Centering

Centering, the frame or mould used in the construction of arches, vaults, groins, cupolas, etc., for the support of the stone, brick, or iron during the course of their formation. The design for the centering of bridge-arches is of great importance. If the passage through the arch is not required during its building, temporary piers may be built to support the centering, considerably facilitating its simple construction. But frequently piers are undesirable, and it becomes necessary to leave head-room above the springing of the arches. Then the design is difficult, especially for long spans. The beams must be arranged, so that the stresses in them are for the most part compressive. The structure must be able to support the incomplete arch, and must neutralise the tendency for the deadweight at the haunches to force the crown upwards. On the completion of the arch the centres are struck, or lowered a little by the removal of certain key-plates. It may then be seen whether the arch is sufficiently strong to hold itself; the sinking at the crown should not exceed 3 or 4 inches. Brick arches should not be struck till the mortar is well hardened.