Chap. XXIII.

CENTERING.

1*103

of changing their form. The ends of the two inclined timbers in the upper part might beframed with bevelled shoulders, thus making them bear upon each other; the binding-pieces would continue to embrace them, and the whole being consolidated by bolts wouldbe more firm.

In forming the centre of a bridge, the weight should be equally borne throughout, andthe parts should be made sufficiently strong to support any portion or the whole of thepressure to which it is submitted ; wherever the timbers abut end to end, a socket of ironshould be used, as was done in the centre of Waterloo Bridge, London . The timbers shouldintersect one another as little as possible, and should never be halved or cut into, as eitherpractice causes a great loss of strength.

The Centres used at the Bridge of Orleans were not sufficiently strong as originallyprojected, and some braces were added to it in the course of the construction, extendingfrom one principal to another. The scantling of the timbers was about 15 inches square,

Fig. 2340.

CENTRES OP BRIDGE OP ORLEANS.

that of the braces 9 inches, and that of the needles 16 inches: these centres were successful,and proved of sufficient resistance, although not very much loaded with timber : they werecomposed of fewer pieces, which rendered them less liable to change form, which a weightof 100 tons was sufficient, when placed on their summit, to prevent.

I ; ig. 2341. represents the centre used at the bridge of Nemours on the Loing: in an arcso flattened, the whole arch bears equallybecomes very difficult: striking the centrewas in this instance effected by destroyingby degrees the extremities of the second andthird rows of inclined timbers, at the pointwhere they framed into the side timbers;the centre, being no longer sustained exceptby the upper row, sunk a little, after which itwas easily taken down. The centres employedin the construction of the bridges of Neuilly and Mantes have but little strength, notwith-standing the quantity of timber employedin them, each course of which may be con-sidered in such centres as an assemblage of several levers, united at their extremities byhinges or turning joints, and loaded with different weights; we know that the equilibriumof such a system depends on certain conditions, which are the same as in the polygon, sothat the length of the levers being given, their respective inclination depends on the weightswhich they support, and are functions determined by those weights : before the centre isloaded by the voussoirs, it is in a state of equilibrium ; but it is no longer so when thefirst voussoirs are laid, and the centre would crush if no attempt were made to establishthe equilibrium, by loading the upper part3 provisionally ; by the advancement of the work,the equilibrium is again broken, and fresh settlements arise, which would become dangerousif the weight on the top were not increased, and the arch completed as quickly as possible :thus at different periods the equilibrium is alternately broken and re-established, and thecentre is in a continual movement during the whole time of the construction.

Several engineers have believed that this movement of the centre during the placing ofthe voussoirs, far from injuring the construction of large arches, serves on the contrary toensure it. A distinction has been made between fixed and movable centres, and the pre-ference has often been given to the latter ; the only reason for this preference must arise fromthe idea that a movable centre yields to the settlement of the arch, which, with the com-

on the centre, and removing the wedges

Fig. 2341. CENTRE OP THE BRIDGE AT NEMOLRS.