Books III and IV of De Architectura will mainly be used to provide an focused analysis of how ‍building principles containing previous knowledge gathered and organized by Vitruvius in combination with his own insights are applied with respect to actual temple structures. Three temples built at different times around the date of completion of De Architectura will be analyzed, in chronological order of date of completion, to determine the influence, relevance and longevity of the treatise. Augustan temples are first analyzed to determine influence and relevance. The Pantheon, completed by Emperor Hadrian, will be analyzed to test the longevity and relevance of De Architectura and see if Vitruvius' building guidelines are still applied about 100 years after the publication of his treatise.


Roman Engineers (Architects)

Classical Roman architecture grew from Greek architectural developments and flourished during the Roman Republic and Empire. This is especially evident with countless architectural feats still preserved and present today, ranging from the universally famous and iconic Colosseum, Pantheon and aqueducts to the lesser known and more common structures for everyday public use such as bridges, roads and houses. As suggested by the Greek etymology of the word “architect” (arkhi-, “chief” + tekton, “builder”), a Roman architect at the time had an impressive repertoire and possessed a variety of skills. This included engineering, urban planning, designing, contracting, supervising, etc., all considered to be completely separate fields today in modern times. The wide variety of topics covered by Vitruvius in De Architectura in relation to architecture (later discussed) is a reflection of the range of knowledge and skills required for the position of an architect at the time. Architects were in charge of basically all aspects of a construction of a building or structure, thus needed to be educated in a variety of subjects. According to Vitruvius, an architect should be knowledgeable in a range of subjects that included geometry, history, philosophy, music, theatre, medicine and astronomy, in order to fully take in his surroundings and construct a successful structure. He stated, "The architect should be equipped with knowledge of many branches of study and varied kinds of learning, for it is by his judgment that all work done by the other arts is put to test."

Vitruvius and De Architectura

Arguably the most famous architect of the time was Vitruvius, due to his treatise on architecture, De Architectura libri decem (Ten Books on Architecture). Born Marcus Vitruvius Pollio around 80-70 BCE and raised and educated in Campania or Rome, Italy, little is actually known about the personal life of the celebrated ancient architect and writer. Bits and pieces are gained from his manuscripts alone. Vitruvius was a freeborn Roman citizen but not necessarily of high (equestrian) class. He had a broad "liberal arts" education, which is evident in his writings. It is presumed that after his liberal arts education, he was apprenticed to an architect, if not several architectural teachers. Vitruvius' actual career may have started while he was in his thirties, which would have been around 50 BCE, about the time of the outbreak of the second Roman civil war between Julius Caesar and Pompey (49 BCE). The next two decades of his life were likely spent in military campaigns, initially working as a Caesarian staff architect. It is also suggested that he was on Caesar's military campaign, as well. According to the Unitarian minister, author and compiler of reference works, Rev. John Platts in his first volume of A New Universal Biography, Vitruvius was "greatly esteemed by Julius Caesar." After the murder of Caesar, he continued his work in military campaigns under the reign of Augustus in a variety of locations including Northern Italy (Gallia Cisalpina), the Adriatic Coast (Fano), the Alps (Larignum), Marseilles, North Africa and Greece. His work included war machines, public buildings and aqueducts. During Vitruvius' later career, he was patronized by Augustus. Ingrid D. Rowland and Thomas Noble Howe, translator and commentator in Vitruvius: Ten Books on Architecture describes this patronage as Vitruvius receiving a "commoda" from Augustus, a sort of regular annuity that would allow him the leisure to study and write. De Architectura was either a product of this patronage or what allowed Vitruvius to receive sponsorship from Augustus.

Figure 1. 1684 depiction of Vitruvius presenting De Architectura to Augustus (Source: Vitruvius on Architecture by Thomas Gordon Smith)
Figure 1. 1684 depiction of Vitruvius presenting De Architectura to Augustus (Source: Vitruvius on Architecture by Thomas Gordon Smith)

De Architectura libra decem is a compilation of previous knowledge as well as a reflection of Vitruvius' own knowledge and understanding of architecture and engineering. Written sometime around 15 BCE, the treatise was written for and dedicated to Augustus after his win over Mark Antony and Cleopatra in the Battle of Actium (31 BCE) which lead to his rise in power and solidified his status as the first Roman Emperor. De Architectura was meant to be a guide for Augustus on building projects. Vitruvius made his dedication in the preface of Book I, for the benefit of Augustus' education and construction projects. Augustus (63 BCE - 14 AD), was the founder and thus, first Emperor of the Roman Empire. Notably famous for bringing peace to the Roman Empire and radically changing the Roman landscape through numerous building projects. Augustus himself more specifically lists his accomplishments in the //Res// //gestae divi Augusti// (The Deeds of the Divine Augustus) and famously said, "I found Rome a city of bricks and left it a city of marble." The reign of Augustus brought a revival in Roman architecture and construction. He began a trend of constructing buildings for the public as an emperor's display and sharing of the wealth, success and strength of the Roman Empire. Vitruvius addressed this and mentioned in his preface to Book I of De Architectura that his writing of the treatise was a means to aide Augustus in serving the public: "But when I saw that you were giving you attention not only to the welfare of society in general and to the establishment of public order, but also to the providing of public buildings intended for utilitarian purposes, so that not only should the State have been enriched with provinces by your means, but that the greatness of its power might likewise be attended with distinguished authority in its public buildings, I thought that I ought to take the first opportunity to lay before you my writings on this theme."

As stated in its title, the treatise is divided into ten sections or "books", each covering different aspects of architecture and city planning. The different books are broken down and categorized into:
  1. City planning, general architectural/civil engineering, and required education of an architect
  2. Origin of dwelling, site selection and building materials
  3. Theory, modes of design of temples and order of architecture
  4. Continuation of Book III; more on Ionic, Doric Corinthian and Tuscan order and temples
  5. Various types of civic buildings (forums)
  6. Domestic buildings in town and country
  7. Finishes: floors, vaults, stucco and painting
  8. Water detection, quality, supplies, storage and transportation
  9. Sciences influencing architecture/civil engineering: astronomy, astrology, clocks and sundials
  10. Construction and use of military engines, mechanical and surveying devices

The treatise has been essential to archaeologists and architectural historians in interpreting remaining physical evidence and literary descriptions of temples and other buildings/structures. Besides its original illustrations, it is the only surviving contemporary source on classical architecture and provides us a window into the times of the Roman Empire. More specifically, Vitruvius' writings also influenced the Renaissance definition of beauty in architecture, with the revival of Classical Rome.

Book III

Book III of De Architectura begins with the derivation of the perfect numbers in relation to each other and circle and square shapes, which Vitruvius uses to determine ideal symmetry and proportions. Ideal human body proportions are derived and used as a scale for measurements; this same idea is also later used for temples. Vitruvius then classifies temples by the arrangement of the colonnades:
  1. temple in antis
  2. prostyle
  3. amphiprostyle
  4. perpteral
  5. pseudodipteral
  6. dipteral
  7. hypaethral

Figure 2. Illustration showing the classes of temples according to arrangement of colonnades (Source: Hellenica World)
Figure 2. Illustration showing the classes of temples according to arrangement of colonnades (Source: Hellenica World)

Temples are then classified into five different styles, based on the determination of the shaft height of the columns and intercolumniation:
  1. pcynostyle: "close-set columns"
  2. systyle: "slightly more ample intercolumnar space"
  3. diastyle: "even more widely spaced"
  4. araeostyle: "columns stand further apart than is desirable"
  5. eustyle: "placement is right"

Figure 3. Illustration showing classification of temples according to intercolumniation, taken from (Source: Hellenica World)
Figure 3. Illustration showing classification of temples according to intercolumniation, taken from (Source: Hellenica World)

Vitruvius continues with listing rules for the dimensions and heights of columns depending on their class as well as column proportions. At the end of this section on columns within Book III, a standout quote from Vitruvius is: "For the eye is always in search of beauty, and if we do not gratify its desire for pleasure by a proportionate enlargement in these measures, and this make compensation for ocular deception, a clumsy and awkward appearance will be presented to the beholder." This tells us that the use of columns in temples play more of a decorative role rather than a functional one as load-bearing structures. The use and size of columns can be a reflection of the wealth and success of the Roman Empire at this time.

The rest of the book finished off with rules on the construction of the foundation and various substructures of temples, along with beginning an in depth discussion of proportions of the base, capitals and entablature of columns of the Ionic order.

Book IV

Book IV of De Architectura picks up where Book III left off by continuing the discussion of the Corinthian and Doric order of columns. He begins with the origin and invention of the three orders and continues with more rules of proportions for Corinthian capitals, as it was explained that the capitals were the only difference between Ionic and Corinthian columns, and Doric temples. Vitruvius also discusses principles for proportions of the cella and pronaos (portico) of the temples and its doorways. Book IV ends with smaller discussions on Tuscan temples, circular temples and altars in general.

Books III and IV had a large emphasis on the idea of proportions. Vitruvius states that these rules based on proportions were fundamental in protecting against structural failure, along with creating an aesthetic that pleases the eye. Vitruvius has also remained true to his initial agenda of aiding Augustus in designing and constructing these buildings for the people. For example, in the chapter on the proportions of intercolumniations and of columns, Vitruvius mentioned, "For the idea of the pteroma and the arrangement of the columns round a temple were devised in order that the intercolumniations might give the imposing effect of high relief; and also, in case a multitude of people should be caught in a heavy shower and detained, that they might have in the temple and round the cella a wide free space in which to wait."

Structure of Columns

It is evident from De Architectura that Roman architects of the time relied on correct and specific proportions and ratios in order to construct a structurally sound building. Although Vitruvius also mentioned foundations and materials, he held a heavy discussion of columns and proportions. As previously stated, Vitruvius found them impertinent in ensuring a structure would not fail. Thus, a very simple modern engineering introduction to columns will be needed to examine the qualifications needed for a column to succeed and how a column could fail. This could also be used as a general comparison to the rules of proportions in De Architectura to understand why these rules for the construction of temples, which were essentially just geometry in combination with observations from hundreds of years of trial and error, worked applying the modern knowledge of engineering and physics we have today.

A column, in engineering terms, is vertical structural element that, through compression, carries the weight of the structure above, through the column itself, and to the other structural elements below. Columns are compression members subject to axial forces. There are three types of columns based on type of failure modes: short columns, long or slender columns and intermediate columns.

Failure Modes

Failure in short columns occur when the direct stresses are dominant and crushing of the material occurs.

Figure 4a
Figure 4a

For a long column, failure occurs by lateral buckling. The buckling occurs before the normal stress reaches the strength of the column material. Buckling occurs when the load exceeds the axial strength of the column and causes it to deform. It originates at the weakest point in the column, which can either be at a point furthest from the supports or at a point where the material has anomalous, or abnormal, microstructure. The stress and strain at the point of deformation will spread until it exceeds the tensile, compressive or shear strength of the material. In other words, buckling occurs when the load applied is greater than the critical load of the column,or the maximum force that it can withstand. Buckling is an important concept as it is one of the major causes of failures in structures. A simple example to explain what buckling looks like would be the effect of when one pushes the ends of a bookmark together.

Figure 4b
Figure 4b

Failure in intermediate columns is a sort of combination of both failure in short and long columns. Failure occurs in both lateral buckling and crushing of the material.
Figure 4c
Figure 4c
Figures 4a-4c, (Source: efunda)

Buckling and Euler's Formula

The Swiss mathematician and physicist Leonhard Euler (1707-1783) determined a formula that predicts the critical buckling load for a straight pinned end column, otherwise known as the ideal column:
external image 1lAFFQK

  • P = critical load on the column
  • E = Young's modulus of the column material (measure of the stiffness of a solid material)
  • I = moment of inertia of cross-section
external image 1PD7R2q

    • where R = base radius of the column
  • K = effective length factor, a dimensionless coefficient determined by the conditions of the end support of columns (can be assumed to be 1 for columns with hinged ends and the sake of simplicity)
  • L = unsupported length of the column
  • KL = effective length of the column

An ideal, or perfect column is long, straight, homogenous and is subjected to concentric compressive axial loads. From this formula, it can be noted that the critical buckling load decreases as the length of the column increases and is dependent on Young's modulus of the material and dimensions. Once the column reaches the critical load, it will buckle.

Critical Stress and Slenderness Ratio

The critical compressive stress is the stress corresponding to the critical load of the column. The formula for the critical stress is obtained by dividing the critical load by A, the cross-sectional area of the column:

external image 1lAF2qd

The effective slenderness ratio, external image 1PD69hr, is the ratio of the effective length of the column (KL) to the least radius of gyration of its cross-section (r). It is a measure of the column's flexibility. Its formula is given by: external image 1PD6tgq, where I is the second moment of inertia and A is the total area of the cross-section. The slenderness ratio can also be used to determine the type of column and when Euler's formula is valid. A short column will have a low slenderness ratio and thus, will crush before it buckles. A large slenderness ratio (long column) will yield a smaller critical load and thus, govern the design strength of the column.

Figure 5. Simple graph displaying relationship between critical load and slenderness ratio (Source: efunda)
Figure 5. Simple graph displaying relationship between critical load and slenderness ratio (Source: efunda)

Temple of Divus Julius


The Temple of Divus Julius (also known as the Temple of the Deified Julius Caesar), is located in the southeast end of the Roman Forum. Augustus was responsible for the dramatic transformation of the Roman Forum after Julius Caesar. In fact, a vast majority of the Forum was a construction zone during his reign as he built or rebuilt nearly all of its buildings with the purpose of restoring its role as the principal civic and religious center of Rome and the Roman world.

Figure 6. Picture of the Roman Forum. Site of the Temple Divus Julius is at the left, evident from the tin roof added in modern times.

The construction of the Temple of Divus Julius occurred during the earlier years of Augustus' reign, on the site where Caesar was cremated. This site was also the location of a previous makeshift memorial to Caesar by a group of freedmen and followers present at the burning of his body. An altar and column had been erected and plans were made to consecrate the site and establish sacrifices to Caesar like other gods. However, it was taken down less than a year after by the consul Dolabella, who had opposed Caesar's deification. When Caesar was finally deified in 49 BCE, plans were begun for the construction for the temple and this site was chose to be the most appropriate. The temple was finally completed and dedicated in 18 August 29 BCE. Later on, the temple was repaired by Hadrian, with no changes made to its architectural style. The temple, obvious dedicated to Caesar, was also used by emperors to deliver public addresses, as the temple was located on the main open square of the forum.

Figure 7. Picture of the Roman Forum today. To the right is the Temple of DIvus Julius and some column remains can be seen.

Construction and Analysis

At the base of the temple was a 3.50 meters high platform, projecting out from the facade columns of the temple. It served as a rostra, decorated with beaks of the ships taken from the Battle of Actium. There were a ramp of stairs on each side of the rostra and at the center of the front wall of the platform was a semicircular niche, a unique feature of the temple at the time. The ramps extend onto the sides of the platform as arcades, or passageways, and are connected behind the temple by an oblique crypto portico. The crypto portico continued along the northeast side of the Regia in the forum. This was the Porticus Julia.

The temple building was placed on a second 2.36 meter high podium. The podium itself was made of opus caementicium (Roman concrete) and faced with travertine blocks and marble revetments. At the front were short stairs passing between the columns of the facade into the temple building.

Figure 8. Illustrated reconstructed of the Temple of Divus Julius, (Source: Roman Religious Experience project)
Figure 8. Illustrated reconstructed of the Temple of Divus Julius, (Source: Roman Religious Experience project)

The composition of the temple was of pcynostyle, as mentioned by Vitruvius himself in De Architectura. This means the columns were closely spaced. The front of the temple contained six closely spaced columns, with blank walls on the sides and rear. The columns are either described as either of Ionic or Corinthian style. The Ionic style is derived from images of the temple on coins from the first century BCE. However, fragments of Corinthian pilaster capitals have also been found on site. One possibility is that the cella walls contained Corinthian pilasters combined with Ionic pronaos. There is also the possibility that the images on the coins showed poorly rendered Corinthian columns and that the columns of the Divus Julius was in fact Corinthian throughout. Basing off the assumption that the columns were indeed Corinthian, then they were probably about 11.8-12.4 meters high.

Figure 9. Denarius struck in 36 BCE depicting a mournful Augustus and the unfinished Temple of Divus Julius, (Source: Coins at Warwick)
Figure 9. Denarius struck in 36 BCE depicting a mournful Augustus and the unfinished Temple of Divus Julius, (Source: Coins at Warwick)

As can seen from Figure 7, the Temple of Divus Julius was of squared nature. As stated by Vitruvius in the beginning of Book III on symmetry and proportions, if the human body was placed on its back with hands and feet extended, and using the navel as the center, a perfect circle could be traced around the body, with its fingers and toes touching the circumference. Aside from a circular outline, a square shape could also yield from the human body; Vitruvius claims that the distance from the feet to the top of the head is equal to the distance from the fingertips of one hand to the other. The temple could have been constructed in its squared nature in order to achieve "the greatest harmony in the symmetrical relations of the different parts to the general magnitude of the whole,", to be a "perfect building". Vitruvius continued saying that this was especially important in temples to the gods, as they were buildings "in which merits and faults usually last forever."

Vitruvius also asserted that temples must face west and altars face east. Augustan temples were found to face in all sorts of directions, except for the Temple of Divus Julius alone. It was the only one of Augustus' temple to adhere to Vitruvius' rule for the orientation of temples. The orientation was purposeful, in that it would allow those who approach the altar of the temple with offerings or sacrifices would be able to face the sunrise. As the sun sets in the east, it would illuminate the statues, or in the case of this temple, the giant statue of Julius Caesar, as people pray and sacrifice.

The Temple of Divus Julius was completed in 29 BCE and De Architectura sometime around 15 BCE. Based on the timing and how much the construction and design of the temple adhered to the principles stated in the treatise, it can be assumed that temple itself could have been an influence for the guidelines on temple design. If anything but, the treatise could also have been written simultaneously as the temple was being built. Vitruvius himself had mentioned the name of the temple in De Architectura. Though it was dedicated to Augustus, Vitruvius' rules were not absolute. Vitruvius was adamant about the orientation of temples and yet, the Temple of Divus Julius was the only one out of all Augustan buildings to adhere to this rule. This brings up the possibility that De Architectura was written too late to have any impact on Augustan architecture. There is also the possibility that Vitruvius and his treatise had not gained enough relevance, or a wider audience yet.

Today, little remains of the temple. Only parts of the cement core of the platform are preserved. The temple had been largely intact until the late 15th century when all the marble and stone had been reused in the construction of new churches and palaces. There is also a low tin roof, as seen in Figure 9, over the remains of Caesar's altar. A low mound is in its place behind a wall, where tourists and visitors toss their coins for some superstitious reason (or at least this was my observation at the time of my visit).

Figure 10. Temple of Divus Julius today in the Roman forum today (Source: Jacob DeBernardi)

Temple of Mars Ultor


The Temple of Mars Ultor is located in the Forum of Augustus. Construction of the forum and temple complex began in the mid-30s BCE, after August avenged the murder of his adopted deified father, Julius Caesar, in the Battle of Phillipi in 42 BCE with the deaths and Brutus and Cassius. Construction continued for three decades; the work of clearing the site and laying the foundations carried out in the decades of the 20s BCE and actual construction of the temple building did not begin until around 10 BCE. It was finally dedicated in 2 BCE, even though it still was not fully completed yet.

The Forum of Augustus was built to further legitimize his rule, and the temple to magnify his role in avenging Caesar's murder. In building a forum, Augustus displayed his dedication in improving public life, showing he was a man of the people, whilst also emphasizing his claim to power and connection to Caesar with the forum location in close proximity to the Forum of Caesar. The Forum of Augustus also had the functional purpose of providing a new space for law courts, where jury selection and prosecutions could take place. However, the primary aim was to symbolize Augustus' authority and power.

It was appropriate that the temple was dedicated to Mars, since Mars was the Roman God that symbolized military might and war and many war and legal proceedings took place here. Commencement ceremonies were held here before military generals set of for war. Many other ceremonies took place here and this was also a meeting place for the Senate to discuss war. Victorious generals would also come here to dedicate spoils and stolen goods from enemies during war to Mars at the altar.

The temple was completely destroyed during the reign of the Ostrogoth king Theodoric (494-526 BCE), except for the external columns to reuse the marble and other construction materials. In the 11th century, the monastery of St. Basil was built in place of the temple. It was ran by eastern monks and after several transformations and changes in the elevation, it was finally demolished in 1924 to build Via del Fori Imperiali. The podium of the temple still remains largely intact as well.

Figure 11. Picture of the remains of the Temple of Mars Ultor today

Figure 12. Close up view of some of column remains intact. Some marbling can still be seen.

Construction and Analysis

The width of the podium of the temple is 36 meters and its length 50 meters. The podium was a combination of opus caementicium (Roman concrete) with tufa and travertine blocks in opus quadratum and faced with a veneer of Carrara marble. There were seventeen flights of marble steps leading up to the podium, where there was a large rectangular altar in the middle.

Opus quadratum is an Ancient Roman construction technique where square blocks of stone are lined up parallel to each other without the use of mortar. Vitruvius described this technique in De Architectura in the section of Book IV discussing the cella and pronaos of the temple: "... If the walls [of the cella]... are to be of dimension stone or marble, the material ought to be of a very moderate and uniform size; for the laying of the stones so as to break joints will make the whole work stronger..."

The columns at the Temple of Mars Ultor were of Corinthian order. The width of the pronaos had eight columns in the front. There was a second row of columns on each side of the main axis of the temple aligned with the cella walls. There were also eight columns on the sides, with the aisles ending in a rear wall. Like the Temple of Divus Julius, the Temple of Mars Ultor was of pycnostyle. The columns were closely spaced in a 1:1.5 ratio of diameter to intercolumniation. The fluted shafts of the columns were made of Lunense marble.

Figure 13. Artist rendition of the complete temple, (Source: The Art of Power project)
Figure 13. Artist rendition of the complete temple, (Source: The Art of Power project)

With their base and capital, the columns were 17.76 meters high (60 Roman feet), and they had a lower diameter of 1.77 meters (6 Roman feet). This yields a height to lower diameter ratio of 10:1, adhering to Vitruvius' rules on proportions. Especially significant is the ratio, as Vitruvius has mentioned himself, the ancients found the number10 to be a perfect number. In addition, the ratio of the columns to the height of their shafts is 6:5. The number six was another "perfect number" mentioned by Vitruvius in De Architectura. The 6:5 ratio was also a commonly used ratio.

With that, it can be assumed that by this time, architecture, or at least Augustan architecture, has been somewhat standardized, using the guidelines from De Architectura. Judging from Figures 10 and 11 above, the Temple of Mars Ultor was squared, or at least almost squared nature, similar to the Temple of Divus Julius. It was also similar in that it was also of pcynostyle, had columns of the Corinthian order, and marble exteriors. The similarities show how the construction and design of temples were pretty generalized. In one way or another, temples at the time essentially all had the same basic large-scale appearance, with the only differences being to what god or deity the temple is dedicated to, its decorations and furnishings, and other smaller details.



The Pantheon is one of the most famous and magnificent Roman sites, along with being the best preserved. The original Pantheon was built by Marcus Agrippa in 27 BCE, during the reign of Augustus but was destroyed in the Great Fire of 80 AD. It was then rebuilt by Domitian only to be burned down again. Hadrian was the patron for the construction for the third, and currently existing, Pantheon. Construction began around 118 AD and was completed about eight or more years later. The original inscription commemorating the original patron, Agrippa, still stands today and was apparently put up by Hadrian as a display of respect for tradition. The Pantheon was reconsecrated by Pope Boniface IV as Saint Maria ad Martyres in 609 AD, which is why the structure is so well preserved today.

Construction and Analysis

The Pantheon is supported by a ring of foundations 7.3 meters wide and 4.5 meters deep. The dome itself has a diameter of 43.3 meters. It has a traditional squared nature appearance when viewing from the front, like any other previous temples built. Five steps from the square lead up to it, so the building looks like a traditional podium temple. The entrance of the Pantheon was designed with the portico containing eight columns across in the front holding up the pediment with Agrippa's inscription.

Figure 14. Portico of the Pantheon

The vestibule of the Pantheon between the portico and rotunda is 33.1 meters wide and 15.5 meters deep. It consists of a gable and 16 Corinthian columns. The outer aisles end in a round niche and were flat-roofed, along with the nave. The central nave was barrel-vaulted.The interior of the rotunda of the Pantheon is considered its "showpiece". The Pantheon was the first temple built with the intent of the interior being mainly used. Cults had previously celebrated outside temples. The archetypal rotunda, consisting of a cylinder and hemisphere, was also created here. The structure was shaped with simple dimensions: the height is exactly equal to the diameter and the dome, a perfect hemisphere, is exactly the same height as the cylinder.

The dome of the Pantheon is the world's largest unreinforced concrete dome. It is of opus caementicium and weighs 4,535 metric tons. The weight is concentrated on a ring of voussoirs of 9.1 meters in diameter that form the oculus. The downward thrust of the dome is carried by eight barrel vaults in the drum wall into eight piers. A simple explanation to help visualize this famous feat, as often mentioned by Professor Steve Muench, is to think of it as eight arches being swiveled around. The thickness of the dome decreases from the base (6.4 meters) up to around the oculus (1.2 meters). The materials used in the concrete of the dome also varied. At its thickest point, it was travertine, then terra-cotta tiles, and at the very top, tufa and pumice. This was an important innovation, having the dome made of lighter material and thus having decreased density, towards the crown. The oculus lightens the load of the dome; this feature makes the dome more durable in the long term because the overall stresses placed on the supports are significantly lower.

Figure 15. Diagram of cross-section of Pantheon showing grading of concrete, (Source: College of the Holy Cross)
Figure 15. Diagram of cross-section of Pantheon showing grading of concrete, (Source: College of the Holy Cross)

Figure 16. Oculus in the rotunda of the Pantheon

The original Pantheon was built in the lifetime of Vitruvius, yet it was never mentioned in De Architectura. The treatise lacked in content concerning vaulted construction, another ignored designs, suggesting the possibility that Vitruvius was not as "in touch" with Roman architecture at the time; that his status now as one of the greatest architects/engineers of the classical world is simply due to the fact that he was the first to have surviving records on the subject.There is also another possibility, that some scholars have argued, that Vitruvius may have also just wanted De Architectura to reflect the simplicity of structural design. Nevertheless, it is evident the Pantheon deviated from traditional temple designs in many ways, showcasing advancement in knowledge and technology as well as reflecting the changes of Roman history.

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