Why an arch needs a keystone, and a wall does not
An arch is a mechanism until the last stone locks the ring. Here is what that means on site, and why we build ours over timber and leave them alone for a week.
People think an arch is strong because it is curved. It is not. A half-built arch is one of the weakest things on a site — a row of loose stones leaning on a bit of timber. What makes it strong is the moment you close the ring.
The ring has to close
Set the voussoirs — the wedge-shaped stones — from both springings inward, resting on the timber centring underneath. Until the keystone drives home at the crown, nothing is carrying anything. Take the centring out at that point and the whole thing folds into the opening.
Drive the keystone home and something changes completely. Now every stone is being squeezed by its neighbours, the load has a continuous path from the crown down to the abutments, and the arch holds itself up. That is why we leave the centring in for a week and strike it last. The keystone is not decoration. It is the switch.
The thrust line has to stay inside the ring
Closing the ring is necessary, but it is not enough. The load takes a particular path through the arch — the line of thrust — and that line has to stay inside the middle third of the ring the whole way round. Where it wanders outside, a joint opens on the far face and the arch develops a hinge. Enough hinges and it is a mechanism again.
This is why a shallow, deep-ringed arch stands happily and a tall, thin one cracks. It is not about how much stone you use. It is about whether the load can find a path that stays inside the stone you have.
A closed ring is not a promise. The thrust line has to agree.
A wall is simpler
A wall does not need a keystone because it never becomes a mechanism. Every course sits flat on the one below and the load runs straight down. You can stop building it half way up and it stands. That is the whole difference: a wall is stable as you go, an arch is only stable when it is finished.
It is also why we would rather build you an arch over a wide opening than hide a steel in the wall. Done right, the arch is the structure and the material, in one, and it will outlast the fixings on any beam. Done wrong, it is a pile of bricks over your head. The difference is knowing where the thrust line goes before you cut the first voussoir.
If you have an opening you want spanned properly, tell us about it.