Rules · Walls · Materials · Structure
Build a Roman wall as two facings of brick or small stone with a core of concrete poured between them in layers; tie the facings into the core with bonding courses of large tiles laid through the whole thickness at intervals.
bonding courses of bipedales (2 ft tiles) every 1–1.5 m of height
The facings served as permanent formwork: courses were raised a little at a time and the space between them filled with mortar and caementa. Vitruvius distrusts the method when the core is thrown in carelessly, because then the wall becomes three parts that do not act together. The imperial builders answered his objection with the bonding courses of large tiles that cross the whole wall, and with brick facings (opus testaceum) of triangular bricks whose points lock into the core.
Brick-faced concrete was the fabric of imperial Rome: of Ostia’s apartment blocks, the Markets of Trajan, the baths and the Pantheon itself. It was cheap, fast and fire-proof, and it could be shaped into any plan.
In a bonded masonry wall the courses above an opening corbel over it of their own accord, so the lintel carries roughly the triangle of wall with sides rising at 60° from its ends; everything above that triangle arches onto the jambs.
load ≈ equilateral triangle on the span: W ≈ 0.43 · L² · t · ρ g
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.
Cite this page Chris Paton, “Facing and core”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/facing-and-core. Text, drawings and models licensed under CC BY-NC 4.0.