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.
Voussoirs. The ring of the arch should be built of the very best kind of ashlar masonry, cut, so that the voussoirs bear evenly and closely against one another, with the thinnest possible joints, as it is desirable to have but little mortar between the stones. The width of the ring stones is seldom less than 1 foot, or more than 2 feet, and the thickness (back into the wall) varies from 1 to 3 feet. The joints of the stonework should be the same width throughout the arch, so that the bearing may be uniform over the entire surface. The thickness of the joints depends somewhat upon the character of the finish. If the work is finely dressed, 3/16 inch is the usual thickness; while in rock-faced work it is seldom made less than 3/8 inch; 1/4 inch is all that is usually allowed for the best work.
105. Usually the arch is divided into an odd number of voussoirs, and the keystone is placed in position last. Except for the convenience of the masons in laying, and for the sake of appearance, there seems to be no special reason for an uneven number of voussoirs, and some authorities claim that an even number makes a better job. Narrow voussoirs, while more economical in the amount of material used, are more expensive in labor, as more cutting and fitting is required than with wider ones.
Sometimes two of the voussoirs are cut from one stone, with a false joint between. Although this is generally done for economy, there are cases when the stability of the arch is thereby increased; as, for example, when the skewbacks are made twice the size of the remaining voussoirs, the number of joints is decreased, thus tending to strengthen the arch. In the case of a pointed arch, as shown in Fig. 52, the keystone should be made in two pieces, as the danger of its cracking or slipping is very much lessened when this is done.
Backing. As a rule the cut-stone arches in buildings are only from 6 to 8 inches thick, having a backing of a less costly kind of stonework. Large arches, especially when both sides are visible, as in some entrances, porches, etc., are often constructed as shown in Fig. 61. In this case, the stone ashlar is backed with brick, and tied together with clamps, as indicated at f.
Beams And Tie-Rods. When an arch is to be built in a position where sufficient abutments to resist the arch thrust cannot be provided, one or more steel beams should be laid on the wall immediately over the arch, with the ends resting on the masonry forming the abutments. Anchor rods, securely embedded, should be used to tie together the beams and the stonework. Immediately below the middle of the beam, a small space, or joint, without mortar, should be left, so that if the beams deflect under the load they will not rest upon the arch. This method relieves the abutments of the arch thrust due to the load, which is, instead, transmitted vertically to the supports.
In building a segmental arch it is a good precaution, if conditions permit, to tie the arch together with steel rods, to take up the thrust until the mortar in the masonry has thoroughly set.