What is a centering segment (schenkel)?
A centering segment (Dutch: schenkel) is a curved component of an arch centre. The centre, or centering, is the temporary timber support on which an arch or vault is built in masonry; once the mortar is strong enough, it is removed. The segment is a short curved assembly of two or more boards sawn to a curve, sometimes thickened with blocks between them.
Unlike a complete centre, a segment covers only part of the curve. In shape it resembles a rib from a ribcage. By joining several segments end to end or setting them side by side, you create a complete centre or a support surface for a vault.
The same word appears in the Dutch schenkelspant, better known as a Philibert truss: a roof truss built up from curved timber pieces.
Application
Centering segments are used when building:
- segmental and round arches over door and window openings;
- vaults, where several segments carry the curve;
- curved walls, where a segment or profile serves as a template to keep the curve true.
Tips:
- Set out full size: draw the inside line of the arch at full scale on a board, for example with a batten and nail as a compass, and cut the boards exactly to this line.
- Build it strong: the segment must carry the weight of wet masonry without sagging. Join the boards with blocks and prop the segment with supports.
- Easy striking: set the segment on folding wedges. Tapping the wedges out lowers the centre slightly so you can remove it without jolting the arch.
- Be patient: only strike the centre once the mortar has gained enough strength. Striking too early is a common cause of cracked arches.
Related Terms
- Arch centering
- Round arch
- Relieving arch
- Vault
- Philibert truss
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