Characteristics
Contenido
La construcción de una amplia sala de planta circular, antecedida por un pórtico rectangular, conformado como un templo clásico, es una innovación en la arquitectura romana. El modelo de espacio circular cubierto por una bóveda se había utilizado por la misma época en las grandes salas termales, pero era una novedad usarlo en un templo. El efecto de sorpresa al cruzar el umbral de la puerta debía de ser notable.
La construcción de una cúpula semiesférica sobre un tambor circular era típico de la arquitectura de la época. Se observa en la Villa Adriana en Tívoli, en las termas de Agripa, y en general en las salas de los primeros tiempos del imperio. Las pechinas no se generalizarían hasta una época más tardía, en tiempos de Diocleciano.
El espacio interno de la rotonda está constituido por un cilindro "Cilindro (geometría)") cubierto por una semiesfera. El cilindro tiene una altura igual al radio, y la altura total es igual al diámetro, por lo que se puede inscribir una esfera completa en el espacio interior. El diámetro de la cúpula es de 43,44 m (150 pies), lo que la convierte en la mayor cúpula de hormigón en masa de la historia. La cúpula de la Basílica de San Pedro fue construida un poco más pequeña.
The pronaos
The octostil pronaos, with eight columns "Column (architecture)") on the façade, and with four columns on the sides, measures 34.20x15.62 m, and is 1.32 m above the level of the plaza, so that it is accessed by climbing five steps. The total height of the order is 14.15 m and the shafts have a diameter of 1.48 m at the base.
The frieze contains Agrippa's inscription in bronze letters. A second inscription[4] relating to the restoration undertaken in the times of Septimius Severus was later engraved on the architrave. The pediment must have been decorated with bronze statues, anchored with bolts, and now lost. Due to the position of the holes, it is speculated that it could have been the figure of an eagle with its wings spread.
Inside, two rows of four columns divide the space into three naves, the central and widest one leads to the large access door to the cella, while the two lateral ones end in two large niches that must have housed statues of Caesar Augustus and Agrippa, moved from the old building. The shaft of the columns was made of gray granite on the façade or red, from the quarries of Egypt. The porticos that surrounded the plaza were also made of red granite, although smaller in size. The Corinthian capitals, bases and interlocking elements are made of Pentelic marble, from Greece. The last column on the eastern side, lost in the 19th century, was replaced by another made of gray granite in the time of Alexander VII. The column at the eastern end of the façade was also replaced with a red granite shaft under the pontificate of Urban VIII. In this way the original alternation of colors was modified.
The gabled roof is supported by wooden trusses, supported by a wall structure supported by arches in the columns. The original coverage was made up of bronze trusses, with U-shaped profiles, as described by Andrea Palladio. One version says that they were looted by Pope Urban VII, who ordered them to be melted down to make one hundred cannons "Cannon (artillery)") for Castel Sant'Angelo. Another version claims that Pope Urban VIII ordered the bronze from the ceiling to be melted down so that it could be used in the baldachin of Saint Peter. The pavement is made of colored marble pieces, placed in circles and squares.
The intermediate body
The intermediate body that connects the pronaos with the cella is built in opera latericia, and consists of two large buttresses attached to the cella. The buttresses flank the access passage to the rotunda, which is the extension of the central nave of the pronaos. On the other hand, between the buttresses and the cella there is a residual space in which two access stairs to the upper part of the dome are located.
On the outside, the structure has the same height as the cylinder of the rotunda, and should have had a stucco and mortar coating "Mortar (construction)"), which has now disappeared. Above this body there is a second brick pediment, taller than the entrance portico, and which was intended to be seen only from a great distance. The cornice lines, which run along the outside of the rotunda marking the floors, continue in this body without a break in continuity.
The difference in level between the two pediments has led to the assumption that the pronaos was planned to be larger, with columns 50 feet high (14.80 m), even larger than those at the northern entrance to Trajan's forum. The project had to be modified, as columns of such large dimensions could not be supplied.
The interior of the roundabout
On the lower level there are seven large exedras, with a trapezoidal plan "Trapezoid (geometry)") and semicircular alternately. The niches are framed by an order of pilasters and columns, with a continuous entablature around the entire perimeter, except in the exedra of the main axis. In these, covered by vaults, the entablature is interrupted since the intermediate columns are not necessary. Between the exedras, in the intermediate wall panels, aedicules appear with alternating triangular and circular capials.
On a second level, from the entablature to the impost of the vault, there is a row of windows. These windows, which open onto an upper gallery, coincide vertically with the niches and aedicules. The original Roman decoration was replaced in the century by the one that can be seen today, probably made between the years 1747-1752. The southwestern sector has undergone several restorations, not entirely appropriate, that have altered the initial appearance.
The pavement of the roundabout is slightly convex, with the central part 30 cm higher than the perimeter, so that rain entering through the oculus flows into the channel located around the perimeter. The covering is made of tiles with a square design in which smaller squares and circles are alternately inscribed.
The dome
The dome is made up of five rows of coffered interior, which decrease in size towards the center, where it is pierced by an oculus 9 m in diameter. This circular window remains open, and light enters through it. The oculus was surrounded by a bronze cornice fixed to the dome in the last row of coffers. The cavities in the factory suggest that both the coffers and the intermediate space were lined with bronze.
On the outside, the dome starts from an elevation of the wall, 8.40 m above the interior start of the vault. It is articulated by seven superimposed rings, the lower one of which still retains its marble coating. The rest was covered by gilded bronze plates, now lost except for the perimeter of the oculus, still in place. The bronze plates were torn off in 663 by order of Constant II, emperor of Byzantium, and a new lead cover was installed in 735.
Another element that reinforces the idea of perfection is that the height of the last second cornice, where the vault begins, is exactly half the height from the highest point of the vault to the ground, and curiously this coincides with the diameter and a perfect complete sphere can be inscribed inside the construction.
Construction technique
Roman construction techniques have allowed the dome to withstand nineteen centuries without the need for reforms or reinforcements. There are several technical factors responsible for the dome reaching our days in perfect condition.
The dome is hemispherical, made of concrete with tuff rubble and volcanic slag. The external parts of the dome were lined with opera latericia, a concrete with brick walls. Bipedal bricks were also used in horizontal layers, like rings.[5] It was reinforced by forming a system of nerves, parallels and meridians, as shown by the shape of the coffers. It was built using successive concentric rings of concrete, resulting in a self-supporting structure, since when each ring sets, the scaffolding can be dismantled and the next ring can be concreted. For this reason, the oculus does not "fall down", as Brunelleschi thought when he entered, since in his time only construction systems were used for domes using voussoirs supported by wooden falsework.
The dome rests on a thick cylindrical opera latericia wall, in which openings were made corresponding to the three compositional levels. In part, these openings were functional, since they formed the exedras, but above all they were structural, because they formed an internal structure of discharge arches. These strong and flexible arches form a framework that is visible on the rear wall now that the original cladding has been lost. Choisy describes this construction process in detail.
Regarding the composition of Roman concrete, the cement was mixed in small quantities, thus draining the excess water. In modern concrete, the more water used in mixing, the greater the porosity once the water evaporates, reducing the resistance capacity. In this way, it was possible to partially or totally eliminate the air bubbles that normally form during setting, giving the material notable resistance. The concrete was poured in thin layers, alternating them with horizontal courses of stone. When placed in small quantities, the shrinkage of the cement is reduced, and therefore the possibility of settlement or cracking.
On the other hand, an attempt was made to reduce the weight of the dome by two means: by lightening the materials, so instead of the travertine used in the foundation, pumice stone was used in the dome, and gradually reducing its thickness from 5.90 m at the base to 1.50 m at the top. Furthermore, the niches, galleries and windows cut into the walls, as well as the coffered ceilings and the oculus of the vault, arranged between the main arches, lighten the construction in the filling areas.