The Pantheon is an architectural wonder. There have actually been three temples in total on this site, and the one that still stands today was built almost 1900 years ago, in approximately 125 AD.

 

To this day, the dome, which has an approximate diameter of 43.4m* (150 Roman feet), is the largest unreinforced dome in the world. If the dome of the rotunda were flipped upside down, it would fit perfectly inside the rotunda.

 

Materials:

Structurally, the Pantheon was ahead of its time. It was constructed using a range of different types of (Roman) concrete. The supporting walls of the rotunda’s lower section are wide and made of concrete containing heavier stones such as travertine. The upper section of the rotunda walls are made from a slightly lighter mix of travertine and tufa and towards the top of the wall the mix is tufa and brick. At the bottom of the dome, the concrete is approx. 6.5m thick (although there is a lot of controversy over this exact measurement). The drums section of the dome is made from all brick. The walls of the dome gradually taper and become thinner to reduce the weight of the dome, so that the area closest to the oculus at the very top, is about 1.18m** thick and the concrete here is lighter in weight as it is mixed with volcanic pumice or tuff.

The Romans developed a very effective concrete by mixing lime with “pozzolana”, a volcanic ash found in the region of Naples and Mt. Vesuvius. This formed very tight crystal structures that were extremely strong and resistant to water. It has been proven that had normal modern concrete been used, the stresses on the arches would have been around 80% greater and the structure would probably have collapsed by now.

The Romans also used bricks to create arches and surface coverings. When built, the Pantheon steps and walls would have been covered in marble and the dome in bronze, but these were removed over the centuries to be used for other buildings and purposes.

 

Architecture:
Discharging arches: The Pantheon rests on eight pillars and barrel vaults and uses discharging arches, many made from brick, to make sure that the pressure from the dome is directed onto these strong pillars. Here is a diagram showing the forces been directed down the structural rib pillars by the discharging arches:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Above: Forces Directed Down on Dischraging Arches   Left: Discharging Arches   Right: Coffers

 

Coffers: The Romans also invented the use of coffers. These are square or rectangular recessed blocks that not only lighten the weight of the dome but visually seem to expand out the space inside the dome rather than just being a heavy, solid concrete cap on the building. 

 

The oculus or “eye” is about 8.2m*** in diameter (30 roman feet) is an important structural aspect of the design at the very top of the dome. It reduces the overall weight of the dome and some engineers think helps to spread out the weight and stress more evenly throughout the dome, although this has not yet been proven. The oculus is still lined with the original Roman bronze and is the main source of light for the whole building. The oculus was never covered and rain falls inside onto the marble floor. The floor is designed to be slightly curved so the rain can run off into the still-working Roman drainpipes underneath.

 

Light grille: The Pantheon was orientated so that sunlight strikes the floor approximately at both the Spring and Autumn equinoxes. Around April 21st, the midday sun strikes the metal grille above the doorway, sending a shaft of light into the main courtyard in front of the Pantheon. The Romans celebrated April 21st  as the founding date of the city, and it is thought that the impressive sight of the Emperor standing at the entrance of the Pantheon highlighted by light coming from inside the Temple of the Gods might have been seen as perhaps raising their Emperor to the level of the Gods.

 

* = ~43.40352 m (142.4 ft)

** = ~1.18872m

*** = ~8.2296m