Tuesday, September 21, 2010

Handily Stepping Out

Beehive Hut in Washington

Dry Stone Cabane in Ontario

Corbelling is a structural method of jutting stones or bricks out from a wall in ascending courses to support more and more material above. Each stone is placed so that the bulk of the stone is deeper into the wall than the part that is projecting. In this way a lot of weight of the building can be suspended over an open space.

In the case of dry stone huts, such as the beehive recently built in Washington and the cabane in Ontario, corbelling the stones towards the centre of the interior space defined by the circular walls eventually creates an enclosed stone ceiling and support for a roof. There are various methods and formulas for doing this. Obviously stepping out too soon or too far each course can make the structure unstable. Stepping out too slowly involves using a lot more material than necessary and makes the structure disproportionately tall.

The beehive corbeling involved long stones straddling the inside angle of another pair of corbelled stones below along the circumference of the opening.The outer corners of the stones are fully supported and it is only the middle overhanging 'delta' of the stone that projects out.

The corbelled stones of the Canadian cabane, on the other hand, have edges (and corners) that project unsupported but with more of their interior length acting as a counter-weight with wall material simultaneously placed on top so as to weigh down each course of corbelling below. The inside corners of these stones touch in a circle at the interior of the structure and the stones slant out and downwards into the wall so that any tendency for the corbel stones to slump is counteracted since each circular course of corbels would have to spread (and the open space get larger) before any stones could begin to drop.

In both methods of construction a functional roof can be created without having either to build a true arched vault or having to use wooden beams.

It is fascinating to see stones used in ways that their weight and mass seem to contradict the force of gravity.