Stone arches are very frequently used both in stone and brick buildings. They may be built in a great variety of styles, and with either circular, elliptical or pointed soffits. The method of calculating the stability of a stone arch is the same as for a brick arch, but a stone arch being constructed in larger pieces, the mortar in the joints adds but very little, if any, to the stability of the arch, and a stone arch of the same size as a brick arch is rather more liable to settle or crack than the brick arch, and should be constructed with greater care. The method of calculating the stability of arches is given in Chapter VIII (Architectural Terra Cotta). of the Architects' and Builders' Pocket Book. In block stone arches each block, or "voussoir," should always be cut wedge-shape and exactly fitted to the place it is to occupy in the arch. The joints between the voussoirs should be of equal width the entire depth and thickness of the arch, that the bearing may be uniform over the entire surface. The thickness of the joint will depend somewhat upon the character of the stonework. In finely dressed work 3/16 of an inch is the usual thickness, while in rock-face work it is seldom made less than 3/8 of an inch. One-fourth of an inch, however, is all that should be allowed in first-class work.
The joints should also radiate from the centre from which the intrados is struck, or, in the case of an elliptical arch, they should be at right angles to a tangent drawn to the intrados at that point. See Fig. 106, Section 198.
The back of the arch may either be concentric with the intrados,. or the ring may be deeper in the centre than at the sides.
The most common stone arch is that shown in Fig. 101, the arch ring being of equal depth and the voussoirs all of the same size, and rock-face, with pitched joints. Occasionally the voussoirs are cut with a narrow margin draft, as shown at B. When the springing line of an arch is below the centre, as shown in Fig. 101, the arch is said to be "stilted," the distance S being called the "stilt." Stilted arches are very common in Romanesque architecture.
A semicircular arch is one of the best shapes for supporting a wall. It must, however, have sufficient abutment, and the depth of the arch ring, or the distance from the intrados to the extrados, in feet, should be at least equal to 2 +
Arches used in connection with coursed ashlar, especially in Renaissance buildings, often have the voussoirs cut to the shapes shown in Figs. 102 and 103.
Such arches are of course more expensive than arches with the intrados and extrados concentric, as there is more waste to the stone and more patterns are required. They have a more pleasing appearance, however, and are also stronger. Voussoirs of the shape shown in Fig. 103 must be cut with extreme accuracy.
In dividing the arch into voussoirs it should be remembered that, as a rule, narrow voussoirs are more economical of material, but more expensive in point of labor.
In most arches the width of the voussoirs at the bottom is about three-eighths of the width of the ring, although they may vary from one-fourth to one-half.
Very often two voussoirs are cut from one stone, with a false joint cut in the centre. This is done generally for economy, although in some cases it may add to the stability of the arch. Generally the arch is divided into an uneven number of voussoirs, so as to give a keystone, the voussoirs being laid from each side and the keystone fitted exactly after the other stones are set. Except that it is more convenient for the masons there appears to be no necessity of having a keystone, and the author has been informed that Sir Gilbert Scott always used an even number of voussoirs, believing that thereby the danger of the voussoirs cracking was decreased.