How it began
Hagia Sophia was built in Constantinople, now Istanbul, for Emperor Justinian I. Work began 39 days after the Nika riots of 532 destroyed the earlier church and took less than six years. The designers were Anthemius, an architect and mathematician, and Isidorus, a professor of physics and stereometry. Some 10,000 men worked on it in two competing teams. Encyclopaedia Britannica calls Hagia Sophia, completed in 537, one of the earliest examples of the use of the pendentive and also one of the largest.
Before it existed
Setting a round dome over a square or many-sided room was a growing concern for Roman builders of the late empire, but according to Britannica it was left to Byzantine architects to see what the pendentive could do and to develop it fully. The two churches that had stood on the site were of an older kind. The first, finished in 360, was a basilica with a wooden roof that burned in 404. The second, rebuilt in 405, stood until 532.
How it works
A pendentive is a triangular piece of a spherical surface that fills an upper corner of a square room. Four of them rise from four piers, their sides join into arches over the four faces of the space, and their upper edges meet in a full circle on which the dome sits. They carry the weight of the dome down to the piers. At Hagia Sophia the first dome was 120.5 feet across and rose about 182.5 feet above the floor, built of brick lighter than stone or concrete. Half domes brace the east and west arches.
Who pushed back
The first dome did not last. An earthquake in 553 weakened the eastern arch and another in 557 split it, and in 558 part of the eastern half dome and much of the main dome fell. The original dome had been too shallow, which pushed load outward onto the walls. Isidorus the Younger, nephew of the first Isidorus, rebuilt it by 562. He raised the crown by 20.5 feet to reduce the sideways thrust, added 40 ribs and ringed the base with windows.
What lasted
The building defined Byzantine architecture and over time influenced Islamic architecture. In the Renaissance and the Baroque the taste for domed churches, above all in Roman Catholic Europe and Latin America, gave the pendentive great importance. The structure itself needed constant care: the Armenian architect Trdat repaired the dome after tenth century earthquakes, the Ottoman architect Sinan added buttresses, and the Fossati brothers bound the dome with iron chains between 1847 and 1849.
Where it stands today
Hagia Sophia still stands in Istanbul under the dome that Isidorus the Younger completed in 562, as later repaired. After a period as a museum it was converted back to a mosque by the Turkish president Recep Tayyip Erdoğan. The American Society of Civil Engineers described it in 2022 as a building that has endured all that 1,500 years of history could throw at it, including repeated earthquakes that brought down parts of its arches and domes.
