The medieval ‘scientia' of structures: the rules of Rodrigo Gil de Hontañón

Abstract

Medieval builders didn't have a scientific structural theory, however gothic cathedrals were not build without a theory. Gothic masters had a ‘scientia', a body of knowledge which permitted the safe design of their buildings. The nature of this theory has not only a historical or erudite interest; perhaps something could be learned from the true masters of masonry architecture.
Literary sources from the gothic period are scarce; in almost all we find structural rules to design the principal structural elements: walls, vaults (ribs and keystones) and, above all, buttresses. These rules (arithmetical or geometrical) conduced in most cases to a certain proportions independently of size, to geometrically similar designs (for example, the depth of a buttress is a fraction of the span). Very rarely, and this is the case with Rodrigo Gil, appeared arithmetical rules which lead to non-proportional designs (following Rodrigo's rule the buttresses become more slender in relation to the span as the size increases).
These rules were a means to register stable forms. Proportional rules are, as has pointed Professor Heyman, essentially correct. It is a problem of stability and not of strength. Non-proportional rules express a finer adjustment to some non-proportional design problems: buttress design for the thin domical cross vaults (bóvedas baídas), boss design for the vaults themselves, and wall design for towers. The rules were deduced empirically, give correct dimensions, but above all draw our attention to some significant facts of design which so far have remained unnoticed.