Groin vault

A groin vault, or cross vault, is formed where two barrel vaults of the same height intersect at right angles. Along the lines of intersection, sharp diagonal edges called groins run from corner to corner. It covers square or nearly square bays, and a row of them can form a long space.

How the load travels

What matters most about a groin vault is how it distributes load. A barrel vault passes its load continuously along a wall; a groin vault gathers it along the groins into the four corners. The walls on each side are largely freed from bearing duty and can be opened up, and the vault rests only on piers or columns at the corners. That gives great freedom for wide openings, windows and passages. In return, the corners carry concentrated vertical load and diagonal thrust, which neighbouring vaults, buttresses or tie rods must resist.

• Groins: the lines where the two vaults intersect
• Webs or cells: the four vault surfaces between the groins
• Corner supports: the piers or columns that collect the load

Groin vault versus barrel vault

A barrel vault needs long, solid walls; a groin vault can leave all four sides open. That is why it is common in arcades, ground-floor rooms and long halls divided into bays. A rib vault can be seen as the same system with its groins expressed as stone ribs.

Survey and 3D modeling

In 3D a groin vault can be built by intersecting two cylinders. In historic buildings, though, the groins are rarely straight and the webs rarely share the same curvature. In survey, record groin heights and corner support positions together.