The Pantheon Today (Photo by author)
The Pantheon is one of Rome’s most iconic and best preserved ancient structures. With massive single stone columns holding up the portico at the entrance, the immense open interior space created by the cylindrical rotunda, and of course, the characteristic concrete dome and open oculus to top it off, the Pantheon is quite the engineering wonder to behold. However, the Pantheon as you see it today is not the Pantheon as it was when it was first built. Nor is it the first version of the Pantheon at that location. The Pantheon you see today is the third rendition of the “church of every god” that had been adapted and modified through the centuries. This is just one example of a piece of the Pantheon’s long history that includes unique ambitiousness, ingenious construction techniques, and examples of remarkable Roman engineering. Because of this rich, and often incompletely recorded history, the story of the Pantheon can be muddled with confusion and mystery. Yet because of its significance and breathtaking wonder, the Pantheon has been the subject of much study and analysis. While not all sources agree on every single point, the goal of this report is to try to sift through the many sources of information to examine some of the history, construction techniques, and evolution involved with its history, in order to better appreciate the marvel that is the Pantheon.
History of the Pantheon
Telescopic view of the interior of the Pantheon (monlithic.org)
The Pantheon is a breathtaking sight simply at first glance. Walk around a corner on a what seems like a typical Roman street and all of a sudden you see the massive columns inviting you in to the towering dome overhead. While it is great to look at and admire the dome, when you start to analyze and breakdown all the engineering involved with its construction, it only gets more impressive. To put it in perspective, the Pantheon is still the largest diameter unreinforced concrete dome in the world, meaning there is no reinforcing structure in or around the concrete, such as the rebar that would be used today. Still, after nearly 2000 years, it holds that record. The tallest building in the world changes about every 10 to 20 years, but the Pantheon has held its record for nearly 2000 years. That alone is a testament to the quality of the Roman engineering techniques that went into creating the Pantheon. After discussing some background information in the next sections, I will present some information to expose some of these techniques and methods that allowed for the creation and survival of the Pantheon.
While it is difficult to determine exactly who built the Pantheon, how, and when, there seems to be a majority consensus on the history as follows. In the year 117, Hadrian began the tremendous undertaking that was the construction of the Pantheon. He was in fact, rebuilding a church on the same site that the first two Pantheons where previously built upon. The first version, built by Marcus Vipsanius Agrippa, was said to have been a T-shaped building, constructed around 27 BC and burned down in the fire of the year 80. Rebuilt by Emperor Domitian, the second Pantheon was struck by lightning and burned down again in the year 110 (Parker, 2009). When Hadrian set out to rebuild the Pantheon again seven years later, the resulting plan included the ambitious 43 meter diameter dome.
Imperial Rome: the Pantheon is in the upper left area (imgkid.com)
Currently in Rome’s business district, the Pantheon is located in Campo Marzo (Campus Martius), surrounded by restaurants and a public square, with Piazza Navona a few blocks to the West. Back in the time of Imperial Rome, as shown in the map above, the Pantheon was located in Agrippa’s personal property in Campus Martius, near the Baths of Nero to the North and the Baths of Agrippa to the South.
The Geometry and Dimensions (in meters) of the rotundaand the dome (Martines, 2009)
Although it is unclear who the architect of the Pantheon was, it is believed most likely not to have been Hadrian himself, but rather someone else with more professional experience. However, at the very least, credit is given to Hadrian for the concept and grand ambitious uniqueness of the Pantheon (MacDonald, 1976). To be further discussed in detail later on, the Pantheon consists of three main parts; a portico entrance with 16 monolith columns supporting it, the cylindrical rotunda that walls the open interior space, and the concrete dome on top, thinning as it gets higher to the open oculus at the top.
Plan views of the Pantheon(Jones, 2009)
The inscription on the portico of the Pantheon (Photo by author)
Built on the site that Romulus, Rome’s mythological founder, was storied to have ascended into heaven (Parker, 2009), Agrippa’s original Pantheon was thought to be used for the glorification of the gens Iulia, one of the most highly dignified patrician families of ancient Rome. Along with statues of Augustus and Caesar, members of gens Iulia, Agrippa’s Pantheon also contained statues of many gods, including Mars and Venus. From such a diverse collection of celebrated gods contained inside, Agrippa’s temple was believed to be given its name “Pantheon” from Greek, meaning roughly “all the gods” (Platner, n.d.). Cassius Dio, an ancient Roman historian who published 80 volumes on Roman history in the early 200’s, acknowledges this commonly accepted interpretation and presents his own ideas on the name of the Pantheon:
“It has this name, perhaps because it received among the images which decorated it the statues of many gods, including Mars and Venus; but my own opinion of the name is that, because of its vaulted roof, it resembles the heavens.” (Thayer, n.d.)
According to Cassius Dio, this was not the original intention, as he describes below, Agrippa initially wanted to dedicate his temple to Augustus:
“Agrippa, for his part, wished to place a statue of Augustus there also and to bestow upon him the honour of having the structure named after him; but when the emperor wouldn’t accept either honour, he placed in the temple itself a statue of the former Caesar and in the ante-room statues of Augustus and himself. This was done, not out of any rivalry or ambition on Agrippa’s part to make himself equal to Augustus, but from his hearty loyalty to him and his constant zeal for the public good; hence Augustus, so far from censuring him for it, honoured them the more.” (Thayer, n.d.)
Agrippa’s Pantheon stood until the fire of 80, and Domitian rebuilt the Pantheon in the same manner and to serve the same purpose as its predecessor. Hadrian built the current Pantheon in 117 with the new cylinder and dome design, but kept the same intention as a temple for all gods. Hadrian even kept Agrippa’s original inscription over the portico of his new Pantheon: M AGRIPPA L F COS TERTIVM FECIT (roughly translating to: Marcus Agrippa the son of Lucius, three times consul, built this). This was considered a most un-emperorlike thing to do; write someone else’s name on your building, but it was possibly a way to give credit to and remember Agrippa’s original concept for the Pantheon. This inscription has caused confusion when trying to date the construction and builder of the Pantheon. But by analyzing the stamps on the bricks that make up the rotunda, most historians feel confident that the current Pantheon was built by Hadrian and not during the reign of Agrippa, as the inscription might seem to suggest (MacDonald 1976).
Evolution of the Pantheon
[LEFT]: Victor Emmanuel’s tomb(photo by author)
[RIGHT]: The Christian altar in the Pantheon(photo by author)
A now barely visible inscription on the architrave acknowledges the restoration of the Pantheon by Severus and Caracalla during the year 202 (Platner, n.d.). The Pantheon remained a temple to all the Roman gods until the 5th century, when In 609, Emperor Phocas gave it to Pope Boniface IV, who consecrated it, dedicated it to St. Mary and all the Christian martyrs, and renamed it Santa Maria ad Martyres (Parker, 2009). This conversion into a Christian church is thought to be one of the main reasons that the Pantheon has stood the test of time and remained so well preserved. Along with other strong structural reasons as described later, the Pantheon’s status as a Christian church provided it with a more consistent and careful maintenance practice than other non-sacred or pagan sites that were left to be pilfered and unmaintained after the fall of Rome and its ensuing long dark ages (MacDonald 1976).
Through the years since then and up to now, the Pantheon has be altered, scavenged from, and used for various things. For example, the Byzantine emperor Constans II robbed the Pantheon of its bronze roof tiles in 663, Pope Urban VIII had two hundred tons of bronze from the porch removed to make eighty cannons for Castel Sant’Angelo in the 1620’s, and in the seventeenth century, twin towers where constructed above the portico and then removed in the 1880’s. The Pantheon has also been used as a burial place for many significant Italians including; the artist Raphael in 1520 at his own request, Victor Emmanuel II in 1878, the first King of Italy, and King Umberto I who was assassinated in 1900 (MacDonald 1976). Today, the Pantheon is open for the public to view and appreciate its wonder, free of charge.
Analysis of the Major Sections of the Pantheon
The Pantheon was built on a location that was naturally marshy, unstable blue clay earth. This clay cycled through wet and dry four times a year due to the Tiber River flooding or changes in water level. This posed the potential to have a very problematic foundation because with such an unstable base, portions of the structure can settle or sink (Moore 1995). It is tolerable if the entire structure settles at a uniform rate and to a uniform depth, but if different parts of the foundation settle at different rates and depths, then the foundation could undergo stresses that it was not designed for. If this were to occur, the walls of the Pantheon would be put under a large amount of bending stress, and this could cause the concrete to crack and fail in shear.
With such a massive structure as the Pantheon, it was important to make sure the foundation was capable of supporting all the weight of the concrete, bricks, and marble above it. The original design for the foundation of the Pantheon consisted of a concrete ring that was 7.2 meters wide, only about 1.2 meters wider than the walls it would support, and 4.7 meters deep into the ground from floor level. However, during a point in the final phases of construction, the foundation cracked, so a second ring was then added in order to hold the first the ring together. The second ring was 3 meters wide and resulted in a final concrete ring foundation of about 10.2 meters (Moore, 1995).
The concrete used to make the foundation is pozzolan concrete, which consists of travertine aggregate in layers, held together by a mortar of lime and pozzolan (Moore, 1995). Roman concrete was made out of three components: pasty hydrated lime, pozzolan and pieces of aggregate. Most often these materials were found in abundance and shipped from relatively nearby to Rome The lime was made from limestone, consisting of mostly calcium carbonate, that was heated in a kiln to undergo a chemical reaction and release the gas in the limestone. After burning for days, the product in the kiln was a soft quicklime that, when mixed with water, becomes pasty and hardens as it dries. The second ingredient of concrete, pozzolan, is a volcanic ash that is composed of an amorphous silica compound. When mixed with the liquid lime slurry, the large holes in the molecular structure of the pozzolan are filled and expand to lock other pieces together. The last ingredient, rock aggregate is added or the concrete is laid directly onto a layer of aggregate for further mass and strength. The processes involved in creating and using concrete require a lot of chemistry; when creating a usable form of lime, when mixing the different amounts of the ingredients, and then letting the concrete dry for the correct time, at the right thickness for the structure to form and harden correctly. The Romans used a system of ratios to determine how to mix the best concrete using certain material
(Moore, 1995). It is fairly humbling considering that the Romans knew nothing of molecular chemistry, their concrete was made through trial and error, yet they were able to come up with concrete comparable to modern concrete, that is in terms of the types of materials used to make it, but not necessarily comparable to modern concrete’s far superior strength.
To construct the foundation they first dug circular trenches and lined them with wooden boards to create the mold for the concrete. They then compacted the concrete over layers of rock pieces and allowed to dry (Parker, 2009). Compaction was a very important step, and Vitruvius showed how detailed it must be when he wrote that “when stamping is finished it must be…three quarters of its initial height” (Moore, 1995). The compaction was important to making the concrete strong and durable because a chemical reaction must take place and the compaction of the concrete pushes the molecules closer together by removing any air gaps and extra water. When in closer proximity and without extra water in the way, the atoms of pozzolan and of lime can better bond by sharing electrons and this created a durable concrete (Moore, 1995).
This original design, where the foundation was only about 1.2 meters wider than the 31.7 meter tall walls it would support, which makes Moore suggest that the Romans may not have fully understood how much sinking could occur and how much of a foundation would be needed. The walls at Pompeii are another example of the Romans sparing use of foundational support, because there is no discernible foundation for the 8 meter high and 5.5 meter thick wall. With the foundation of a structure being arguably the most vital element for longevity and stability, considering that the planned foundation of the Pantheon seemed to be somewhat meager and built on top of wet clay, it is amazing that the structure has stood to be as stable as long it has. Of course, they did add more foundation after the first ring cracked, but it is uncertain what has prevented the destruction of the structure, whether it be the lack of stress concentrations points on the foundation, very strong concrete, and/or something else (Moore 1995).
The Rotunda Walls
The exterior of the rotunda walls. See the ledges marking the cornices just below the rectangular openings (photo by author)
From the exterior, the rotunda looks like a solid wall of bricks, but it is in fact more than that. There are openings at various levels, chambers, and passageways throughout the the rotunda wall. Also, while the wall looks like it is made of bricks, the bricks are just a thin outer layer, the majority of the material in the walls is concrete, which provides the structural support and strength of the walls. The outside of the walls were covered with white marble which hid all the brick and provided a cleaner finished look after construction. There are three cornices in the walls that separate the wall into sections or levels (MacDonald, 1976). The walls contain internal and external relieving arches that, along with 8 very large niches in the interior, divide the wall into a series of concrete piers. The interior of the rotunda has a diameter of 43.4 meters, the same as the dome above, and is 31.7 meters tall (Moore, 1995).
Types of Roman scaffolding (Moore, 1995)
To construct the rotunda, a repetitive cycle was used: the brick walls would be built up slightly, layering bricks and mortar, then aggregate would be placed in a layer, lime and pozzolan mortar would be placed on top, the concrete would be compacted, then let to dry. They built up like this in 20 centimeter thick layers, adding the relieving arches and leaving empty cavities as they moved up. The relieving arches were made by erecting a temporary semicircular wooden form over the opening, laying a thin layer of mortar on top to make a bed for the bricks, and then the bricks were stacked on end over the form. The bricks in the arches were bipedales, a Roman type of square brick, that were about 60 centimeters in length and width, and about 2 centimeters thick. The relieving arches at the bottom of the rotunda are one layer of bipedales thick, but are two or three layers thick in the upper sections. After the form was removed, the void below would be filled in with brick and concrete. There were also bonding courses, layers of bricks 2 bricks thick placed horizontally, all the way through the wall about every 1.1 meters up the wall, thought to be a way for the constructors to keep the wall level and straight as they built up. (Moore 1995)
To construct the walls, the builders used wood scaffolding that was very light and tied together with rope. The scaffolding could be of three types: independent scaffolding that stood on its own and did not need to be supported by another structure, dependent scaffolding which was inserted into holes strategically placed in the structure and was fully supported by the structure that they were using the scaffolding to build, and semi-dependent scaffolding that was a mixture of both; supported by both the ground and the structure being built. The constructors of the Pantheon likely used a combination of these types of scaffolding as they built the wall and dome. See the pictures above for visual explanation of the types of scaffolding (Moore 1995).
How bricks undergo chemical changes with heat. From page 88 of David Moore’s “The Roman Pantheon” discussing the influence of heat in the brick making process
The concrete in the rotunda wall was the same type as is described in the Foundation Materials section above. The brick layers as seen on the outside were constructed with the typical type of Roman brick. These bricks were made of burnt clay that undergoes a chemical reaction through the heating process, changing the chemical structure of the material. The bricks are heated at high temperature for about 2 hours to complete the chemical transformation. The colors of the bricks are determined by the temperature it is burned at and the chemical composition, specifically the concentration of iron oxide, alumina, and calcium. Above is a brief breakdown of the changes that occur in a brick during heating (Moore 1995).
Different sizes and cuts of standard Roman bricks (Acocella, 2014)
The bricks would be formed into standard shapes as shown above; bessales, sesquipedales, and biedales. The different types of bricks would also be broken up into triangles, as shown by the lines across the bricks in the picture above, and this was done to provide various different sized bricks to fit certain jobs. Broken brick pieces were also used as part of the aggregate in the concrete walls of the Pantheon, an example of the common Roman practice of the reuse and re-purposing of materials.
In this photo you can see the outer brick layer (bottom), the exposed inner concrete (middle to top), and the holes used to support scaffolding and framework during construction. (Photo by author)
[LEFT]: Cross section of wall. See thick pier areas vs. the niches and cavities (MacDonald, 1976)
[RIGHT]: A diagram of the arches directing load to the piers (Lancaster, 2006)
The rotunda walls hold up the massive dome above and provides the necessary interior space for the public space inside. With all the niches, cavities and relieving arches to provide for these features, the walls of the Pantheon don’t necessarily behave like a typical solid wall. The cavities and niches in the walls divide the rotunda into what is essentially a series of eight concrete piers, where the concrete is thickest, strongest, and supports the majority of the load. The relieving arches, framing the niches and cavities, are in place to divert the load from the area near the structurally weak cavities into the piers. The arches transfer some of the load in vertical weight into a diagonal support reaction at the base of the arches in the piers. The vertical component of this transferred load is directed through the pier and into the ground, while the lateral thrust from the arches is directed toward the adjacent piers and arches. The lateral thrusts in adjacent arches point in opposite directions and essentially “cancel” each other out, and because the arches in the Pantheon are in the shape of the circular rotunda, all the forces distributed by the arches are fully supported and “cancelled” by the completed loop (Lancaster, 2006)
[LEFT]: Relieving arches in the brick walls of the rotunda (photo by author)
[RIGHT]: Relieving arches in the brick walls of the rotunda (photo by author)
The relieving arches also helped prevent against and manage settlement of the mortar as the walls dried during construction. Due to the shape of the arch and the way it redirects load, the arches were able to shift slightly and better absorb some of the effects of mortar settlement and also allowed them to build faster because they didn’t have to wait for the mortar to cure completely before moving onto the next layer (Lancaster, 2006). However, this did not stop all the settlement and mortar creep in the Pantheon walls. There is an interesting theory about the structure attached to the rear of the rotunda that suggests that the Pantheon dealt with some major settlement at some point during construction and they needed to improvise a way to remedy it. Attached at the rear of the Pantheon is a small structure, called the grottoni, which is a brick building consisting of 6 interior walls, 2 floors, overhead vaults, and a bridge-like connection to the rotunda about 2/3 of the way up. The grottoni appears not to have served any ceremonial or utilitarian purpose, and along with the fact that it looks as though the grottoni was built quite quickly, there is some mystery as to why it is there. Mark William Jone’s theory is that because of the clay foundation, portions of the Pantheon’s walls began to settle at a significantly different rates, and this caused the walls to begin separating during construction. As a way to quickly fix this, they constructed the grottoni to hold up and support the walls against leaning and collapse due to further settlement. Based on the cracks in the rotunda walls and the dates stamped on the bricks indicating when the grottoni was constructed, Mark Williams Jones describes this as the most probable reason for the existence of the grottoni:
“…whatever may be the exact causation, the very existence of the grottoni and the manner in which they were constructed undoubtedly speaks of a crisis that was related to structural distress or constructional difficulty.” (Jones, 2009)
The grottoni in the rear of the Pantheon (photo by author)
At the entrance to the Pantheon is a large portico; an area roofed by a triangular pediment structure supported by 16 monolithic (one single stone) columns above a slightly raised porch floor. The portico serves as a grand entrance-way to the Pantheon and includes some quite interesting features, specifically the columns.
How the columns may have been erected (Sahotsky, n.d.)
The Pantheon’s columns were of typical Roman column style, but were larger than many columns used at this period of time, so it was quite the feat to create, transport, and erect them. They most likely erected the columns by the typical Roman method as described by Brian Sahotsky (n.d) below.
Another method of column raising (Sahotsky, n.d.)
“The raising of the columns employed a variety of equipment, including cranes, lift towers, and other simple machines. In Vitruvius’ tenth book on machines, the author describes many cranes, and elaborates that these machines are used for hoisting heavy loads during “the completion of temples and public works,” and also for loading and unloading ships. He mentions that some of these machines are set upright in a stationary position, while some have revolving booms. Vitruvius also describes an instrument of laminated wood and supporting cords that resembles a fulcrum lever mechanism. Capstans at the ground level would feed the cords through pulleys to effectively tension the wood beams, and pull the column from a lying horizontal position to its vertical standing position. Large wooden cranes, would then lift the column into its place on each podium. Depending on the size of the loads, these cranes employed single or double boom arms. The largest of loads would require reduction gear, including the use of capstans to tension the boom arms. The most difficult and unwieldy of loads would be handled by treadmill cranes, as illuminated in a scene from the Haterii Relief (shown above).These cranes have been depicted with up to eight workers inside the bowels of the treadmill, which provide the necessary power to manipulate the loads.” (Sahotsky, n.d.)
The columns of the Pantheon’s portico are arranged in three rows; 8 in the front row and then two rows of 4 columns behind them. The columns are all monoliths carved out of Egyptian granodiorate. A stone very similar to granite, granodiorate is formed in the same way, by slow cooling underground magma, but granodiorate has more calcium and sodium and is darker than granite. The rosetta stone and Half Dome in Yosemite are also made of granodiorate (National Parks). Each column is 11.8 m tall, 1.5 m in diameter, and weigh about 60 tons. The columns were mined out in one piece each from Egypt’s mountainous quarries of Mons Claudianus. From there, they had to be transported to Rome through the Nile, Mediterranean, and Tiber River by means of wooden sledges, barges, and larger sea-going vessels (Parker, 2009). Simple wood trusses are now used underneath to support the roof of pediment, but there once was a bronze roof structure in the roof that has since been removed by Pope Urban VIII.(Macdonald, 1976).
The underside of the pediment. Note the columns supporting the stone beams and arches, and the wood trusses holding up the roof. (Photo by author)
When in place, these columns were used to support the large triangular pediment on top of the entrance. The pediment is made of cut stone and supported by wood beams in the interior. There used to be bronze in the roof of the portico, said to be in the form of trusses, but that implies that the ancient bronze had a greater structural strength that it may actually have had. The Bronze was likely used just to cover timber supports like the ones used now (Macdonald, 1976). The columns of the Pantheon’s portico serve the same functions as most all other columns; they are load bearing structures used to transfer the weight of the structure above, through compression, to the ground. As a fairly simple structure, the massive columns of the Pantheon support the beams that make up the underside of the portico roof above. Though their functions i simple, the sheer size of the columns and the fact that they are each made out of single stones make the story and creation of the columns quite complex.
Reconstruction of Pantheon as it may have looked, showing portico and statues (Hudelson, n.d.)
One last thing to note about the portico and columns is the existence of a visible second outline for a pediment on the intermediate block of the rotunda, slightly above the roof of the existing pediment. As is seen in the picture above, this strange triangular outline, mirroring the shape of the current portico just below it, has led to much speculation as to why it is there. Mark Wilson Jones (1987)offers a possibly explanation to this odd aesthetic; they originally planned to use even larger columns. Jones argues that it is possible that the Pantheon was originally designed to have columns 50 Roman feet and capitals 10 Roman feet tall, but for some reason they were not able to use the columns as designed. They therefore had to use the the 40 Roman foot columns and 8 Roman foot capitals that are currently in place. Whether it was a mistake during cutting at the quarry, the taller columns broke, or some other reason, it is unknown why they had to use these shorter columns. They would have already built the rotunda walls when they made this construction change, so they had already made the outline for the designed pediment, hence the second, taller outline you can see today (Jones, 1987). This is an intriguing example of the ingenuity and adaptability of Roman Engineering and another one the unique challenges presented during the construction of the Pantheon that they had to overcome.
The Pantheon today, see the second cornice outlining the planned pediment above the existing pediment (romeonsegway.com)
A perfect half circle of coffered concrete in the interior leading up to the oculus open to the sky, the dome probably the most defining feature of the Pantheon. To construct the dome took a lot of planning and Roman engineering technique, work that paid off, as the dome still stands today.
The theoretical sphere, cube, and cylinder of the Pantheon (toolonginthisplace.wordpress.com)
The entire Pantheon is saturated with geometrically inspired design and the dome is a good example of this. The dome, a perfect hemisphere from the interior, has a diameter of 43.3 meters and rests on top of the rotunda walls which have an equal height. This means that the Pantheon is designed to theoretically hold a sphere of equal diameter to that of the dome, and the sphere is encased in or defined by the dimensions of the cylinder or cube that could be used to represent the rotunda walls (MacDonald, 1976). Such geometrical relation draws to the mind Leonardo da Vinci’s Vitruvian man, the ideal human form defined by a circle and square. This circle and square pattern of design is prevalent throughout the Pantheon, as also seen by the design of the tiles in the floor. See the pictures below for visual explanation of these geometries.
Leonardo da Vinci’s Vitruvian Man (leonardodavinci.stanford.edu)
Pantheon floor and hte circle and square geometry (Photo by author)
Geometry and dimensions (meters) of rotunda and dome. See the stepping of the exterior and the coffering of the interior. (Martines, 2009)
The dome was created from poured concrete using a system of interior scaffolding and framework. Once the rotunda wall was completed, the builders could start affixing wooden platforms to the walls and begin building up the dome. The dome started from the top of the rotunda as a series of 7 concrete rings that go halfway up the dome decreasing in width and diameter until it transitions to a smooth circular line up to the 5.9 meter diameter oculus. The interior of the dome is lined with 5 bands of coffers, rectangular spaces left out of the dome to save weight and material (Moore, 1995).
Figure of stepping rings and lead plates on exterior. (Moore 1995)
To lay the concrete for the dome, the builders worked up in levels, constructing wooden work platforms and forms to shape the concrete. The rings were laid and dried on top of each other, then forms were built the lay the circular dome on as they built up, creating special forms for the coffers as they went. Above are some pictures on how this may have looked. They used the heaviest aggregate, mostly basalt, at the bottom and lighter materials, such as pumice, at the top. This was done to lighten the weight of the dome while using the necessary materials to provide enough support where needed; at the bottom, and save weight where less load is supported; near the top (Parker, 2009). When they got to the oculus, it was not as simple as just leaving an empty hole, the top of the dome is under compression, so they had to install a compression ring to prevent the oculus from collapsing inward. The compression ring at the center of the dome is 5.9 meters in diameter 1.4 meters thick. The ring is made of 3 horizontal rings of tile, 2 bricks thick, set upright, and one above the other. This brick ring and a bronze ring lining the interior of the oculus properly distribute the compression forces at this point. (Moore, 1995)
Possible methods of scaffolding: Dependent scaffolding (Moore, 1995)
Originally the dome was covered with a layer of bronze plates, but those were since removed and replaced with lead plates. In the pictures above, you can see the stepped ring pattern seen at the exterior base of the dome, the interior coffering, and some possible representations of how they could have constructed the scaffolding and framework for laying the concrete of the dome.
The dome of the Pantheon is made of concrete, but it is not uniform throughout. Different mixes of concrete were used at different levels as you move up the dome, heavier materials lower at the base and lighter weight materials up near the top. The concrete was similar to as I described it in the Foundation Materials section above, but they changed what type of aggregate they used. At the bottom they used heavier rocks like basalt and at the top they used lighter rocks such as pumice (Parker, 2009).
The stresses in a spherical dome: Tension in the bottom causes meridional cracks from the base (Isler and Balz, 1980)
The stepped rings provided most of the buttressing support of the lateral thrust from the dome. By adding enough mass at the base, where the force is concentrated, the rings act like buttresses. These buttressing rings are the reason that the dome does not look like a perfect half circle from the outside, but rather more of a bowl shape. The reason they varied the weight of the concrete of the dome at different levels was to make the dome lighter with cheaper material while still providing enough structural support. By making the dome weigh less near the top, but by making the bottom of the dome heavier, it provided more support, at the bottom where it was needed, for the lateral thrust from the load of the rest of the dome above (Lancaster, 2006). Other attempts at lightening the dome are seen in the coffering of the interior of the dome and the thinning of the dome from 7 meters at the base to 2 meters thick at the top. The 140 coffers are rectangular slots left out of the dome to save a little weight and material, but also add an artistic perspective scene.
Despite these many attempts to avoid excessive stress in the dome by reducing the weight, the Pantheon’s dome is cracked in many places. Due to the fact that the concrete is in the shape of a dome, it is subject to tension by means of hoop stress and, as concrete performs more poorly in tension than in compression, the hoop stress has resulted in cracks in the dome and walls of the Pantheon. The cracking of the Pantheon has been heavily documented and mapped and a design study by Mark and Hutchinson has this to say about it:
“Terenzio [an Italian superintendent of monuments who documented the cracks in 1930] also identified fractures reaching from the base of the rotunda to the summit of the dome that he thought were brought about by differential settlement from uneven loading of the wall, particularly near the entrance of the rotunda in the principal niche. Rather than finding vertical differential settlement, we have observed only traces of lateral openings across the cracks-corresponding to the effect of hoop tension.” (Moore, 1995)
Mapping of the cracks in the dome of the Pantheon. (Moore, 1995)
The cracking occurs in the lower half of the dome, starting at the point on the dome where the stress changes from compression to tension. The cracks are in the meridional direction, rather than laterally or horizontally and do not reach up to the oculus. The cracking creates an interesting situation in which the cracks create sections that act as a series of arches that share the uncracked oculus as a common keystone. While the dome is obviously still standing, the cracks have contributed to the top of the dome having slumped about 60 centimeters. (Jones, 2009). According to Moore, a model produced results finding that the maximum tensile bending stress of the concrete in the dome is 18.5 psi and, using a sample of similar Roman concrete from Libya, he provided a tensile strength of 213 psi for the concrete. So, it appears that the cracks in the dome are not a major concern as the stresses are within a safe design limit, but the cracks are still being monitored and there is discussion of adding a protective steel band around the base to prevent dislocation during an event such as an earthquake.(Moore, 1995)
Whether it was talking about its marshy foundation, the design of the dome, the cracking, or all the improvisation that occurred with features such as the columns being too short or the additional grottini structure in the rear, I couldn’t help but be drawn into the depth and range of engineering genius that went into designing and constructing the Pantheon. But beside the history and stories you can read about and listen to, the one thing that solidified my choice to research the Pantheon was the awe I experienced seeing it in person, walking around it, touching it, and going inside. You walk around a street corner just like any other Roman street corner and then, Bam!, the Pantheon is right there. The sheer size, complexity, and appealing aesthetics are undeniably jaw-dropping. I got a chance to go in early one morning when it was raining, and it was unforgettable to see the rain fall through the oculus, seemingly in slow motion, hear the rain hit the uncrowded and silent floor, and then just take a deep breath to feel the great open space inside the Pantheon, the gateway to heaven. There is no stairway available to climb up to that gateway, but as you listen to the music in the square outside, it feels nice to imagine there is one
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