by Unknown
The relieving arches embedded in the body of the rotunda wall are made of bipedales, with a minority of sesquipedales in some cases. These tile-shaped bricks are arranged in one, two, or three superimposed concentric rings, depending on the strength required (Figs. 4.3 and 4.4). The relieving arches that form part of the eight piers have only one ring of bipedales on the ground-level tier; their intrados correspond to the semidomes of the chambers within (see Fig. 1.13). The arches directly overhead have two rings of bipedales, again arranged to coincide with the chambers behind. The great relieving arches with triple rings (two of bipedales plus one of sesquipedales), also on the second level, correspond to the crown of the vaults over the exedrae; their internal diameter is 11.80 meters.40 On the third story, arches with double bipedales relate to the barrel vaults of the top tier of chambers (Figs. 4.5 and 4.6).
4.5. Perspective rendering of the structure over an exedra of the rotunda. (Pier Olinto Armanini, in Beltrami 1898, Plate IV)
4.6. Interior elevations projected flat, showing the bare structure (a) and the structure in relation to principal marble elements (b). (Drawn for the author by Roberta Zaccara in 2007)
As just mentioned, the Romans did not generally build arches from whole sesquipedales or bipedales alone, but broke many of them in two for reasons of economy and for better bonding, with the nonbroken edges in view on the exterior faces. As Gene Waddell explains in Chapter Five, whole bipedales often alternate with portions of concrete in between that may be likened to voussoirs. This can be seen in many ruined Roman buildings, but we can be less sure of this aspect in the Pantheon because it is intact. We can only catch a glimpse inside of the barrel vaults through a few cracks in the dome. I have, however, been able to inspect a high-level chamber of the Basilica Neptuni, immediately to the south of the Pantheon. Where some of the linings have fallen off, it is possible to observe the structure behind. The visible surfaces are made up entirely of bipedales, without rubble, as suggested in the drawings by Joseph Durm (Fig. 4.4). Durm draws the ribs of the dome as if they were made of brick, but this is not certain because we do not see inside the arches. The impressive arches that once bridged the Pantheon and the basilica at high level are in any case definitely made of solid bipedales (see Fig. 7.5). Perhaps all third-story barrel vaults were built with a high density of bipedales and relatively little rubble.
The lower level of chambers in the piers open onto the exterior of the rotunda with apertures crowned by flat tile arches; above each of these there is a semicircular relieving arch (Fig. 4.3). The second-story chambers have no openings toward the outside, except for the ones that correspond to the stairs.41 At the same time all of the chambers sit behind the blind attic windows. The third-story semicircular chambers have an outside aperture that is not in the center so as to avoid the radial wall that bisects them.
Inside the rotunda, the trapezoidal exedrae on the diagonal axes have barrel vaults, while the exedrae on the cross axes have semidomes. The extrados of both attain a level just beneath the springing of the cupola. The relation between the architectural volumes and the fabric at attic level can best be seen in drawings that relate to the works that Pope Benedict XIV authorized after the Jubilee of 1750 to repair damage to the interior of the dome, which had been caused by infiltration from rain. During 1756–1758, adjustable scaffolding was put up, and Giovan Battista Piranesi had an opportunity to make firsthand observations.42 His studies were collated by his son Francesco (Fig. 4.7), which appeared as part of a set of engravings in 1790.43 The upper part of the plate contains reproductions of two brickstamps from Hadrian’s time, found in the dome.44 In the center there is a drawing of the brickwork in the attic, which was temporarily visible on account of the removal of areas of defective covering. The attic wall has a continuous series of main and secondary relieving arches, relating to the exedrae and the piers, respectively. Under the intrados of each main arch there is a framework of minor arches. This system lightens the load while directing it over the columns below (Fig. 4.5).45
4.7. Engraving illustrating brick stamps (top), construction of attic level (middle), brick skeleton for a portion of the dome, and an elevation detail of the oculus (bottom) by Francesco Piranesi. (Rome 1790, Plate XXVIII)
The function of the relieving arches of the Pantheon, like those of numerous other Roman buildings, is rather enigmatic. Having been built over and filled with masonry, they cannot behave like real arches. What purpose, then, do they serve?
One of our sources for such arches is the sixth book of Vitruvius, on private buildings:
Likewise, make certain that arches relieve the weight of the walls [ut levent onus parietum fornicationes] onto their voussoirs [cuneorum divisionibus], and that they are centered over the opening. For if arches spring from voussoirs that begin beyond the wooden beam or the head of a stone lintel, in the first place the wood will not bend because its load has been relieved, and secondly, if in time it begins to develop flaws, it can be replaced easily, and without piling up braces.46
Toward the end of the period of classical Roman architecture Procopius of Cesarea provides us with some interesting information on the construction of Hagia Sophia in Constantinople. He writes of the decision of the Emperor Justinian to complete one of the four main arches in the face of the danger that the centering would collapse:
And the props [Greek pessoi], above which the structure was being built, unable to carry the mass which bore down upon them, somehow or other suddenly began to crack, and they seemed on the point of collapsing. ... And straightway the Emperor ... commanded them [Anthemius of Tralles and Isidorus] to carry the curve of this arch to its final completion. “For when it rests upon itself,” he said, “it will no longer need the props [pessoi] beneath it.”47
In 1985, I had new bipedales made for the restoration of arches in the Domus Tiberiana on the Palatine.48 A dry bipedalis weighs 30 kilos but, when laid wet, and so much heavier due to water absorption, two bricklayers are required to shift it. As the wall dries, bipedales gradually release water, and this helps to even out the curing process and reduce the extent of shrinkage cracks. Due to the viscous quality of the mortar and the capillary nature of the terra-cotta, the union between wet bipedales and wet concrete is extremely strong. The porosity and surface area (0.36 m2) of a bipedalis is far greater than ordinary bricks. As a matter of fact, when an arch is closed by laying the final, key bipedalis, even though the masonry of the arch is still wet it immediately acquires stiffness (which gradually increases as the mortar dries). Once the crown of the arch is in place, the weight on the props diminishes as the thrust on the haunches begins, just as described by Procopius of Cesarea.
It is also significant that the amount of mortar used with the bipedales is much less than in the main body of concrete, this being another reason why relieving arches gain strength more quickly. This made relieving arches particularly effective in terms of constructional procedure, since the centering could be struck earlier than otherwise would be the case. It can further be supposed that having the vaults gain their strength quickly would have been a great advantage for proceeding with the construction at a higher level.49
Relieving arches confer greater stiffness to a wall because an arch of bipedales is stiffer than an equal mass of either normal opus testaceum or concrete. And as Lynne Lancaster notes: “The idea of using arches to control how a structure supports its load is related to the idea of reinforcing the parts of a building that support the greatest loads with the most durable materials.”50
Furthermore, stress is placed along a line going from one extremity to the other, as in a bridge. This advantage is useful in cases of differential subsidence of the terrain under the foundations. Some of the cracking in the Pantheon is almost certainly due to settlement.51 It can be surmised that Roman architects believed that such cracks, which slowly get bigger over the life span of a building, would be checked or even averted by the use of relieving arches.
Thus, we have seen how inside
the drum, voids are overlaid by either semidomes or barrel vaults, which on the outside are echoed by series of relieving arches. Built on three levels, these trace the logic of the 20-foot-thick diaphragmatic structure: arch-pier-arch, like a bridge or an aqueduct (Fig. 4.6).52 The voids give the structure lightness while the vaults in bipedales confer stiffness. The relieving arches brought advantages, too, in terms of both the performance of the structure and the speed with which it was put up.
About the Dome
Ammianus Marcellinus, describing a view of Rome during a visit by Costantius II in AD 357, expressed his admiration for the Pantheon: “The Pantheon is like a rounded city–district, vaulted over in lofty beauty.”53 This feat of engineering is underlined by the great weight of the masonry above the springing of the dome (shell of the dome + the third story of the drum), which, according to MacDonald, is about five thousand metric tons.54 The wonder of the building also raises other questions: what theoretical notions lie behind the construction of the dome? What technical criteria were used? Were there any precedents?
A book written during the greatest period of Roman architecture is The Mechanics of Hero of Alexandria, mentioned earlier. It is not a treatise like Vitruvius’s De Architectura but a textbook for students of architecture and engineering, which includes study exercises. It includes a foreword on geometry and kinematics, and describes lifting equipment for heavy weights on building sites and the straightening of a wall twisted by an earthquake.55 The heavy weights to be lifted are columns and stone blocks of opus quadratum, and so the book is not concerned with constructions using mortar, brick, and caementa. Hero writes with precision, and his work contains a formula relating to inclined planes that was not improved upon until Galileo Galilei.56 Unfortunately, we do not have an analogous source for the arch and the vault as yet, but Hero gives us an idea of Roman engineering science, which was evidently empirical but neither improvised nor approximate. We also know that Hero wrote a treatise On Vaulting, which was the subject of a commentary in the sixth century AD by Isidorus of Miletus, one of the architects of Hagia Sophia.57 Hero and Isidorus lie at either end of five centuries of written tradition on building vaults.
According to Robert Mark, an important source of Roman engineers’ knowledge was the observation of constructional failures – which may have been quite numerous – along with any remedies employed.58 Theory, therefore, was based on observation of deformation, displacement, and collapse, as in modern limit analysis. Reading Hero, we can appreciate how the concept of statics was based on mechanics and, furthermore, how architectural techniques made use of experience from a variety of other contexts including stone quarries, olive presses, and docks.
The biggest domes in the Roman world apart from the Pantheon have interior spans as follows:
The Temple of Mercury in Baiae, diameter 21.4 m
Temple of Venus in Baiae, 26.2 m
Temple of Diana in Baiae, 29.8 m
Temple of Apollo at Lake Avernus, 35.5 m59
Caldarium of the Baths of Caracalla, over 35 m.
Four of these buildings are located above Capo Miseno, near Naples, in the same volcanic region, from which derived their function as part of thermal complexes. Furthermore, the common constructional feature is that all of these domes are made from light volcanic material.
A most unusual vault is that of the Octagonal Hall in Nero’s Domus Aurea.60 There is no comparison in size with the Pantheon because it has a diameter of only 13.35 meters. Yet it is worth dwelling on the Octagonal Hall because its vault is exceptional in the history of Roman architecture (Fig. 4.8, a, b, c).61 The rotunda of the Octagonal Hall must have produced quite an effect on its privileged visitors. It was highly unusual in many ways beyond the extensive use of gilding,62 but what would really have impressed Nero’s guests was the way light was scooped in from above. This was something totally unexpected and must have produced great wonder: the walls were empty, whereas in other rotundae they were full; there was light where others were dark.
4.8. Section (a), plan (b), and model (c) by Filippo M. Martines of Octagonal Hall in the Domus Aurea, Rome. (Drawn for the author in 2005–2006)
The Neronian vault did not rest on a full drum but on an octagon with solid corners and open walls. The vault is very slender, and where it does thicken toward the base this is not achieved by means of external concentric stepped rings. Despite these differences, some structural elements of the Octagonal Hall of the Domus Aurea are similar to those of the Pantheon. The dome rests on eight piers, which are connected by a system of bipedales, that is, eight flat arches. The crown is lightened by an oculus 5.6 meters wide. Examination reveals that the concrete vault is formed by two different, superimposed, geometrical figures, a calotte on top of a domical vault. In the lower half, eight webs, separated by clearly visible groins over the piers, spring from the eight flatarches. The calotte begins where the groins end. We have already mentioned that the dome of the Pantheon is also made up of two superimposed systems: eight barrel vaults over the third-story ring chambers supporting a calotte with an oculus. Thus the structural idea is similar to that of the Octagonal Hall.
The architects Severus and Celer, magistri et machinatores, built the Domus Aurea before AD 68; Tacitus praises their talents: “[T]hey had the ingenuity and the courage to test the force of art even against the veto of nature.”63 Contemporaries said that Nero’s architects had surpassed nature in feats of construction, just as Parrasius had done in painting.64 This marvel was no longer visible by the time the Pantheon was built. In fact, in AD 104 a great mound of earth sealed the damnatio memoriae of Nero and his Domus Aurea. But as the architect who, ancient sources tell us, was responsible for creating the Baths of Trajan, the complex that subsumed Nero’s Domus into its massive substructures, Apollodorus of Damascus could have seen the renowned octagonal hall. What is more, he would have seen it without the decorations that camouflaged its constructive inventions when the Domus Aurea had been in use. In a highly original study in 1975, Wolf-Dieter Heilmeyer attributed to Apollodorus the design of the Pantheon as well. His hypothesis has found subsequent support and amplification (see Chapter Seven), though we still await definitive proof of it.65
MacDonald has suggested a comparison with another innovative piece of Roman vaulting:
There is a certain similarity between the structure of the aula of the Markets of Trajan and that of the Pantheon in spite of the basic difference in plan. The abutting tabernae barrel vaults and the gallery arches of the aula appear at the Pantheon in a more complex arrangement, disposed around a central vertical axis.66
Nero’s Octagonal Hall, the aula of the Markets, and the Pantheon share a static concept: central vault equilibrium is produced by a system of barrel vaults and abutments. This concept is different from the great domes of Baiae; it reminds us of the statics of gothic cathedrals. However, arches of bipedales in Roman vaults are not autonomous ribs but, as we have seen, are embedded in the concrete.67
Apollodorus’s best-preserved work is Trajan’s Column, the structure of which may be attributed to him as part of his authorship of Trajan’s Forum as a whole.68 The Column and the Pantheon are very different, yet they have similar characteristics. Firstly, they are gigantic, the biggest examples of their kind. Apollodorus’s experience of large-scale structures also included authorship of Trajan’s renowned bridge over the Danube.69 The Column and bridge set two world records in vertical and horizontal dimensions.70 The bearing structure of the Column is, like the drum of the Pantheon, a diaphragm (Fig. 4.2).
Both monuments are gigantic constructions with tiny passages (the spiral in the Column, the honeycomb in the drum). This relationship (giant building/tiny space) is mentioned in the Poliorketica, Apollodorus’s treatise on siege warfare, in a passage describing the excavation of niches to undermine enemy walls.71 Finally, with its height of approximately 130 Roman feet, without the statue,72 the Column would fit neatly into the Pantheon, with its clear height of 147 feet. These obse
rvations are no proof for attributing the Pantheon to Apollodorus but they are indicators. Two very different buildings use similar ideas. The architect of the Pantheon was certainly closer in artistic sensibility to Trajan’s era than Hadrian’s, and he must have seen the great construction sites at the beginning of the second century AD: the dome of the Venus’ Temple in Baiae, and Trajan’s Forum and Baths in Rome. On the basis of these indications, it may be assumed that he belonged to Apollodorus’s circle if he were not indeed Apollodorus himself.
Beneath the Plasterwork of the Dome
Plaster covers the inside surface of the dome of the Pantheon, while the outside is protected by a dressing of lead that replaced the original system of bronze tiles. Unlike that of many other ancient domes, which lie in ruins, its structure cannot be directly observed. We must therefore rely on data from previous inspections to an even greater degree than for the drum, many parts of which are still accessible today.73
The earliest document we have is a drawing, U 69A, by Antonio da Sangallo the Younger, which shows a section of the Pantheon on the right, while on the left there are studies for Villa Madama and, below, for St. Peter’s.74 Antonio’s interest in the construction of the Pantheon evidently sprang from his concerns for the building of St. Peter’s, in which he had been involved since 1507, some years before he made this drawing. The study of the Pantheon highlights two important high-level structural elements, relieving arches at the internal springing of the dome and bonding courses (of bipedales) capping the arches.
