Pantheon – The Pinnacle

Even though Roman dome engineering progressed during the 3rd and 4th centuries their perfect dome had already been constructed. The iconic Pantheon built in the 2nd century AD is still the largest unreinforced concrete dome in the world at a diameter of 44 meters and it is unlikely that the Roman’s built any that were bigger as no record of one has been discovered (Moore, 1995). Today it is one of the most monumental structures of the world and analysis of the dome is still being done as the engineering complexities of it are not fully understood. Dome construction breakthroughs from previous works were employed in the Pantheon and together they resulted in the pinnacle of all Roman Domes.

Pantheon Formwork

The massive size of the Pantheon has fueled discussion regarding the centering system used to construct it. Did it use a ground supported centering system or a completely self supporting system with all radial trusses framing into a central compression ring at the top? In order for the dome to be constructed without many column supports upholding individual formwork pieces, radial trusses would have to span a distance from the base of the dome curvature to the edge of the oculus. This angular distance is approximately 26 meters as seen in Figure 1 below and coincides with the centering span required to build the Basilica Ulpia, which was constructed at roughly the same time as the Pantheon (Lancaster, 2005). The Romans likely designed both buildings so that the same centering could be used which would save time and materials (Lancaster, 2005). The wood construction knowledge at the time would have allowed radial trusses of this dimension to be built so it would have been unnecessary to have many columns supporting the centering from the ground (Lancaster, 2005). The next question is whether or not the centering had a large central tower or not. According to Vitruvius, towers reaching 53 meters high and having a base of 10.4 meters had been designed and built by Diades, an engineer for Alexander the Great, so the technology to build a column supported centering system was available to the Romans at the time of the Pantheon’s construction (Lancaster, 2005). The alternative method of a completely self supporting centering system would also have already been developed. This method would require a large central compression ring against which the radial trusses would rest as seen in Figure 2 below.

Figure 1: Required radial frame dimensions for the Pantheon construction (Lancaster, 2005)

Figure 2: Self supporting radial frame centering system for Pantheon. Section above and plan below (Lancaster, 2005)

Of the centering systems proposed for the Pantheon, it is likely that the Romans used the easier and less complicated method, which would suggest the large central tower system. The use of a self supporting system would have been overly complex and required extra supports or cranes to hold the system in place until all the radial frames were positioned and equilibrium was achieved. Any of the systems discussed could have been used but with the construction of a large central tower also came the ability to easily get to the top of the centering for construction and material transport by ladders or elevators (Lancaster, 2005). It seems logical that for a dome as large as the Pantheon the Romans would choose a more stable and safe method that was still economical.

Concrete Weight

Because the dome of the Pantheon was twice as large as any earlier dome the Romans constructed, they were very concerned with the forces it would exert on the rotunda walls supporting it (Lancaster, 2005).