87. Stone arches are generally used in both stone and brick structures, over door and window openings, for porches, etc. They are also erected over streams and roads for highway and railway bridges and aqueducts. Stone arches of long span are not as frequently built now as formerly, iron and steel haying been very largely substituted for stone. In some ways, a stone arch is not as satisfactory as a brick one. Being composed of a few large pieces, instead of many small ones - as is a brick arch - the bond is not so perfect; and consequently, of the two, the stone arch is somewhat more liable to settle and crack.

The amount of masonry in heavy piers, etc., can, without injuring the stability of the structure, often be considerably diminished by the use of arches, provided the stone and the footings are capable of carrying the increased load. The pressure on the soil may, if necessary, be decreased by using inverted arches. (See Arts. 91-95, Masonry, § 7.)

88. The principal parts of an arch are as follows: The abutments are the piers from which the arch springs, as at a, Fig. 48. The inner edge of the top of the abutment is called the springing line; the stones resting on the abutments, shown at b, are called skewbacks. The arch itself consists of wedge-shaped stones, called voussoirs, or ring stones. These are sometimes of varying sizes, but for the same arch are generally made as nearly uniform as possible; the depth (back into the wall), however, may vary as much as may be necessary for proper bonding. The voussoirs are shown at c. The ring stones between the keystone and the skewbacks are collectively known as the haunches of the arch. The masonry resting on the arch ring, from the piers to a horizontal line touching the highest point of the upper curve, form the spandrels. The under surface of the arch is called the soffit, and a line representing the curve of the soffit is the intrados; the one parallel to it at the outer end of the voussoirs is called the extrados. The span of an arch is the distance between the abutments; and the rise is the extreme vertical height from the springing line to the intrados.

Stone Arches 175

Fig. 48.

89. In building construction, it is not customary to determine the proportions of arches of small span by calculation. The appearance is often the controlling factor in designing such arches. But when the arches are of considerable span, the position of the line of resistance should be determined. As that is somewhat beyond the scope of this section, merely the conditions necessary for stability will be here mentioned.

In relation to arches for engineering purposes, the well known authority, Professor Rankine, says: "The best course in practice is to assume a depth for the keystone based on the dimensions of good existing examples." This statement holds good in connection with the construction of the arches which an architect ordinarily has to design.

90. Having fixed the depth of the keystone, the voussoirs are all made the same height, in arches of small span, while in longer ones the ring stones vary in depth, increasing gradually from the crown to the skewbacks, so as to preserve a uniform pressure on the stones as the load becomes greater. The resistance to crushing of any kind of stone may be readily determined, and a large margin of safety must be allowed over the greatest pressure to which it will be subjected in the arch.

91. To insure the stability of an arch, there are two conditions, besides the one just mentioned, which must be satisfied. One is that the pressure shall not cause the opening of the joints; the other, that the direction of the pressure shall not be such as to cause one ring stone to slide on another.

In order to prevent rotation on the edge of any stone, the line of pressure - through which the load is assumed to act - must not be above or below the arch ring at any point, but must cut the abutting surfaces of the stones as near as possible to the center of the joint, and always within the middle third of the arch, so as to prevent the opening of the joints. To obviate the liability of sliding at any joint, the pressure tending to move one stone on another must not be sufficient, nor in such direction as to overcome the friction between the surfaces.

These requirements are met by making the arch ring of proper depth, and generally do not need to be determined theoretically for small arches.

92. Flat arches - those having but little rise - give way by breaking the four parts, opening at the crown of the intrados and at some joint on the extrados. When a flat arch breaks, the two upper parts fall inwards and press the lower parts outwards. In pointed arches, the reverse is the case, the lower portions tending to fall into the opening, and to force the upper parts outwards.