114. The Problem of Spanning
If now our attention be confined to large buildings of the more massive and permanent sort, it becomes clear that one of the chief problems which all their constructors have had to grapple with, is that of roofing large spaces and spanning wide openings in walls. A pyramid can be heaped up, or a wall reared to a great height, without much other than quantitative difficulties being encountered. A four hundred foot pyramid does not differ in principle from the waist-high one that a child might pile up. The problems which it involves are essentially the economic and political ones of providing and controlling the needed multitudes of workers. Architecture as such is in abeyance and the engineering problems involved are mainly those of transporting and raising large blocks of stone. Much the same holds of walls. The Incas, for instance, reared masonry of astounding massiveness and exactness without ever seriously attempting to solve architectural problems.
Once, however, a structure is planned to cover a wide space, it becomes architecturally ambitious. The roof of a large dwelling can be made easily of poles and thatch by such collaborators as a family might muster. But to span a clear space of some size in stone requires more than numbers of workers. The accomplishment also yields definite sense of achievement which is strong in proportion as the extent of the ceiling is great. The difficulties are diminished in proportion as the mass of the structure is large and the clear space is small, but the satisfying effect is correspondingly decreased. A vault whose walls are thicker than its interior is wide, produces as chief impression an effect of massiveness. One feels the solidity of the structure, the amount of labor that has gone into it; but one is left without the sense of a worth-while difficulty having been self-imposed and mastered. Sooner or later, therefore, after men began to hold themselves available for co-operative enterprises in numbers, adventurous minds must have been fired with a desire to grapple with problems of æsthetic construction, and to leave behind them monuments of triumphant solution. The story of these voluntary and imaginative endeavors is the history of monumental art.
Two principal methods have been followed in the solution of the problem of covering large free spaces. The first is the method of the column and the lintel; the second that of the arch or vault. The column and lintel do not differ fundamentally from the idea of the wall with superimposed roof beams. The elements of both are vertical support and horizontal beam. In the arch, however, this simple scheme is departed from, and the covering elements take on a curved or sloping form. The apparently free float of the span is stimulatingly impressive, especially when executed in a heavy and thoroughly rigid material. The beam is subject to bending stress. Timber makes a good material because of its strength against breakage by bending. Stone is unreliable or outrightly weak against a bending stress, besides adding to the stress by its own weight. There are therefore inherent limitations on the space that can be covered by a horizontal stone beam.