What is a scissor arch?
A scissor arch is a compound arch consisting of two pointed arches, one upright and the other mirrored, upside down, resting on top of it. Seen from the side, the shape resembles an open pair of scissors or an hourglass, with two circular openings on either side in the middle. They are also called strainer arches: arches that resist sideways forces.
The best-known example: Wells
The most famous scissor arches stand beneath the crossing of Wells Cathedral in Somerset, England. After the crossing tower was heightened in the fourteenth century, the piers beneath it began to subside and crack. From 1338, master mason William Joy built these large scissor arches between the piers on three sides of the crossing. They hold the piers in place and spread the load of the tower. They still stand today and are one of the best-known images of English Gothic.
How it works
- The lower arch carries load and braces itself against the piers.
- The inverted arch above it holds the piers apart at height, so that they cannot tilt inwards.
- Together they act as a rigid strut between two supports: roughly what a brace or tie rod does in a modern structure, but in masonry and working in compression.
Applications
Scissor arches are rare and mainly a solution to a structural problem after the event, as at Wells. The principle behind them does recur more often: where walls are subsiding or piers are spreading, a connection is made between two supports, in historic buildings with arches or buttresses and in modern restoration with steel tie rods or braces. Cracks in piers and vaults beneath a tower always call for structural advice.
Related terms
- Crossing (architecture)
- Gothic architecture
- Flying buttress
- Round arch
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