Rules · Vaults · Structure · Façades
Build galleries (tribunes) over the aisles and cover them with quarter-circle (quadrant) vaults that lean against the nave wall just below the springing of its barrel, so that the gallery vaults carry the barrel’s thrust down to the outer walls.
The great churches on the roads to Santiago share a type: barrel-vaulted naves without clerestories, galleries over the aisles, transepts with aisles, and an ambulatory with radiating chapels so that pilgrims could circulate past the relics. The quadrant vault is an internal flying buttress, the same idea the Gothic masons later lifted above the roofs and into the open air.
Build an arch of wedge-shaped stones (voussoirs) whose joints all point towards the centre of the curve: each stone can then fall only by forcing its neighbours apart, and so the ring holds itself up once the last stone, the keystone, is in place.
joints ⟂ intrados (radial to the centre)
An arch pushes outward on its supports as well as down. For the same load and span the horizontal push grows as the rise falls: halve the rise and you roughly double the thrust.
H ≈ w·s² ⁄ 8f (uniform load w per metre, span s, rise f)
Trace the path of the resultant force through the stones from crown to foundations. If a line can be found that stays everywhere within the masonry, the arch will stand; where it touches an edge, a hinge will open, and four hinges make it collapse.
A round arch must rise half its span, so arches of different spans on one springing line have crowns at different heights. A pointed arch can be given any rise at least half its span, so arches of different widths can all be made to reach the same height.
round: f = s⁄2 (fixed) · pointed: r = (s²⁄4 + f²) ⁄ s for any f ≥ s⁄2
An arch cannot support itself until it is complete. Build it on a timber centring shaped to the intrados, strike the centring only when the keystone is set and the mortar has taken, and reuse it from bay to bay, which is one reason to keep the spans of an arcade equal.
In a row of arches the thrusts of neighbouring arches cancel over each intermediate pier, which may therefore be slender; at the ends of the row nothing balances them, so the outermost piers must be made wider or buttressed.
Cite this page Chris Paton, “Galleries that buttress the nave”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/rom-gallery-buttress. Text, drawings and models licensed under CC BY-NC 4.0.