Rules · Façades · Domes · Structure
Let the exterior show the structure: pile semidomes, exedrae, corner domes and weight towers round the central dome so that the masses rise in steps to the crown.
Seen from the Golden Horn, an imperial mosque is a pyramid of lead-covered domes. The composition is not decoration: each lower dome or semidome buttresses the one above, and the turrets over the main piers load them to steady the thrust of the dome. The ring of windows at the base of the dome, separated by small buttresses, makes the dome seem to float inside and to sit crisply on its drum outside.
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
Build the tower for the call to prayer in the form of the regional tradition: square and stepped in the Maghreb and Spain, cylindrical and tapering in Iran and Central Asia, needle-like and polygonal in the Ottoman lands.
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 cascade of domes”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/ottoman-cascade. Text, drawings and models licensed under CC BY-NC 4.0.