Roman builders had an intuitive understanding of vault behavior and structural form as seen through their use of lightweight caementa, vaulting ribs, and iron tie bars. They had no means of quantifying and calculating vault thrusts, but they had developed ways of controlling behavior through long experience with the problems that could occur. Because most readers do not have the same benefits of firsthand experience of vault construction, I present some basic principles of vault behavior and examine how the Roman builders and designers developed techniques to control it. In Chapter 8, I then give an overview of the historical development of the modern understanding of vault behavior and methods of analysis that have been developed to study it.

The way Roman architects and builders approached design would have influenced how they determined the appropriate size and form for their vaulted structures. M. Wilson Jones has recently examined the design methods used by Roman architects and has pointed out that they typically used rules based on numerical proportions and/or geometrical relationships, both of which were principles used by Vitruvius for attaining symmetria, or mathematical harmony. Vitruvius's concept of symmetria was an aesthetic principle rather than a structural one, but when Roman architects and engineers were faced with determining the form of an arch or the appropriate wall thickness for a given structure, they would have likely resorted to the same type of proportional system that was used to ensure symmetria.