ABSTRACT
The master builders, the designers, and the constructors of the Gothic cathedrals of the Middle Ages used intuition and experience to develop design rules based on simple force equilibrium and treated the material as rigid. This solution process provided the equivalent of what is now known as the lower-bound theorem of plastic limit analysis. This theorem was not proved until more than 500 years later. Modern lower-bound theorem shows that these design rules are safe. These simple design rules have existed from the earliest times for building Greek temples, Roman aqueducts and arch bridges, and domes and vaults. However, tests on real structures showed that the stresses calculated by designers with these rules could not actually be measured in practice.
