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.
Bonding. Whenever arches are carried on piers or columns, care must be taken in cutting the springing stones, so that they will bond properly into the spandrel masonry. In Fig. 59 are shown two arches springing from a pier; if the stones a are so cut that the wedge-shaped piece b is necessary to fill up the space between them, there is danger that the weight of masonry over b will force it down, displacing the springing stones a; and similarly with the stones c. To prevent this, the stones a should be cut in one piece, while those marked c should be cut so as to make the joint come at d.
A somewhat similar case is represented in Fig. 60. Here the back of the arch extends almost to the corner of the wall, as shown at b. It is evident that, if the brick wall rests on such a small footing, it is liable to separate from the arch, thrusting out of place some of the lower bricks. In such a case, the lower voussoir a should be cut so as to extend to the corner of the wall, until the distance c d is at least 8 inches; more than this should be allowed if the wall is very heavy.
Molding. Arches are often decorated with more or less elaborately dressed stone, known as label and soffit moldings. The former is sometimes cut in the ring .stones, but oftener forms a separate course of thin stone. If such is the case, the stability of the arch should not depend on the strength of the stone in the molding.
The soffit molding is frequently in the form of a bead and cone, or three-quarter round and cone, or some similar shape. Entrance arches are often decorated with various devices cut in the soffits, especially in entrances to cathedrals, public and office buildings, etc.
In Fig. 6.1, the label mold is shown at a; the arch rings, at b and e; the soffit mold, at c; the brick backing, or filling, at d; and the voussoir joints, at g. Every alternate pair of voussoirs should be tied together with galvanized-iron clamps doweled into the stones, as shown at f.
Centers. In building an arch, it is carried up from both piers or abutments at the same time. During construction, the stones must "be supported until the ring is completed. For this purpose, a framework, made of planks having one side cut to exactly fit the curve of the arch, is used. This framing, known as a center, is supported on posts; it is usual to insert wedges between the center framing and the posts supporting it, which, when the arch is completed and the mortar has set, are driven out gradually, so as to bring the load on the arch ring without shock. The center should be strong enough to support the weight of the arch and a share of the wall above, as no weight should be put on the arch until the mortar in the joints has become hard.
111. Fig. 62 represents a form of center suitable for arches of small span. At a are shown the bearers, which are cut out of 2-inch plank, to a radius about 1 inch less than that of the intrados of the arch. At c are indicated pieces of plank, nailed at the crown of the center to splice and stiffen it. Small bearing strips b, about 1 in. X 2 in. in section, are nailed to the curved pieces a. At d are the longitudinal braces; at e are the plates under the center and on top of the posts; at f are the wedges; and at g, the posts, which, if quite long, should be braced at the middle by struts.
112. For arches of considerable span, centers more strongly built are necessary. Fig. 63 shows a good form of construction. At a and b are represented the bearers, breaking joint as shown; c indicates the bearing strips; e, the upright; d, the inclined braces; f, the tie piece; i, the bearing plates, with wedges g between; and h, the side and center posts.