The Roman Pantheon was constructed in the 2nd century BC to serve as a temple to all the gods. The building combined two different building designs- a standard rectangular building with a triangular roof for the temple's entryway, and a spectacular circular room covered by a huge dome to serve as the main portion of the Pantheon. At the top of the dome is a 27ft. in-diameter hole (called an occulus), that allows the people to look up at the heavens, and also let the Roman Gods “oversee business” within the temple. This webpage will focus on the physics behind the domed portion of the temple.
Distributing the Pantheon's Weight
The massive size of the Pantheon is accompanied by a tremendous weight. Roman architects used ingenious design to create a stable structure without the use of internal supports.
The tremendous weight of the stone on top of the entryways, windows, and passages would cause them to collapse. The architects solved this problem with the use of arches. Arches take the tremendous force of the stone above it and redirect this force through its sides to the Pantheon’s support walls and piers. These support walls and pillars provide a horizontal normal force to counteract the force of the stone above the arches. The structure’s weight is channeled through the piers to its foundation.
Design of the Structure's Dome
Domes maintain their shape through a balance of tension and compression forces. Let us look at a model to better understand how domes work.
The vertical lines of the dome are called meridians. At the top of the dome, the meridians push together under the domes weight, creating compression forces. Towards the bottom of the dome, the meridians are pushing outwards, stretching the dome apart with horizontal tension forces.
At a certain level on all domes (indicated in the model by the dotted line), there is an area that is neither in tension or compression. The tension and compression forces must both be dealt with to enable the dome to stand.
To deal with the massive tension forces, the Roman architects poured several layers of concrete around the base of the dome. These layers are called step rings, and they provide a normal force to push inward against the tension forces that push out. The rings also help to redirect the tension forces down into the walls below. The Romans had a ingenious way of dealing with the compression forces, which leads us to the next section...
The Occulus of the Building
Surprisingly, right where the compression forces are the greatest on the dome (the top), the Roman architects chose to have nothing but air.
The occulus, or eyehole, is 27 feet in diameter, and made up of 4.5 foot thick ring of bricks. The ring acts exactly like an arch, except that its ends are joined together. Compressive forces are redirected along the ring’s body. The only difference between the ring and the arch is that the compression forces come from all directions. These forces wedge the ring stones tightly together. The compression forces cancel each other out and the top of the dome is left in a state of horizontal equilibrium.
