Plain, Common, or Rough Brick Arches are those in which the bricks are not cut, or rubbed, so as to form voussoirs accurately radiating to a centre. The joints are therefore wider at the "extrados" than they are at the "intrados." Such arches are used for ordinary brickwork in tunnels, and concealed work generally.
Bough arches of small span are generally turned in half-brick rings, 4 1/2 inches thick, as shown at hh in Fig. 92. In arches of quick curve, with not more than 3 or 4 feet radius, this is absolutely necessary to prevent very large joints at the extrados.
Fig. 9 2 is the section of portions of small arches, of which one, w w, is turned in 9 inch rings consisting of headers. It will be seen that the mortar joints in this are much wider at the extrados than those of the portion hh, built in rings half a brick in thickness.
Fig. 92. Brick Arch, showing Opening of Joints at Extrados.
When wide joints necessarily occur on the extrados of an arch, they are often filled in with pieces of slate, and made as tight as practicable.
The plain or rough arches most usually required in buildings are those for relieving lintels over window and door openings (see Figs. 99, 543, 549), trimmer arches to support hearthstones (Fig. 290), the arches in chimney breasts (Part. II.), etc.
Rough-cut or Axed Arches have the bricks roughly cut with a bricklayer's axe to a wedge form, and are used over openings when the work is to be plastered, as relieving arches at the back of window and door heads (see Figs. 525, 545), or in some cases as face arches.
Gauged Arches are built with bricks accurately cut, and rubbed down so as to radiate from the centre. They are used chiefly for external face arches over openings and recesses in superior work (see Figs. 98, 549).
Bricklayers frequently carefully rub only the portion of the joint near the face, cutting the back part right away, so that the arch does not bear equally, except just on the front edges of the bricks. This leads to the arch bulging forward, or to bricks dropping out of it altogether, and the pressure being all on the edges they readily splinter off.
These arches are generally built with special bricks, easier to rub, and of a larger size than common bricks.
When bricks of the ordinary size are used for straight arches, they are not long enough to be splayed at the ends to form the horizontal joints parallel to the soffit, as shown in Fig. 98. In such a case, the ends of the bricks are left square, the real joints daubed over to conceal them, and false joints made for appearance to look like those in Fig. 98. This, however, is bad work, and should be avoided.