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1404

THEORY AND PRACTICE OF ENGINEERING. Book II.

pression of the joints, begins before the striking of the centre, and even before the arch isclosed; by the time the centre is struck, the mortar has already acquired much con-sistency, and no sensible change can happen in the figure of the arch : but it is veryeasy to imagine that a centre composed so as to remain fixed during construction shouldhave the same properties ; for whatever may be the system of the carpentry of a centre, ingradually destroying the points of support on which it is placed, its resistance may alwaysbe insensibly diminished, leaving it the necessary strength for supplying that which thearch has not yet acquired; which resistance should continue until after the joints havingbeen compressed as much as possible, the arch can sustain itself, and is no longer susceptibleof any movement. The only precaution to be used is not conducting this operation with toogreat rapidity.

The question of fixed and movable centres was examined by the engineers of the Fontset Chaussees in France , on the occasion of the construction of the arch of the bridge ofJena, and they decided in favour of the first; the success attending the execution of thesearches confirmed the propriety of this decision. With respect to the system of carpentryto be employed for fixed centres, it appears that, in order to have requisite solidity, theyshould be composed of three parts at most, the two inferior ones immediately resting oncorbels placed at the springings, to carry the upper parts; by such an arrangement therewill be no tendency to a change of form, at any epoch of the construction of the arch ;for it is evident that the only condition necessary for the equilibrium is, that the numberof the voussoirs at the springing should be the same at each side.

Fig. 2342. represents a centre formed after these principles, for the construction ofthe arches of the bridge of

Moulins , which was built on ageneral radius, on which wasestablished a centre, on pointsof support taken on the sameradius, evidently producingthe greatest solidity: the prin-cipals were distanced at 7 feetfrom centre to centre, and main-tained by horizontal binding-pieces, and by St. Andrew ’scrosses; the timbers were 13inches square.

The centres of West-minster Bridge, constructedby Mr. King, were an im-provement upon those de-signed by Perronet. Twotimbers resting on theabutments incline, andmeet at the top of the arch,forming a large triangle;this is crossed and bracedagain in various directions,forming as it were sevenlarge voussoirs: there isconsiderable strength

Fig. 2342.

CENTRK AT MOULINS.

Fig. 2343.

CENTRE AT WESTMINSTER.

such an arrangement, and it combines a portion of the principles afterwards adopted withso much success.

Centres of 130 feet span have been formed by vertical and horizontal timbers bracedby’ others in a diagonal position: they are exceedingly simple, and are easily puttogether and taken down ; the upper portion is contrived to rest on wedges, and can belowered without disturbing the rest; and by striking the inclined supports at bottom, theirposition can be altered when required.

Fig. 2344.

CENTRES OF 130 FEET SPAN