C E N

139

CEN

can < ^ r ' 1 i Uerals ’ S* vea nothing but immutability of figure. ItJoint ° ^ ^ er ' ve i ts stiffness from the resistance of the

P. *

Polvrf*^ 6 ® shows the manner of forming a centre by twoi n thig 11 }’ 0p ^Eieh the interior one is secured to the exterior:Parts f i . * s no occas i° n for double trussing-pieces, as thethat r .e , inscribed polygon act either as strutts or ties toSi 1 6 circu mscrihing one.

t Wo ^ is the manner of forming the rib for a centre, byno oc ' • en( i er| t trusses; in this form of centering there isof ^t ^ f° r bridles, or double trussing-pieces, as in thosep-° ’ op same construction,to pf re is the manner of constructing a centre, accordingbut wit]° net ? four polygons, independent of each other,ring 0 f 1 ^*' s improvement, that the lower extremities of eachgives I P ° lygons are framed into the two abutments ; thisBareg , Inu ch firmer base than if they were all to meet at thethe an^ 6 ’ ant ^ ren( iers the centre much stronger, by makinghave h° '*0 rnore acu te. In this it becomes also necessary toeff ec tiv ^ US ’ °ti |er 'vise the exterior polygon only would be

p°ly£^ re 6 is the manner of constructing a centre with threepiece s 7’ wb ’ c ^ ar e all secured to each other. In this, truss-P< %£on e20rne necessary, otherwise the angles of the innerJg ut ?°uld bend the sides of that next to it.

St tb” F M re is the design of a centre, its principlee Xarn t) ] lat , op two roofs intersecting each other. In thisPart s of tt! 16 Porces which are communicated to the various^ s ^on 0 7±l me : are resisted longitudinally, either by com-any or extension; and no force is exerted transverselyk° a r<lin Ut ’ exce Pting the curved pieces in contact with theSa PPorting the arch-stones.

ar ge trng^ 18 tbe design of a centre ; it is first framed in onet a cofi 8 ’ a common roof, with two principal rafters,t ' v ° srn a u ar ' beam ’ eacb op tbe raPters becomes a tie for theti r °°f, w- t ,’ USSes a ' 30Ve > which are framed in the manner ofti Prittc' 1 \ t l ueen- P°sts and braces. The lower angles of• * e Posts Pa i, rafters are braced from the lower queen-posts to* ts Parts ' b b ' s truss is free from transverse strains in allv? 1 ?’ *f \v 7 ce P t the curve, which supports the arch-stones ;ri , <; secured at the abutments, an arch of immensea® 8 " 8tai “ed by it.

'-iti7p >e r' ! , is tbe celebrated centre used at Blackfriars4 . 0 rf. ibe names of the timbers are as follows :

which support the centering.

D. lytJpper an d i ower striking-plates, cased with copper.

between striking - plates, for lowering the

trussing-pieces, to confine braces.

^tre.

*• H

liyj j Jn .~P* eees 5 to strengthen rib of centre,j tt, IH 0c p' n ® S laid on the back of the ribs.a Hce s _ ’ s between bridgings, to keep them at equal dis-

^'°n con fine the r ihs tight.

**• J a ^ s holted to trussing-pieces and apron-pieces.

, m at the feet of truss-pieces.

iu>5 Palbrace8 -

tj^PendenJ® u sed at “Westminster Bridge was formed bya cr* S a b sun russes > consisting of two rafters ; the intersec-k,,° Ss the 7° se( f to be halved together, and firmly strappedb? f <* th e notclli - ~ '

huble

^P>'on-

Poi'

aese t] ln S s - Double truss-pieces were also used,be f i; ere Was evidently no occasion, as the pres-

. 4..—two oppose

m tg 1)6 directed to the abutments,

tho „ e arch in the same level. tlie centering

4 ,7 r ',” Xetl P late is a perspective vie magnificent

hfitW J tl '-e arches of Waterloo Bridge. yp r . John

86 built under the direction of the late

Bennie, and is a noble specimen of simplicity of design, skil-ful arrangement, and solidity of execution. The centre wascomposed of eight frames or trusses, and, though somewhatcomplicated, was on the whole a judicious combination ;exhibiting rather an excess than a deficiency of strength.

In the erection of Chester Bridge, finished in 1832, anentirely different principle was adopted in the constructionand the mode of relieving the centre; it is thus described inthe Transactions of the Institution of Civil Engineers ,Yol. I.:—

“ The centre on which the stupendous arch of Chester newbridge was raised, and which is stated by Mr. Hartley, (theengineer of the bridge,) to have been exclusively designed byMr. Trubshaw, claims a detailed notice, from the novelty ofthe principle it was formed on, the efficiency with which itdid its work, and the economy that attended its use. Thecentre consisted of six ribs in width, and the span of the archwas 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-lihe towards the soffit, so as to take theirload endwise. The lower extremities of these radiatingbeams rested in cast-iron shoe-plates on the tops of the piers,and the upper ends were bound together by two thicknessesof 4-inch planking bending round, as nearly as they could bemade, in the true curve of the arch. On the rim thus formed,the lagging, or covering, which was 41? inches thick, wassupported over each rib by a pair of folding wedges, 15 or16 inches long, by 10 or 12 inches broad, and tapering about1J inch ; for every course of arch-stones in the bridge, therewere therefore six pairs of striking wedges. The horizontaltimber of tbe centre was only 13 inches deep, and tbe sixribs were tied together transversely near the top, by thoroughholts of inch iron, but with a view not to weaken and injurethe timber more than was absolutely necessary, the least pos-sible of iron was used.”

This centre thus differs essentially from any other hithertoemployed; each rib, instead of forming one connected pieceof frame-work, consisting in this of four independent parts,and hardly any transverse strain has to be resisted. It hasalso this advantage, that the bearings may be graduallyrelieved, 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 bythe wedges.

In striking centres it is of great advantage to be able tosuffer them to rest at any part of the operation ; for it isimportant that the arch in taking its proper bearing do notacquire any sensible degree of velocity, or settle too rapidly.The centre, says Alberti, should always be eased a little assoon as the arch is completed, in order that the arch-stonesmay take their proper bearings before the mortar becomeshard. If the mortar be suffered to dry before the centre belowered, the arch will break at the joints in settling, and theconnection of the arch will be destroyed. In small centres,the wedges are driven back with mauls, men being stationedat each pair of wedges for that purpose. But in larger worksa beam is mounted, as a battering-ram, to drive the wedge-formed blocks back. The French engineers, in removingcentres, destroy, by little and little, the ends of the principalsupports ; a work of difficulty, as well as danger, and whichcannot be done with so much regularity in this way as bywedges. See Ikon Bridge, Stone Bridge, and SuspensionBridge .

Centre, in geometry, a point in a figure or solid, such thatif any straight line be supposed to pass through the pointuntil it terminate on both sides of the figure or solid, the linewill be bisected. Figures of this property are infinite. Some

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