Rules · Ornament · Arches · Materials
Alternate voussoirs of two colours (red brick and pale stone, or black and white stone) to make the structure of the arch legible as ornament.
At Córdoba the voussoirs of both tiers alternate red brick and cream limestone. The striping multiplies the rhythm of the arcades and, because every stone of the arch is shown, it lets the eye read how the arch is built. Syrian and Egyptian builders developed the idea into ablaq, courses and voussoirs of alternately dark and light stone, which became a hallmark of Mamluk Cairo and Damascus and passed into Romanesque Italy and Spain.
Draw a pointed arch from two centres on the springing line. With the centres at the springings, the arch is equilateral; with centres beyond them, a lancet; with centres between them, a drop arch. Late Gothic added arches of four centres and the reversed curve of the ogee.
r = s (equilateral) · r > s (lancet) · s⁄2 < r < s (drop)
Classify arches by the geometry of the intrados: the number and position of the centres (one for round and segmental, two for pointed, three or four for basket and Tudor arches), or a single continuous curve (ellipse, parabola, catenary), and distinguish true arches, with voussoirs, from corbelled “arches” of oversailing courses.
Where the available columns are too short for the height you want, stand a pier on each column and carry the roof on a second, upper tier of arches, bracing the piers with a lower tier of horseshoe arches.
height ≈ column + pier + upper arch; lower arch = brace
Carry the curve of the arch beyond the semicircle, so that the opening at the springing is narrower than its greatest width.
springing width = 2R cos β, where the arc continues β below the centre line
Frame an important arch with a rectangular moulding or band rising from its springing, so that the curved opening is set within a square field.
To draw a star pattern, cover the surface with a tiling of polygons, draw two lines through the midpoint of every edge at the same angle to it, continue each line until it meets another, and erase the tiling.
pattern = tiling + contact angle θ at every edge midpoint
Cite this page Chris Paton, “Striped voussoirs”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/ablaq-voussoirs. Text, drawings and models licensed under CC BY-NC 4.0.