Centering, the framework upon which an arch or vault of stone, brick, or iron is supported during its construction. The simplest form of centering is that used by masons and bricklayers for the arches of common windows and doors. This is merely a deal-board of the required shape, upon the curved edge of which the bricks or stones of the arch are supported until they are keyed in. In building bridges or other structures where arches of great span are to be constructed, the centering is usually made of framed timbers, or timbers and iron combined. The arrangement of the timbers should be such that the strain upon each shall be mainly a thrust in the direction of its length, for if the strain were transverse, a comparatively slight force would snap it, and if a longitudinal pull, the whole structure would be no stronger than the joints holding the pieces of timber together. In arches of great span, a longitudinal pulling strain is almost inevitable in some parts, as a beam of great length would bend to some extent under a thrusting strain. In such cases great skill and care are demanded in the designing and construction of the joints. As an arch is built from the piers towards the keystone, the weight upon the haunches during construction tends to push the crown upwards, and therefore the problem of designing a framed centering involves the resistance of this tendency, as well as the supporting of the weight of the materials. Occasionally, when a very great span is required, and the navigation will permit, piers are built on the bed of the river, or piles are driven into it, to support the centering directly, simplifying it, and at the same time facilitating a more rigid disposition than in centering supported only from the sides. See article BRIDGE for descriptions and illustrations of three types of centering: (1) that for the bridge over the Dee at Chester, Vol. II., page 437, supported directly from the bed of the river; also the centering for the Ballochmyle Bridge; (2) inclined struts in pairs supported from the sides, as Rennie's centering for Waterloo Bridge, page 438; (3) trussed wooden girders supported from the sides, as Rennie's centering for London Bridge, page 438.
Cupolas, like those of the Pantheon and St Peter's at Rome, St Paul's in London, or the flat domes of the Turkish mosques, require very effective centerings.