Pantheon
The Pantheon is a former Roman temple, now a church, in Rome, Italy, on the site of an earlier temple commissioned by Marcus Agrippa during the reign of Augustus (27 BC – 14 AD). The present building was completed by the emperor Hadrian and probably dedicated about 126 AD. He retained Agrippa's original inscription, which has confused its date of construction as the original Pantheon burnt down and we are not completely sure when the one standing now was built.
The building is circular with a portico of large granite Corinthian columns (eight in the first rank and two groups of four behind) under a pediment. A rectangular vestibule links the porch to the rotunda, which is under a coffered concrete dome, with a central opening (oculus) to the sky. Almost two thousand years after it was built, the Pantheon's dome is still the world's largest unreinforced concrete dome. The height to the oculus and the diameter of the interior circle are the same, 43.3 metres (142 ft).
It is one of the best-preserved of all Ancient Roman buildings, in large part because it has been in continuous use throughout its history, and since the 7th century, the Pantheon has been used as a church dedicated to "St. Mary and the Martyrs" (Latin: Santa Maria ad Martyres) but informally known as "Santa Maria Rotonda". The square in front of the Pantheon is called Piazza della Rotonda. The Pantheon is a state property, ruled by Italy's Ministry of Cultural Heritage and Activities and Tourism through the Polo Museale del Lazio.
Though the Pantheon's large circular domed cella, with a conventional temple portico front, is "unique" in Roman architecture, it was much the largest and most accessible complete classical temple front known to the Italian Renaissance, and was the standard exemplar when classical styles were revived. It has therefore been copied many times by modern architects.
Rotunda
The 4,535 metric tons (4,999 short tons) weight of the Roman concrete dome is concentrated on a ring of voussoirs 9.1 metres (30 ft) in diameter that form the oculus, while the downward thrust of the dome is carried by eight barrel vaults in the 6.4 metres (21 ft) thick drum wall into eight piers. The thickness of the dome varies from 6.4 metres (21 ft) at the base of the dome to 1.2 metres (3.9 ft) around the oculus. The materials used in the concrete of the dome also varies. At its thickest point, the aggregate is travertine, then terracotta tiles, then at the very top, tufa and pumice, both porous light stones. At the very top, where the dome would be at its weakest and vulnerable to collapse, the oculus actually lightens the load.
No tensile test results are available on the concrete used in the Pantheon; however, Cowan discussed tests on ancient concrete from Roman ruins in Libya, which gave a compressive strength of 20 MPa (2,900 psi). An empirical relationship gives a tensile strength of 1.47 MPa (213 psi) for this specimen. Finite element analysis of the structure by Mark and Hutchison found a maximum tensile stress of only 128 kPa (18.5 psi) at the point where the dome joins the raised outer wall.
The stresses in the dome were found to be substantially reduced by the use of successively less dense aggregate stones, such as small pots or pieces of pumice, in higher layers of the dome. Mark and Hutchison estimated that, if normal weight concrete had been used throughout, the stresses in the arch would have been some 80% greater.
Interior
The interior of the dome was possibly intended to symbolize the arched vault of the heavens. The oculus at the dome's apex and the entry door are the only natural sources of light in the interior. Throughout the day, the light from the oculus moves around this space in a reverse sundial effect. The oculus also serves as a cooling and ventilation method. During storms, a drainage system below the floor handles the rain that falls through the oculus.
The dome features sunken panels (coffers), in five rings of 28. This evenly spaced layout was difficult to achieve and, it is presumed, had symbolic meaning, either numerical, geometric, or lunar. In antiquity, the coffers may have contained bronze stars, rosettes, or other ornaments.
Circles and squares form the unifying theme of the interior design. The checkerboard floor pattern contrasts with the concentric circles of square coffers in the dome. Each zone of the interior, from floor to ceiling, is subdivided according to a different scheme. As a result, the interior decorative zones do not line up. The overall effect is immediate viewer orientation according to the major axis of the building, even though the cylindrical space topped by a hemispherical dome is inherently ambiguous. This discordance has not always been appreciated, and the attic level was redone according to Neoclassical taste in the 18th century.
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