CLASSIFICATION OF AROHZS. Masonry ' arches may be divided into voussoir arches and monolithic arches, according to whether the arch ring is composed of several separate stones or con sists of a monolithic mass of concrete. The separate arch stones or voussoirs may be blocks of natural stone dressed to the required shape or of concrete moulded to proper form.
Arches may be divided also into hinged and hingeless, according to whether or not there are one or more joints or hinges in the arch ring. Of course, a hingeless arch is one having fixed ends. Hinged arches may have one hinge at the crown, or one at each abutment; or one at each abutment and one at the crown. However, hinged arches usually have either two hinges or three. Both voussoir and monolithic arches may be built with or without hinges; but hinges are used only in very large arches. The chief advantage of the hinges is that their use permits a more accurate analysis of the stresses, and consequently makes possible a saving of material; and the disadvantages are that the hinges themselves are expensive, and the hinged arch is not as stable nor as durable as an arch without hinges. Hinged masonry arches are somewhat common in Europe, but are hardly used at all in America. Hinged arches will be briefly considered in the next chapter.
Abutment. A skewback' and the masonry which supports it. Arch Sheeting. The arch stones which do not show at the ends of the arch.
Backing. The masonry outside and above the arch stones, which usually has joints horizontal or nearly so.
Crown. The highest part of the arch.
Coursing Joint. The joint between two adjoining string courses. It is continuous from one end of the arch to the other.
Extrados. The convex curve which bounds the outer extremities of the joints between the voussoirs.
Haunch. The indefinite part of the arch between the crown and the skewback.
Heading Joint. A joint in a plane at right angles to the axis of the arch. It is not continuous.
Intrados. The concave line of intersection of a vertical plane with the lower surface of the arch. See Fig. 185.
Keystone. The center or highest voussoir or arch stone.
Ring Course. The stones between two consecutive series of heading joints.
Ring Stones: The arch stones which show at the ends of the arch.
Rise. The vertical distance between the highest part of the intrados and the plane of the springing lines.
Skewback. The inclined surface or joint upon which the end of the arch rests.
Springing Line. The inner edge of the skewback.
Spandrel. The indefinite space between the extrados and the roadway. The wall at the end of the arch above the extrados is called the spandrel wall; and the material between the end walls and above the extrados is called the spandrel filling.
String Course. A course of voussoirs extending from one end of the arch to the other.
Voussoir. One of the wedge-shaped stones of which the arch is composed; also called an arch stone. The voussoirs which show at the ends of the arch are called ring stones; and those which do not thus show are called arch sheeting.
According to whether or not the end walls are at right angles to the axis, arches are divided into right arches and skew arches. A right arch is one terminated by two planes, termed heads, at right angles to the axis of the arch; and a skew arch is one whose heads are oblique to the axis. Voussoir skew arches were never very com mon, on account of the difficulty of dressing the complicated vous soirs required; and since the introduction of concrete in arch con struction, they are never built.