Ancient Structures in Rome: The Colosseum & Pantheon

Note: All pictures taken by the author

Introduction

Romans are known for their engineering success and powerful civilization. They took bold and simple ideas and built a whole civilization upon them. Many Roman structures seen nowadays were built more than a thousand years ago and they are still standing. Of course, some restorations have been made to preserve them. Some structures have been restored more than others and some of them stayed as a ruin for a long time until finally getting attention to being preserved.

Romans made a revolution in the civil engineering world by inventing the “Roman Concrete”. Until the discovery of Portland cement in the 19th century, it was the strongest and best building material(11). Roman concrete was primarily “pit sand”, which is a form of grained volcanic sand combined with limestone (11). The main difference between modern ready mix concrete and Roman concrete is that the Romans mixed mortar and aggregates in place and compacted it within a framework (12).

As concrete acts better in compression than tension, compression members like domes, arches, and columns were mostly used in Roman structures. Arches can be seen everywhere in Rome. The idea of an arch is distributing the load equally across it allowing materials like rocks and unreinforced concrete to act effectively. Granting that, the arch properties and the effective use of it enabled the Roman structures to survive throughout the centuries.  

This article discusses two interesting Roman structures, the Colosseum and the Pantheon. Even though both faced natural disasters throughout the centuries they are still standing today. Understanding how they have been used and damaged over the centuries adds to their powerful design.

Colosseum

The Colosseum is the largest Roman amphitheater in the world. It took ten years to be fully constructed between 70 AD to 80 AD. Being that it is located in the middle of the city, it was called the “Heart of Rome”. Built from travertine and brick, its outer elevation comprised four stories reaching up to a total of 159 feet (11). The Colosseum stayed as a ruin for more than 1,500 years in ancient times (1). Due to urban development in the 1900s, it started to get attention again and was marked as an important place that should be preserved and maintained(1).

It can be seen on the map that the colosseum is located in the area surrounding the Tiber river. This is considered a “Wetland” area and the soils around it might be considered risky for a massive structure like the colosseum to be built on. In view of this, how did Romans overcome this geological issue and were able to successfully build a structure that is still standing today?

The substructure of the colosseum is one of the main reasons why it’s still standing(7). Due to the poor condition of soils, a deep and strong foundation was required to stabilize the structure. In general, a solid foundation is achieved by excavating down to bedrock or a strong layer of clay (9). Figure 2 shows a geological profile under the northern and southern sides of the Colosseum (8). The foundation was embedded into the sandy gravel deposits from Paleotiber units of the Middle Pleistocene layer that contained Basal fluvial gravels, sands, pelites rich in organic matter, Aurelia units of pebbles, volcanic sediments and Tufo layers(8). A solid foundation with more than 12 m of thickness had been constructed by two layers made with ancient Roman Concrete(12).

Travertine and Tuff were the two types of stones that were used to build the Colosseum, Travertine is durable and attractive enough to serve as an exterior finish(11). Additionally, can be shaped into neat large blocks. Travertine is mainly brought from Tivoli quarries and is still used to this day as a building material(11). Tuff was also used and is softer than Travertine which makes it easier to be shaped into blocks(11). The materials used by Romans like Travertine and Roman concrete are very strong in compression and were a factor for the survival of most of their structures, including the Colosseum, that depended on compression members. Also, Tuff is resistant against weathering which is a problem that causes failure through time for structures made of concrete and rocks. Table 1 shows a list of the materials that were used to build the Colosseum(11).

Looking at the Colosseum, it can be noticed how there are holes in the structure. Those holes are due to the removal of iron clamps throughout the centuries. When the Colosseum was a ruin, iron clamps were all taken out and used somewhere else. Figure 3 and 4 shows pictures taken inside and outside the Colosseum. It can be seen that the holes are all at the edges of the concrete pieces where the iron clamps existed.

The 76 arches in the lower story served as the entrances for the spectators, with four main arches at the ends of the axes(10). The structure was divided vertically into four arcaded stories. The first three stories consist of arches flanked by pillars, with Doric, Ionic, and Corinthian columns, which also support the decorative overhangs above them(10).

Due to natural disasters, several arches on the south side were completely demolished due to an earthquake in 1349 (2). The earthquake that happened in 1231 caused part of the wall on the south-west side to collapse (2). Due to that collapse, the whole structure became weaker (2). Tables 2 and 3 lists the dates and descriptions of natural disasters that affected the Colosseum(10). Also, it lists the restorations that were made to the Colosseum to save it from collapsing.

Nowadays, only the northern part of the structure still fully exists at a height of 159′(11). Figure 5 shows the current plan of the colosseum. For a long time, the monument was used as a limitless stone quarry. Its material like marble, Iron, and travertine was either stolen or taken to be used in other structures. Figure 6 shows little of the marble left that was collected in that part of the seating area for tourists to see.

Referring to figure 7 & 8 that show pictures of the buttresses at two sides of the Colosseum, it can be seen how there is a difference between the material used for the old and new structure of the Colosseum. They used the same design but different materials with a darker color. The addition that they constructed is just enough to save the structure from failure. On the northern side, radial walls and seven arches were rebuilt on the first level(3). On the second level, eight arches were rebuilt. On the third and attic level, vaults attached to the external walls and some internal walls were restored (3). Those restorations were enough to stabilize the structure at that time.

Pantheon

The Pantheon is the best-preserved ancient structure and has the biggest dome (4). The original Pantheon was built in 27 BC but it was damaged due to a fire that occurred in 80 AD(6). It’s not clear whether it was fully restored or rebuilt after the fire. The exact date of construction of the current pantheon is not yet determined.

The Pantheon has the most perfect interior space from the ancient world and its design has been copied in many other buildings. Figures 11 and 12 shows pictures taken inside the Pantheon. The pantheon design is impressive yet simple. It is a 143 feet diameter rotunda that supports a big dome and has free-standing exterior columns that provide extra support for the structure(6). It is built entirely out of concrete without the support of any steel. The Pantheon dome is among the largest unreinforced masonry dome ever built in the world (13). Moreover, the height of the building is the same as its diameter which makes the Pantheon notable for its proportionality(6).

Figure 12 shows the western side of the pantheon. It can be seen how there are arches inside the exterior walls. The massive dome required strong support to withstand its weight, regarding this, thick exterior walls with arches being part of the walls were built to provide enough support against the compressive forces imposed by the dome. The walls of the pantheon are made of brick-faced concrete which is a material that is used widely by Romans for big buildings(5). Figure 12 shows one side of the pantheon the arches inside the walls are distributed throughout the structure.

On the frontage of the pantheon, it is supported by 16 free-standing columns that are made of Egyptian granite. Three of those columns on the left side of the Pantheon have been taken from somewhere else and it can be seen how the design of those three columns is different. Also, the columns look like they came in different pieces and were stacked on top of each other. Figures 14 and 15 show the difference between the three columns on the left side of the pantheon and the other columns.

The pantheon dome is still considered the largest unreinforced concrete dome (10). To be able to construct the dome it was built in different layers using a single casting of concrete (5).  The engineering aspect behind is using lighter stones mixed with concrete to reduce the load and stresses in higher layers of the dome. Aggregates sizes range from a thickness of 19.4 feet at the heavy bottom layer to 4.92 feet at the top lighter layer(11). The design of the layers was also chosen carefully. As shown in figure 17, the layers are not constructed as a flat surface. They used a design of more than one square stacked on top of each other. The design is copied on each layer around the dome. This design helped them reduce the weight of the dome since less material was used.

As the dome structure pushes outward towards the base, they built 20 feet thick walls to be able to transfer the load to the ground and stabilize the structure. Due to the massive weight of the dome, they made an opening in it to make it lighter and also use it as a light source (9). The dome weakest point is the middle top point of it. Making an opening in the dome would prevent the stresses and the self-weight from accumulating at that point. Due to the opening, floods can occur in the pantheon when it rains. Romans were able to find a solution for this by making a slight incline in the base of structure so that water can naturally runoff (9).

Conclusion

After all the natural disasters, taking material out of the structure and staying as a ruin for a long time major part of the colosseum still stands. Pantheon interior is still conserved until now and its design has been an inspiration ever since. Its dome that is still considered the biggest unreinforced dome in the world has been able to survive the natural disasters without collapsing. Restorations through time helped to save those structures from demolishing but without the strong construction materials and the smart design, they wouldn’t be able to survive. All in all, Roman’s exceptional structures are still an inspiration to architects and engineers all around the world and their development made an impact on the civil engineering world.

References:

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6. Godwin, W. (1809). The Pantheon; or, Ancient History of the Gods of Greece and Rome… For the use of schools, and young persons of both sexes… With engravings, etc. MJ Godwin.
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10. Tan, A. H. (2015). While stands the colosseum: A ground-up exploration of ancient roman construction techniques using virtual reality (Order No. 3710424). Available from ProQuest Dissertations & Theses Global. (1702720303). Retrieved from https://search.proquest.com/docview/1702720303?accountid=14784
11. Tan, A. (2012). A Computer-Generated Model of the Construction of the Roman Colosseum. (Electronic Thesis or Dissertation). Retrieved from https://etd.ohiolink.edu/
12. Nakamura, Y., Saita, J., Sato, T., & Valente, G. (2015). Dynamic Characteristics of the Colosseum at the Pillar# 40 Comparing the Results of Microtremor Measurement in 1998 and 2013. Proceedings of the DISS_15 in Rome.
13. Plunkett, J. W. (2016). The Roman Pantheon: scale-model collapse analyses (Doctoral dissertation, Massachusetts Institute of Technology).