Tectonics and the Craft of Construction
When the Pantheon was completed in about A.D. 128, it stood in resemblance of Roman's genius in ingenuity and craftsmanship of construction. The architectural elements used were remarkable to the ancient Romans, for building domes was not new, but combining a domed cylinder with a traditional, Greek temple entrance was new. The building was designed in three parts. The first was the rotunda, an enormous brick and concrete cylinder with a dome. The second, the entrance, was a traditional porch with columns and the third was a rectangular concrete block connecting the porch to the domed cylinder. The most significant architectural and structural accomplishment of the Pantheon was dome. It was the largest anyone had ever built, stretching 142 feet across (Mark 24). Getting the dome to stand up was a remarkable feat for ancient builders, for it had no support beams or columns to hold it up.
The entry to the Pantheon began with five marble steps leading into the traditional Roman high architectural porch. The entry porch consists of sixteen unfluted façade columns, arranged in three abbreviated aisles aligned on a north-south axis. The middle aisle is the widest and leads to the doors of the rotunda and the narrower aisles to the side terminate in large semi-domed niches. The porch then leads you to connection block where there were massive doors to the entry of the rotunda. The connection block contains the niches and has staircase enclosures behind them. It rises above the level of the porch roof to form a right angled intermediate block.
The dome rotunda is the third element, which is almost twice as high as the porch. It can only be entered through the porch and then through the large doors in the intermediate block. The initial design “was based upon a circle and square relationship: the dome rises from a height above the paving that is exactly equal to its own height, so that in vertical section the rotunda is composed of half of a circle inscribed in the upper halve of a square.” Proportions of the pantheon can relate back to Vitruvius, who wrote on proportions in both architecture and the human figure, and their sympathetic relationships, in something quite like these circle-and square terms. “These concepts appear dramatically enlarged in the Pantheon, where the sweep of the limbs of the Vitruvian figure are expanded to colossal dimensions” (70 Mac Donald). The geometry of the interior of the rotunda is created from a vertical center line, an invisible axis that rises from the center of the interior floor and passes up through the center of the oculus above (28 MacDonald). This kind of design is called a centralized building. The rotunda consist of two horizontal zones, which incase statues in concave vaults. These two zones also contribute towards the support of the dome above. The dome itself has one hundred and forty coffers, as they rise towards the oculus they diminish in size and depth (33 MacDonald). The oculus is the round opening at top-center of the dome and it is 30 feet across (33 MacDonald).
“About nine-tenths of the structure of the intermediate block and the rotunda is of concrete” (33 MacDonald). The concrete used was created by mixing sand, lime, and aggregate together. Aggregate is broken rock and is the heaviest material. It made the concrete stronger and increases the mass. As the height of the dome increases the need to support its weight is much less than it is in the foundations and supporting walls below. Furthermore, concrete could be made quickly and also dried quickly. Even more important than its ease of use was the fact that concrete assumed the shape of whatever mold it was poured into. Because of the new invention, the Romans were able to develop a new architectural style.In the Pantheon the weight of the kinds of aggregate used decrease regularly in layered zones as the height of the building increases. This can be visually seen for the color of the concrete is darker and the base and is increasingly lighter at the top of the dome. “The heaviest appears in the foundation, the next lightest in the lower walls, and so on to the upper part of the dome, where pumice, which is very porous and light, was used” (43 MacDonald).
When it came time to construct the dome of the pantheon, master carpenters became the focus of the operation. They were needed to construct the dome's mold out of wood. The wood beams had to be strong and the construction had to be sturdy in order to support the weight of the mold and the concrete that would eventually be poured over it. The construction of the mold itself was especially difficult because the interior of the dome was going to be covered with an intricate waffle-shaped pattern of indentations called coffers. The coffers had to be placed on the exterior of the wooden dome mold so that they would later be embossed into the concrete as it was poured. The pattern was so complex that the coffer molds could only be built by master carpenters. The concrete was poured in sections; there are five rings section of coffers on the interior of the dome, each ring smaller than the one beneath it. When a whole ring of coffer mold was in place, the workers would pour concrete over that ring. While the concrete hardened, the workers prepared the next ring of coffer molds.
In order to keep the dome in the air, the architects used several techniques to make sure the dome would stay up. The coffers were used as a technique in keeping the dome in the air. Each coffer reduces the weight of the dome. The Pantheon's architects also had a strong understanding of various construction materials, especially concrete. They figured out that by changing the aggregate, they could change the weight of the concrete. As the dome of the Pantheon rises, each level is lighter than the one beneath it because the architect changes the aggregate in each level. Because of the dome's shape, it's weight naturally pushes outward as well as downward. The weight pushing outward is called hoop stress. Hoop stress complicates the problem of keeping the dome from crumbling. The architects counteracted the hoop stress by starting the inward slope of the bottom of the dome inside the rotunda wall. The outside of the rotunda wall is higher than the inside. From the outside, people can't tell where the bottom of the dome is, because it starts inside the rotunda wall. The higher outside wall acts as shoulders for the dome, holding it in. The architects also manipulated the thickness of the dome They made the bottom thicker than the top. To do this, they built several stepped rings at the bottom of the outside of the dome. By adding weight to the outside edge of the dome, the stepped rings act as buttresses, or supports. The weight of the stepped rings pushes downwards into the thick drum, which helps counteract the tendency of the dome to push outward. At the ring level, the dome is nearly as thick as the drum. But as the dome rises, it becomes thinner. At the top of the dome, the concrete is only 5 to 6 feet thick, making the top of the dome by far the lightest part of the structure.
MacDonald, William. The Pantheon. Cambridge, MA: Havard University Press, 1976
Mark, Robert, and Paul Hutchinson. "On the Structure of the Roman Pantheon." College Art Association 68.1 (1986): 24-34. Web. 12 Oct 2009. <http://www.jstor.org/stable/3050861>.
Ruggieri, Gianfranco. Guide to the Pantheon. Rome: Editorial Museum, 1990.