Centering

Part of Bridges

Designing and building the temporary timber framework that supports an arch during construction.

Why This Matters

An arch cannot support itself until it is complete. Every stone or brick placed during construction is an independent block with nothing but friction holding it in place — and friction is not enough to resist the forces involved. The centering is the temporary wooden framework that supports every voussoir from the first placed to the last, holding the arch curve precisely until the keystone is driven home and the arch becomes a self-supporting compression ring.

Centering failure during arch construction is catastrophic. The entire partly-built arch collapses, workers on and around it are at severe risk, and materials may be damaged beyond reuse. This is not a component to improvise. A properly designed centering is as important an engineering task as the arch itself — perhaps more so, because it must be designed, built, and removed safely under the constraint that the arch it supports cannot be wrong.

Centering is temporary, but it must be built to a high standard. The Roman engineers who built the Pont du Gard and similar structures spent enormous effort on centering. Their arches still stand two millennia later partly because they were built on accurate, solid centering that held the voussoirs in exactly the right position while the mortar cured.

Geometry: Matching the Arch Curve

The primary function of centering is to hold the intrados (inner face) of the arch in the correct curve. Any error in the centering curve becomes an error in the arch — permanent, locked into stone.

Laying out the curve. For a semicircular arch, the centering edge follows a half-circle. Establish the center point and radius, and use a string compass (a pin, a string, and a pencil) to mark the curve on a flat surface. For segmental arches, calculate the radius using the span and rise relationship (R = S²/8r + r/2) and lay it out the same way.

Building curved ribs. Centering ribs are the curved members that define the arch shape. Options:

• Sawn curved ribs: If you have a sawpit and good timber, ribs can be sawn to the required curve from wide planks. This wastes timber but produces a strong, accurate rib.
• Laminated ribs: Thin strips of flexible timber (20–30 mm thick) bent around a template and spiked or pegged together. Multiple thin layers follow the curve more naturally than a single thick member. Use 3–5 layers for a rib 100–150 mm total thickness.
• Built-up truss ribs: For large spans, the centering rib is itself a small truss or king-post structure, curved to match the arch. More complex to build but very strong and economical in material.

The outer edge of the centering ribs must match the intrados curve to within ±5 mm. Check with your string compass before proceeding.

Centering Structure Design

A centering for a modest arch (span 3–6 m) consists of:

Ribs: Typically two to four ribs across the arch width, spaced to support the lagging. Each rib spans the full width of the arch opening from spring to spring.

Lagging: Longitudinal planks or poles running parallel to the arch axis (along the bridge length), laid across the ribs to create a continuous bearing surface for the voussoirs. Lagging should be close-spaced — gaps of 50–100 mm are acceptable for large voussoirs, but smaller stones require tighter lagging or continuous planking.

Framing: The ribs are supported by a timber framing system that transfers load to the ground, river bed, or temporary piers. This can be:

• A simple A-frame or tripod at each rib
• A series of posts bearing on timber sill beams placed across the river bed
• Cantilevered from the abutments if they are built first and are strong enough

Diagonal bracing: The centering must not rack sideways. Install diagonal bracing between adjacent rib frames in the horizontal plane. Without bracing, a centering can twist and collapse under the asymmetric loading of voussoirs placed from one side.

Striking the Centering

Striking — removing the centering — must be done carefully to avoid damaging the newly completed arch. The arch mortar must be adequately cured first. Minimum cure times:

• Lime mortar: 14–21 days in warm weather, longer in cold
• Natural cement or pozzolanic mortar: 7–14 days
• Portland cement: 7 days minimum (this was not available in traditional construction but may be in a rebuilding context)

Gradual lowering. The centering should be lowered gradually, not suddenly pulled away. The arch needs to undergo elastic settling — slight compression of mortar joints as load transfers from centering to arch structure. A sudden removal would cause a momentary dynamic loading.

The traditional method uses sand boxes or wedge systems. Sand-filled timber boxes sit beneath the centering frame. Allowing sand to drain slowly out of a hole lowers the centering gradually and uniformly. Alternatively, wedge pairs beneath each support are driven back slightly in rotation around all supports, each time removing a few millimeters.

Order of striking. Lower both sides simultaneously and evenly. Begin at the crown and work toward the springings. Monitor the arch intrados throughout for cracking.

What to watch for. Minor hairline cracks in mortar joints at the crown (intrados) after striking are normal — the arch is settling into its working position. Significant cracking — especially cracks that penetrate the full ring thickness, or opening at the springings — indicates a problem with arch geometry, mortar strength, or premature loading.

Reuse and Efficiency

Good centering can be reused multiple times. If you are building several arch spans in sequence (a multi-arch bridge), design the centering to be dismantled and reassembled in the next opening. The curved ribs are the valuable part — protect them from damage during striking and store them dry.

For a single arch, the centering timber becomes available for other purposes after striking. Plan this ahead — the centering materials are part of the project resource budget. Laminated ribs may be worth delaminating and reusing the individual strips; sawn ribs make good structural timber for building construction.

Improvised Centering for Emergency Crossings

For an urgent crossing that cannot wait for proper centering construction, a packed-earth or sand embankment can serve as improvised centering for small arches (under 2 m span). Build an earthen arch form, lay the masonry directly on the compacted earth, and then excavate the earth out from inside after curing.

This works because the earth is essentially a mold rather than a structural support. The arch must be thick enough and the masonry carefully laid so it is self-supporting before any settlement occurs. This method is used in some traditional vernacular construction for small culverts and doorway arches. It is not appropriate for larger spans where accurate geometry is critical.