Rules · Walls · Structure · Light
Carry vaults on walls thick enough to absorb their thrust within their own mass; keep the windows small and set them low, below the springing of the vault, and strengthen the walls outside with shallow pilaster buttresses.
walls commonly 1½–2 m thick; buttresses shallow, window openings narrow and splayed
Romanesque structure is the structure of mass. The thrust of a barrel or groin vault is resisted by the weight and width of the wall, not by a separate system of buttresses; openings weaken the wall, so they are few, and splayed inward to spread what light they admit. The result is the characteristic Romanesque interior: sober, heavy, dim, its light falling in shafts.
Many Romanesque walls are not solid: two faces of dressed stone enclose a rubble core. Their strength depended on their thickness and on the quality of the core, and failures were common when builders ventured higher.
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.
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.
Over a doorway or window spanned by a lintel, turn an arch in the wall above so that the weight of the masonry is carried round the opening onto the jambs, leaving the lintel to carry only the small triangle of wall beneath the arch.
Cite this page Chris Paton, “Mass resists thrust”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/rom-thick-walls. Text, drawings and models licensed under CC BY-NC 4.0.