Permanent

Centering, although traditionally thought as a carpentry temporary structure, may be embedded in the final building structure, in what we call a permanent centering.

In the Temple of Diana, only six arches are built with a typical centering, probably reusing the same wood truss. These arches have a stone wide space between them, with a supporting dent in which a course of stone slabs was put in place. When the complete second set of stones created an arch, the weight was discharged in the wall instead of the first set of stones.

Instead of an interleaved permanent centering, a continuous form of permanent centering may be found in the baths of Sufetula in which a first barrel vault made of a different material (ceramic tubes) is built with a quick setting plaster and on top of this surface, the heavier voussoirs are put in a radial fashion thus creating the final arch (Adam, 2005).

In concrete construction, the concept of lost formwork is commonly used, and could be also applied to stereotomic construction. A centering made with a material easier to work with than stone would give support for these stones to be put in position, and should cater to their own weight. The added weight of people travelling, roof construction or others, would already be supported by the compressive discharge between stones. This concept is effectively illustrated in the “Viollet le Duc” Arch (Fallacara and Stigliano, 2012) where a metallic falsework efficiently dictates each voussoir’s position. The Flex Bridge stone arch visible in (Fallacara, 2016) only uses a top membrane attached to the blocks to keep their extrados common edges in position, so it depends on a perfect contact between voussoirs for the final shape of the work; this is also the case of the pre cast concrete voussoirs FlexiArch (Taylor et al., 2006) bridge system, which has been extensively applied in Ireland.