page_title: Pantheon, Rome
section_title: Rotunda
hierarchical_section_title: Pantheon, Rome / Structure / Rotunda
caption_attribution_description: Pantheon dome
context_page_description: The Pantheon is a former Roman temple, now a Catholic church, in Rome, Italy, on the site of an earlier temple commissioned by Marcus Agrippa during the reign of Augustus. It was rebuilt by the emperor Hadrian and probably dedicated about 126 AD. Its date of construction is uncertain, because Hadrian chose not to inscribe the new temple but rather to retain the inscription of Agrippa's older temple, which had burned down.
  The building is cylindrical with a portico of large granite Corinthian columns under a pediment. A rectangular vestibule links the porch to the rotunda, which is under a coffered concrete dome, with a central opening 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 metres.
  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 in use as a church dedicated to "St. Mary and the Martyrs" but informally known as "Santa Maria Rotonda".
context_section_description: The 4,535-tonne 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-metre-thick (21 ft) 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 vary. 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 0.128 MPa (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. Hidden chambers engineered within the rotunda form a sophisticated structural system. This reduced the weight of the roof, as did the elimination of the apex by means of the oculus.
  The top of the rotunda wall features a series of brick relieving arches, visible on the outside and built into the mass of the brickwork. The Pantheon is full of such devices – for example, there are relieving arches over the recesses inside – but all these arches were hidden by marble facing on the interior and possibly by stone revetment or stucco on the exterior.
  The height to the oculus and the diameter of the interior circle are the same, 43.3 metres (142 ft), so the whole interior would fit exactly within a cube (or, a 43.3-m sphere could fit within the interior). These dimensions make more sense when expressed in ancient Roman units of measurement: The dome spans 150 Roman feet; the oculus is 30 Roman feet in diameter; the doorway is 40 Roman feet high. The Pantheon still holds the record for the world's largest unreinforced concrete dome. It is also substantially larger than earlier domes. It is the only masonry dome to not require reinforcement. All other extant ancient domes were either designed with tie-rods, chains and banding or have been retrofitted with such devices to prevent collapse.
  Though often drawn as a free-standing building, there was a building at its rear which abutted it. While this building helped buttress the rotunda, there was no interior passage from one to the other.
caption: Beam in the dome of the Pantheon
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