Arches are generally built over a centring made to the shape of the arch needed. The centring is a temporary structure which has to support the arch till it develops strengths. For small spans, mud masonry is used. The masonry is made to the shape of the soffit of the arch to be built and plastered, over which the arch is constructed. After attainment of adequate strength, the masonry can be dismantled.
Timber centring is preferred generally as it is easier to erect and dismantle. Further, the material can be re-used. Thick wooden planks are used to get the shape of the soffit and are supported by vertical timber posts. Wedges are used to tighten or loosen the centring.
For heavy arches, the shape is got by suitable braces and struts. In such cases it is necessary to support the posts on boxes containing sand. A hole is kept plugged into the box. The post sinks slowly when the plug is removed and the sand flows out, thereby relieving the arch centring without any shock.
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