Centering is the temporary structure required to support brick, stone or concrete arch during its construction, till it has gained sufficient strength. The centering is installed in such a way that its upper surface corresponds with the intrados of the arch. For minor works, centering may be made of mud masonry constructed to match with the inner soffit of the arch, and then plastered. This masonry is dismantled later when the arch has been constructed and cured.
The usual centering is made of timber or steel. Wooden centering is the simplest and cheapest, used for moderate span. It is easy to construct and easy to dismantle and it can be used several times. A plank with horizontal bottom and the upper surface shaped to the underside of the soffit is known as centre or turning piece. Its width is normally 10 cm, and is supported on vertical timber posts called props, with wooden wedges to tighten or loosen the centering.
If the soffit is wider than 10 cm, two ribs, suitably spaced and suitably shaped at the top may be used. These ribs may be connected by 4 x 2 cm wooden sections called laggings. At the ends, the ribs are supported by bearers, wedges and posts as shown in figure 20 below.
For wider soffits, and for larger spans, a built up centering of cut wood ribs is used. The upper surface of the ribs is given the shape of the soffit of the arch. Laggings (or cross – battens) are nailed across the ribs at close intervals to support the voussoirs at its top. Ribs are kept 25 to 40 mm thick, with width varying from 20 to 30 cm. The distance between ribs depends upon the thickness of the wall supporting the arch. The ribs are connected by braces and struts to strengthen them. Horizontal ties are provided at the lower ends of the ribs to prevent them from spreading. The ribs are supported on bearers, and a pair of folding wedges is provided at the top of each prop to tighten or loosen the centering.
The centering is installed in such a way that its upper surface corresponds with the intrados of the arch. For minor works, centering may be made of mud masonry constructed to match with the inner soffit of the arch, and then plastered. This masonry is dismantled later when the arch has been constructed and cured.
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