Rules · Domes · Plans · Structure
To make the domed space as nearly one room as possible, set the dome on eight piers inscribed in a square, and fill the four corners with semidomes on the diagonals.
In the Selimiye, which he called his masterpiece, Sinan raised a dome of about 31 metres diameter (comparable to Hagia Sophia’s, a comparison he deliberately invited) on eight piers standing close to the outer walls. The piers are slender and the space between them almost uninterrupted; the thrusts are taken outward by buttresses that are partly hidden in the walls. The octagonal baldachin had been tried at smaller scale before; the hexagonal variant appears in mosques such as that of Sokollu Mehmed Pasha at Kadırga (1571–72).
To set a round dome on a square bay, imagine a sphere passing through the four corners of the square; cut it by the four vertical planes of the sides and by the horizontal plane of their crowns. The four spherical triangles left in the corners are the pendentives; the circle at their top carries the dome.
sphere radius R = S⁄√2; arches = sphere ∩ side planes (radius S⁄2); ring at h₀ + S⁄2 with radius S⁄2
Pierce the base of the dome with a close ring of windows between buttressing ribs, so that the shell seems to float on light rather than to rest on the arches.
Hagia Sophia: 40 windows and 40 ribs; dome ≈ 31 m across, crown 55.6 m above the floor
To turn a domed square into a long church, open two of its great arches into semidomes of the same radius, and open the semidomes in turn into smaller exedrae, so that the space expands by stages from the central dome.
Hagia Sophia: central square 31 m; two semidomes of radius ≈ 15.5 m; four exedrae; interior ≈ 31 × 70 m
Orient the prayer hall so that its principal wall, the qibla wall, stands square to the direction of Mecca, and mark the middle of that wall with a niche, the mihrab.
qibla wall ⟂ direction of the Kaaba; mihrab on the axis
Fill a half-dome, squinch or cornice with tiers of small concave cells, each tier corbelled forward from the one below and each cell set over the junction of two cells beneath.
height = tiers × module; each tier projects inward
Cover the prayer hall with a single central dome on four piers and extend the unified space by semidomes on two or all four sides, with smaller half-domes (exedrae) in their corners.
Cite this page Chris Paton, “The dome on eight piers”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/sinan-octagon. Text, drawings and models licensed under CC BY-NC 4.0.