The Evolution of Structural Design of Monumental Vaulting in Opus Caementicium in Imperial Rome

We present a numerical study of the structural behavior of monumental Roman cross-vaulted halls in opus caementicium under static gravitational loads. The study is based on linear elastic FEM stress analysis and is focused on the Great Hall of Trajan's Markets and the Frigidarium of the Baths of Diocletian. Both cross vaults were designed following a similar supporting scheme based on contrasting arches, transverse shear walls, and supporting blocks. There are, however, critical differences in the two structures, which allow us to evaluate the shift in design paradigms that took place after the construction of the Great Hall. The analysis of the Great Hall reveals the inherent weakness of the support system. The contrasting arches play no significant role in the static equilibrium of the vault. The shear walls and, in particular, the supporting blocks are the critical elements on which the stability of the vault hinges. In fact, motion of the blocks might have caused a near collapse of the vault. The analysis of the Frigidarium shows a much improved structural configuration. The shear wall is extended upward, the contrasting arch is lowered and becomes an integral part of the shear wall, and, most importantly, the supporting blocks are now completely encased in the opus caementicium. This suggests that Roman engineers were able to detect and correctly interpret the structural deficiencies of the Great Hall, thus developing the knowledge necessary to build the gigantic hall of the Frigidarium.