CENTRING (from centre, Fr. centre, from Lat. centrum, Gk. Kerrpov, kentron. centre, froth kellteill, to prick). The framework upon which an arch or vault of stone, brick, or iron is supported during its construction. The simplest form of centring is that used by ma sons and bricklayers for the arches of and doors. which consist of boards cut to the required shape. upon the curved edge of which the bricks or stones of the arch are supported mild they are keyed in. In building bridges or other structures where arches of great span are to be constructed, the centring is usually made of framed timbers„ or timbers and iron Nan bined. The arrangement of the timbers should be such that the strain upon each shall lie main ly a thrust in the direction of its length, for if the strain were a comparatively slight force W0111‘1 snap Or bend it. and if a longitu dinal pull, the whole structure would be no stronger than the joints holding, the pieces of timber together. In arches of great span. such as that of Waterloo Bridge. London. the Cabin ;John Bridge. Washington. or the Luxembourg );ridge ( see Manner(. a longitudinal pull ing strain is almost inevitable in some parts. as a beam of great 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 joint-. As an arch is built from the piers toward the keystone, the weight upon the haunches during construction tend- to push the crown upward, and therefore the prob m of designing a framed centring involves the resistance of this tendency, as well as the support ing of the weight of the materials.
The centring of Waterloo Bridge, over the 1 Names at. London. designed by Rennie, pre sents a line example of the fulfillment of these requirements. The weight is resisted by direct thrust upon beams passing obliquely down ward from various parts; one of each pair of these oblique beams thrusts outward, and is directly supported by the abutments; the other thrusts inward, toward a support equidistant from the almtments, the yielding of which is prevented by the longitudinal pull of lower and longer oblique beams. in this and other modern structures cast-iron shoes have been successfully used for the tying joints subject to the longi tudinal pulling strain. The flexible centring. so called from its yielding at the joints, and varying its form with the load put upon it, is now aban doned. It was chiefly used by French engineers. That of Pcrronet fur the bridge of Neuilly is a celebrated example. Occasionally, when a very great span is required, and the navigation will permit. piers are built or piles are driven to support the centring, and the design is much simplified thereby. Cupolas like the Pantheon and Saint Peter's in Rome. Saint Paul's in Lon don, or the flat domes of the Turkish mosques, require very effective eentrings. See BRIDGE BUILDING; TUaNEL.