Does the Middle Stone Hold the Arch Up? Not in the Way You Think

An arch turns a load pressing down into forces pushing outward along its curve, so every stone is squeezed against its neighbours and the whole ring is in compression. Each stone is cut as a wedge, wider on the outside than the inside, so that it cannot fall inward through the ring, and the forces travel down through the wedges to the supports at each side, which must resist a substantial outward push. The material never needs to resist being pulled apart, which is why an arch can be built from stone and brick, neither of which is any good in tension.

Its significance is procedural rather than structural:

• •It is the last wedge placed, which completes the ring
• •Until it is in, the two halves are held up by a timber frame
• •Placing it lets that frame be removed and the arch stand alone
• •It is therefore the visible moment the structure becomes real
• •It is the most conspicuous stone and is frequently carved or dated
• •Structurally it does the same job as every other wedge

The common claim that removing the top stone collapses the arch while removing another does not is not right, and the truth is more interesting. The ring works as a whole and removing any wedge from a simple arch is serious, since the compression path is broken wherever the gap is. Real arches are usually more forgiving than that, because thickness and the weight of masonry above allow the line of force to shift within the structure, so an arch can survive losing material, and many standing ruins have. What actually causes failure is the supports spreading apart, which lets the ring flatten and hinge, and that is how most arch collapses happen.

The curve chosen changes how the forces run and what the arch demands of its supports. A semicircular Roman arch pushes outward strongly and needs heavy masonry at the sides to contain it. A pointed Gothic arch directs more of the force downward, which allows thinner walls and taller buildings and is the structural reason for the style's appearance. A segmental arch, a shallow slice of a circle, is flatter and pushes outward hardest of all, which suits bridges with solid abutments. A flat arch is built from wedges arranged in a straight line and works, precariously, on the same principle.

Taking down the timber frame that supported the arch during building is the critical moment and was managed carefully. Lowering it too fast drops the arch onto its own supports as a shock rather than letting it settle, and lowering it unevenly loads one side first. Builders therefore used wedges or sand boxes under the frame, which could be released gradually and evenly, letting the arch take its load slowly while the masonry settled and the mortar adjusted. An arch settles measurably as it takes up its load, dropping at the crown and spreading slightly at the springing, and allowing for that movement is part of the design rather than a defect.

Wedge-shaped stones squeezed against each other turn a downward load into compression running around the ring and an outward push at the supports, which is why stone works in an arch. The top stone is the last one placed and lets the supporting timber frame come out, which is why it is conspicuous and decorated, and structurally it does what every other wedge does. Arches fail when their supports spread.