How did Hagia Sophia's dome seem to float, and why did it fall?
A brick dome 32.6 metres wide and about 61 centimetres thick, ringed with windows, was finished about five years after work began, and part of its vaulting fell some twenty years later.
▶ Start the storyHagia Sophia's dome seems to float because a ring of forty windows sits around its base, and it fell because it was too flat for the load it put on its piers. The great dome is 32.6 metres in diameter and only 0.61 metres thick. The light reflecting around the nave gives the dome the appearance of hovering, an effect achieved by inserting forty windows around the base, which also reduced its weight.
0.61 m
The key device is the pendentive. A pendentive lets a circular dome sit over a square room: triangular segments of a sphere, tapering to points below and spreading at the top into a circular base, concentrate the dome's weight at the four corners where piers receive it. Hagia Sophia, completed in AD 537, was among the first buildings with a fully pendentive dome, and for a long time its dome was the largest pendentive dome in the world.
Then why did it fall? Earthquakes in 553 and 557 cracked the dome and eastern semi-dome, and in 558, according to the chronicle of John Malalas, the eastern semi-dome collapsed. The collapse is attributed mainly to the excessive bearing load and the enormous shear load of a dome that was too flat, which deformed the piers. Mortar also played a part: the builders did not let it cure, and the weight of the dome made the walls lean outward over the wet mortar.
Justinian ordered a rebuild with lighter materials by Isidorus the Younger. The vault was taken down and rebuilt about 6.25 metres higher, which gave the building its current interior height of 55.6 metres, and it was given a ribbed shape that lets the weight flow down between the windows.
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Recap
The dome seems to float because its weight is gathered at four corners by pendentives and its base is ringed by windows, and it fell once because it was too flat.
💡 A trick to remember it · Four sails catch the dome's weight, and forty windows make it float.
Surprising fact · Its dome is 32.6 metres wide and only 0.61 metres thick.
Connects to
- 🕌 How did a Janissary engineer become the Ottomans' chief architect?
- ⛪ How did a ribbed stone ceiling help Gothic walls turn into windows?
- ⛪ How did Brunelleschi build Florence's giant dome without a wooden frame?
- 🌉 How can a pile of loose stones stand up as an arch?
- 🏛️ How has the Pantheon's concrete dome survived for nearly 2,000 years?
- Byzantine iconoclasm
- Pointed arch
Sources (2)
No source, no claim. Every fact in this lesson (16 claims) cites at least one of these.