It has been stated in Part I. that small rough arches of brickwork are generally turned in half-brick rings, and that this is especially necessary when the arch is of a quick curve, in order to avoid large joints upon the extrados.
Some authorities, however, recommend that flatter arches, especially those of larger span, should be built in 9-inch rings. This may be done in two or three different ways.
The section being like that in Fig. 43, and the soffit showing the same arrangment as the face of a wall built in Flemish bond.
The ring throughout consisting of headers as in the section, Fig. 44, excepting at the ends of the arch, where three-quarter bricks are introduced to break the bond, in the same manner as is done in the face of a wall built in heading bond.
Of the above varieties the heading bond is the strongest, as the voussoirs are each in one piece and no bats are required ; but it is very difficult to make neat work with such a bond, and it is therefore very seldom adopted.
Arches 1½ Brick thick may be built as shown in Fig. 4 5, which represents a section; the end elevation of the arch is the same, and the plan is like the face of a wall in English bond.
1. Arches Turned in Whole-brick rings consist of rings like that in Fig. 44 superposed one over the other.
2. Arches in Half-brick rings (see Fig. 46) are very commonly used, and are easily built; they should not, however, be adopted for spans exceeding 30 feet. The rings have a tendency to settle unequally; in such a case the whole weight may be thrown for a moment upon a single ring; if this is crushed, the pressure comes upon the next ring, and so on, resulting in the failure of the whole arch.
In building arches with half-brick rings it is advisable to build the undermost ring with thin joints and gradually to thicken the mortar joints as the extrados is approached; this prevents the lowest ring from settling while those above remain in position, which would cause an ugly fissure.
To avoid the disadvantages above mentioned, arches have been built with blocks, B B, set in cement, running through their thickness at intervals, so as to form a bond right through the thickness of the arch.1 Stone bonds may be used instead, cut to the shape of a voussoir. These bond blocks should be placed at the points where the joints of the various rings coincide: those points will be determined by the radius of curvature of the arch, the thickness of the bricks, and of the joints.