Vaults: barrel to fan
Nine ways to roof a bay in stone, from the Roman barrel and groin to the ribbed, tierceron, lierne and fan vaults of the Gothic masons.
The rules it illustrates
Break a corridor into bays. Keep a corridor’s height to no more than one and a half times its width, and divide its length into bays about as long as it is wide, marked by pilasters and by the divisions of the ceiling or vault.
A barrel vault needs a continuous wall. A barrel (tunnel) vault is an arch extruded along its axis. It thrusts outward all along both sides, so it must rest on continuous thick walls, which can be pierced only sparingly, and only below its springing.
Two barrels crossed gather their thrust at the corners. Intersect two barrel vaults of equal height at right angles: the result is a groin vault, whose weight and thrust are gathered along the diagonal groins to the four corners, leaving the sides free for openings.
The cloister vault: the groin turned inside out. Where the groin vault is the union of two barrels, the cloister (domical) vault is their intersection: four curved cells rise from all four walls and meet in valleys along the diagonals. It needs walls on every side.
The problem of the oblong bay. Two semicircular barrels of different spans rise to different heights. To groin-vault an oblong bay the narrower barrel must be stilted or flattened, and the groins then wander out of plane: the difficulty that pushed builders towards ribs and pointed arches.
Ribs first, webs after. Build the arches of the vault first (transverse, wall and diagonal ribs) on light centring, then fill the triangular cells between them with thin webs of stone laid course by course from rib to rib.
Level crowns over an oblong bay. Over an oblong bay, make the diagonal ribs semicircular and strike the transverse and wall arches as pointed arches of the heights needed to bring every crown nearly level with the central boss.
The sexpartite vault spans a double bay. Cover two bays of the nave with a single square vault divided into six cells by its diagonals and an extra transverse rib through the centre. The main piers carry three ribs each, the intermediate piers only one, so the supports may alternate strong and weak.
Where it appears in the book
In the styles Romanesque, Gothic.
In the chapters Vaults.
The terms it shows
Barrel vault · Fan vault · Groin vault · Lierne · Quadripartite vault · Rib vault · Sexpartite vault · Tierceron