finely rubbed to the form and dimensions required. They are usually set in "putty," with very fine joints; and care should be taken that the joint of each brick is perfectly true, and at right angles with and within its face; so that the pressure may be equally distributed, and the edges of the bricks not liable to crack and split away from any undue pressure at the outside, where they are bound to show a line. Some arches are built with special-made bricks of the required wedge-shape and size.
The student will notice that, in openings with rubbed or axed arches outside and a rough arch inside, the inner arch is to be set back to the full depth of the jamb, outside the line of soffit of the outer arch as shown in figs. 140 and 141; or as already shown in fig. 135, when applied to lintels and discharging or relieving arches. There is only one, difference by which the various kinds of arches can be distinguished on drawings, common brick arches showing thick wedge-shaped joints, and each brick measuring 3 inches at the intrados, while the bricks in both axed and rubbed and gauged arches show 3 inches on the extrados; and the difference between the two is further shown by two grades of the lines, thin lines indicating the superior kind, while the thick lines denote common work.
Inverted arches are those turned upside down, for the purposes of spreading the weights and pressure from piers, and in special places, on to a wider area of foundation; as in such instances as big chimney-breasts and arcades, shown in fig. 142.
Section Fig, 141.