The Roman structural program of arch, vault, and concrete was new to the world. None of these individual elements was without precedent: The Egyptians and Mesopotamians had employed primitive arch forms; the Greeks had experimented with the arch and even with concrete. As early as the fourth century B. C., the Etruscans were constructing impressive, if simple, vaulting forms. But the Romans were the first to combine these structural media and fully exploit their inherent potential.
Arches are deceptively simple forms. Structurally, they are far more intricate than the elementary post-and-lintel ( a system of constuction in which two or more uprights support a horizontal beam; also caled, trabeated), or trabeated, system (an architectural system using a horizontal beam over supports, as opposed to an arched or arcuated system; synonymous with post and lintel) that formed the basis or pre-Roman architecture, in which a rigid slab rests on upright supports. As a rule, an arch also takes upright supports, but the true arch is not rigid. Rather, it is formed of a multitude of small elements that arch over space by resting against each other in delicate balance. A lintel is inert, but an arch forms an organic system. An arch is alive.
The small elements of which an arch is built are called voussoirs (definition: a wedge-shaped stone used in construction of an arch or vault). In stone arch, the voussoirs take the shape of trapezoidal blocks, but in a brick arch it is generally the mortar between the rectilinear bricks that forms trapezoids, which make the curve. The wedge shape not only gives the arch its curve but holds the arch up by keeping the voussoirs from slipping downward. However, each voussoir exerts a downward impulse due to gravity, and this impulse is partly transformed into a lateral impulse, or thrust ( the outward force exerted by an arch or vault), because of wedge shape. The downward load of the arch is not a problem, because it is taken up by the supports in the same manner as the trabeated system. But the accumulated lateral thrust is the bane of arch builders. Its forces, as they pass from vouissoir to voussoir, must be kept within the arch itself. The tendency for the arch to fall inward toward its crown or to break outward at its hauches becasue of "escaping" thrust must be avoided. And even when the thrust is contained safely along the entirety of the arch, it must once more be accounted for when it emerges from the base of the arch, where it must be taken up safely by the vertical supports and finally transmitted to the ground. In practice, this means massive supports for the arch, or supports augmented along the direction of the arch in the form of buttresses( a projecting mass of masonry serving to provide additional strength for the wall as it resists the lateral thrust exerted by an arch or vault).
The constructing of an arch is no less painstaking and intricate than its design. Only a complete arch is self-supporting. Therefore, until the arch is complete, the voussoirs must be supported in their proper position as they are laid up. This temporary support is called centering and usually is built of wood. It may be built up from the ground or, more economically, from supports at the base of the arch. The critical aspects are that the centering must be strong enough to take the weight of the voussoirs without deformation and, of course, that its upper surface
(known as the extrados) precisely follows the intended curve of the lower surface (known as the intrados) of the arch. Once the supports are standing and the centering is in place, the construction of the arch begins. The first pair of voussoirs are set at the springing line, and the rest follow up to the top, where the keystone is finally set, completing the arch. Strictly speaking, the keystone is nothing but the top voussoir. An arch need not even have a keystone; it may simply be completed with a final pair or ordinary voussoirs. When keystones are present, they are frequently accentuated in size and decorated with a sculpted boss. They figure importantly in the visual calculus of the arch design by stressing is axiality, symmetry and sturdiness. Once the keystone or the top voussoirs are in place and the mortar is set, the centering may be removed. Presumably all has been done correctly and the airborne ring of masonry will stand. >>>Cont'd
