The dome of the temple of Diana in Baiae: geometry, mechanics and architecture

The purpose of this paper, which is part of a multi-disciplinary research project started in 2018, is to investigate both the geometry and stability of the dome of the temple of Diana, which partially survived for many centuries. Important observations made by De Angelis d’Ossat and results of the architectonic survey performed by Rakob will be presented and discussed; they introduce the delicate issue concerning the identification of the geometric profile of the dome, on which its structural stability depends. It is the Authors’ opinion that ascertaining the geometric profile of a dome is not only a matter of measuring, but also of historical context, geometric knowledge, traditions in construction and technical problems occurring in building practice. Thus, by taking all the above issues into account and making use of data from a recent architectonic survey, this paper identifies the most probable theoretical design: the dome’s meridian profile is a unique curve – to be precise, a parabola, the best approximation of a catenary – which immerses itself into the drum and continues to be fully contained in it as far as the dome springing. The polycentric intrados curve first proposed by Rakob thus appears to be an architectonic trompe-l’oeil, since the lower arc-shaped profile mainly constitutes the intrados of the drum, which acts as an extended buttress that can counteract possible tensile stresses generated along the lower parallels of the dome itself. Thus, once the profile of the dome has been identified, structural analysis confirms not only the stability of the dome, but also that the Romans were aware of the dome’s behaviour in relation to the construction solutions adopted.