Rules · Capitals · Columns · Structure
Crown the Persian column with two kneeling bulls (or lions, or griffins) set back to back; lay the main beam in the saddle between their necks, and the cross-beams on their heads.
The composite Persepolitan capital is built up of a bell base, a fluted shaft, a floral crown of upturned and drooping leaves, a block with vertical double volutes, and finally the animal protome. It combines motifs from the whole empire (Egyptian palm leaves, Ionian volutes and fluting, Mesopotamian bulls), as the Persian kings’ inscriptions boast that craftsmen came from every land.
The protome is not merely ornamental: the saddle positions the beam and the heads receive the joists, so the capital works as a bracket spreading the bearing.
Weight for weight, timber is many times stronger in bending than stone; long spans in trabeated buildings are therefore roofed with timber beams resting on stone walls and columns.
bending strength ÷ density: oak ≈ 70 · limestone ≈ 3 (kPa per kg ⁄ m³)
Where the roof is of timber, make the columns tall, slender (about twelve diameters) and widely spaced, and fill the hall with them on a square grid.
height ≈ 12 D · spacing ≈ 5 D (Apadana: ≈ 19 m, D ≈ 1.6 m, axes ≈ 8.7 m)
Throw an arch diagonally across each upper corner of the square, backed by a small conch or cone; the four squinches and the four walls between them make an octagon, on which a round dome can sit.
octagon side = S (√2 − 1) ≈ 0.414 S; squinch span = octagon side
Dig the foundations down to bedrock and fill them with rubble; then build with the best material of every province, each worked by the people who know it best.
Make the hall square and fill it with columns on a square grid; put a portico two columns deep across the front, between solid corner towers; build the walls of mud brick and keep stone for columns, door and window frames.
Cite this page Chris Paton, “Beams in the saddle of the double bull”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/double-bull-capital. Text, drawings and models licensed under CC BY-NC 4.0.