:1 is the celebrated centre used at Blackfriars Bridge. The names of the timbers are as follows : A. Timbers which support the centering.
B, C. Upper and lower striking-plates, cased with copper.
D. Wedge between striking-plates, for lowering the centre.
E. Double trussing-pieces, to confine braces, F Apron-pieces, to strengthen rib of centre.
o. laid on the back of the ribs.
11. Blocks bridgings, to keep them at equal dis tances.
1. Small braces, to confine the ribs tight.
x. Iron straps bolted to trussing-pieces and apron-pieces. L. Ends of beam at the feet of truss-pieces.
m. Principal braces.
The centre used at Westminster Bridge was formed by independent trusses, consisting of two rafters; the intersec tions all supposed to be halved together, and firmly strapped across the notchings. Double truss-pieces were also used, but for these there was evidently no occasion, as the pres sure would be directed to the abutments, or to two opposite points of the arch in the same level.
The annexed plate is a perspective view of the centering of one of the arches of Waterloo Bridge. This magnificent bridge was built under the direction of the late Mr. John Rennie, and is a noble specimen of simplicity of design, skil ful arrangement, and solidity of execution. The centre was composed of eight frames or trusses, and. though sotnewhat complicated, was on the whole a judicious combination ; ex hibiting rather an excess than a deficiency if' strength.
In the erection of Chester Bridge, finished in IS.";?. an entirely different principle was adopted in the construction and the mode of relieving the centre ; it is thus described in the Transactions of the Institution of Civil Engineers. Vol. 1. :— The centre on which the stupendous arch of Chester new bridge was raised, and %%filch is stated by Mr. Hartley, (the engineer of the bridge.) to have been exclusively designed by Mm'. Trubsbaw, claims a detailed notice, from the novelty of the principle it was formed on, the efficiency with which it did its work, and the economy that attended its use. The centre consisted of six ribs in width, and the span of the arch was divided into four spaces by means of three nearly equi distant piers of stone built in the river, from which the tim bers spread fan-like towards the soffit, so as to take their load endwise. The lower extremities of these radiating beams rested in cast-iron shoe-plates on the tops of the piers, and the upper ends were bound together by two thicknesses of 4-inch planking bending round, as nearly as they could be made, in the true curve of the arch. ( )n the rim thus
the lugging, or covering, which was 41 inches thick, was supported over each rib by a pair id' fidding wedges, 15 or 16 inches long, by 10 or 1:1 inches broad, and tapering about 11- inch; for every course of arch-stones in the brid:re, there were therelore six has of wedges. The horizontal timber of the centre was only 13 inch is deep, and the six ribs were tied together transversely near the top, by thorough bolts of inch iron, but with a view not to weaken and injure the timber more than was absolutely necessary, the least pos sible of iron was used." This centre thus differs essentially from any other hitherto employed ; each rib. instead of firming one connected piece If frame-work, consisting in this of fun' independent parts, and hardly any transverse strain has to be resisted. It has also this advantage, that the bearings may be gradually re lieved. or tightened at one place, and slackened at another, as may be necessary, because the wedges are in this construc tion borne by the centre, instead of the centre being borne by the wedges, In striking centres it is of great advantage to be able to suffer them to rest at any part of the operation ; for it is important that the arch in taking its proper bearing do not acquire any sensible degree of velocity, or settle too rapidly. The centre, says Alberti, should always be eased a little as soon as the arch is completed, in order that the arch-stones may take their proper bearings before the mortar becomes hard. It' the mortar be suffered to dry before the centre be lowered, the arch will break at the joints in settling, and the connection of the arch will be destroyed. In small centres, the wedges are driven back with mauls, men being stationed at each pair of wedges for that purpose. But in larger works a beam is mounted, as a battering-rani, to drive the wedge formed blocks hack. The French engineers, in removing centres, destroy, by little and little, the ends of the principal supports ; a work of difficulty, as well as danger, and which cannot be done with so much regularity in this way as by wedges. See IRON BRIDGE, STONE BRIDGE, and SUSPENSION BRIDGE.