729 



CENTERING. 



CENTERING. 



730 



cisely in accordance with the importance of the stream to be bridged 

 will be the difficulty of constructing the centres to be used over it. 

 In designing a centre, it must also be borne in mind, that the peculiar 

 construction in question is distinctly a temporary one ; and that there- 

 fore it should be put together in such wise as to allow the timber 

 which enters into its composition to be reconverted, without material 

 loss to the contractor from the waste of material, or the deep cuttings 

 made into the said timber. 



Centres may be described, nearly in the language of Tredgold, as 

 being composed, in ordinary practice, " of several separate vertical 

 frames, or trusses, connected together with horizontal ties, and stiffened 

 by braces, in order to decompose the action of the vertical weights 

 thrown upon the respective temporary structures intended to support 

 them. It thence follows, from the peculiar nature of the resistances of 

 timber to permanent efforts, that the various pieces of a centre should 

 be made to receive their ultimate strains in a direction vertical to their 

 length, as far as it is possible so to combine the framing; because, 

 from the mere fact of the elasticity of the kinds of wood usually 

 employed, the outline of the centre would be liable to deformation 

 under the action of the permanent load, if that load should act either 

 transversely or in a markedly inclined direction to the fibres of the 

 wood employed. Even the Romans themselves seem to have been 

 aware of this law, for they evidently made the centres of their arches of 

 the very smallest possible span, in order to reduce the thrusts upon those 

 centres to the most direct vertical action ; and in the erection of the Pons 

 Celsius of Rome, and in the Pont du Gard in France, it is evident that 

 every advantage was taken of the dynamical resistance of the springing 

 of the arches, to diminish the effort thrown upon the wooden framing 

 intended to support the weight of the vault in the intermediate part. 

 In both these cases it would appear, from the projections left upon the 

 voussoirs, that the centres were made to rest upon the stones laid with 

 an inclination of about 30 to the horizon, an inclination at which the 

 friction upon the bed was only just balanced by the effects of gravity ; 

 and indeed it is more than probable, from the constructive details still 

 to be observed in these bridges, that the Roman engineers adopted 

 nearly the same modes of proceeding which are even now followed by 

 practical builders in the execution of similar works. Amongst other 

 facts to be gathered from the remains of antiquity which still survive 

 in a tolerably perfect state, it would appear that even the modern sub- 

 claseification of close-boarded centres and of laggings was known to the 

 Romans; and in the subsequent part of this article we shall have 

 occasion to refer to the practical distinction between the two. 



In small arches it is, of course, easy to make the centres of sufficient 

 strength to resist the superincumbent weights, without producing any 

 change of form in the outline of the vault ; but in large arches the 

 centres are so much exposed to vary in their outline, that it has been 

 found advisable to admit at once the distinction between the flexible 

 and the inflexible centres ; and, in the cases of both these varieties, to 

 further subdivide them into centres designed to receive the whole arc 

 from the springings to the crown, and into the centres having inter- 

 mediate supports, or, in other words, into entire or into partial 

 centres. 



At the present day, the flexible centres. used by Perrounet, in the 

 construction of the bridge at Neuilly, are almost entirely abandoned ; 

 because the shape of the intrados of an arch, raised upon such a centre, 

 must inevitably be exposed to changes of the most extraordinary and 

 irregular nature. In fact, a centre of this description must always be 

 liable to change its form, from the mere pressure of the voussoirs which 

 are progressively added, and the only mode of obviating this incon- 

 venience would consist in weighting down those parts of the centre 



Centre of Neuilly Bridge. 



which did not immediately support the load. The success of any such 

 operation must of course be always equivocal, and the great subsidence 

 which took place in the masonry erected upon the centres of the 

 Neuilly Bridge, may be referred to as proving the danger attending 

 the application of the particular centre there used. In the article 



BRIDGE, this has been already alluded to ; and the extent of the settle" 

 ment recorded in that case must be considered to be the most marked 

 condemnation of the system adopted ; but it may also be observed 

 that in previous experiments the same results had been obtained, as 

 in the case of the old bridge of Moulins over the Allier, erected by 

 Mansard. The flexibility of centres like those used by Perronnet and 

 Mansard, in the cases thus cited, arises from the perfect freedom of 

 the timbers of which they are composed to move upon one another ; 

 but, by the same rule, it follows that precisely in proportion as the 

 haunches are loaded in an arch of this description, the centre must 

 rise at the crown, unless some artificial means be adopted to keep it 

 from rising. As it is impossible to regulate these movements, it 

 follows that the use of flexible centres must be abandoned in all cases 

 where it is desired to secure the regularity of the curvature of the 

 arches, and a rigid system of framing must be introduced, which, as 

 before said, may either be supported upon the springing of the arches 

 themselves, or upon intermediate points of support in the opening 

 they are designed to span. In some cases centres are framed, designedly, 

 to be supported in the commencement upon the springings only ; and 

 to be subsequently strengthened by intermediate struts, inserted during 

 the progress of the work ; and there appear to be really some important 

 advantages in this system, for the centres can, under it, be more easily 

 slackened than when there are no intermediate supports. 



In the more modern centres, such as those used for the London and 

 Waterloo bridges, one of the most serious sources of danger (namely, 

 that arising from the number of butt joints) which occurred in the 

 earlier descriptions of centres, was obviated by the use of cast-iron 

 shoes ; and, as will be observed, the timbers are made to support strains 



Centre of London Bridge. 



Centre of Waterloo Bridge. 



acting~principally in the direction of their length. The cast-iron shoes 

 also acted as ties to the whole of the framing, and thus prevented the 

 centre from rising in any dangerous manner at the crown. But even 

 in these cases it was necessary to carry up the arches evenly on either 

 side, and to weight the centres temporarily at the summit. 



