QUESTIONS RESPECTING TH2 CONSTRUCTION OF A CAST IRON BRIDGE, OF A SINGLE ARCH, 600 FEET IN THE SPAN, AND 65 FEET HIRE. (Plate XLII. fig.7.) • 1. What parts of the bridge should be considered as wedges, which act on each other gravity and pressure, and what parts as weight, acting by gravity •only, similar to the walls and other long, usually ereded upon 'the arches of stone beidges. Or doei the whole act •as one frame of iron, which can only be de. stroyed by crushing its parts f The distribution of the resistance of a bridge may be considered as in some measure optional, since it may be transferred from one part of the structure to another, by wedging together most firmly those parts which we wish to be most materially concerned in it. But there is also a natural principle of adjustment, by which the resistance has.a be thrown where it can best be supported ; for the materials 'being always more or less compressible, a very small change of form, supposed to be equal throughout the structure,'will relieve those parts most which are the most strained, and the accommodation will be still more effectual when the pasts most strained un dergo the greatest change of form. Thus, if the flatter ribs, seen at the upper part of the proposed structure, supported any material part Of its weight, they would undergo a considerable longitudinal com pression, and being shortened a little, would natural ly descend very rapidly upon the more curved, and stronger parts below, which would soon relieve them from the load improperly allotted to them ; the abutment would also give way a little, and be' forced out, by the greater pressure at its upper part, while the lower part remained almost entirely unchanged..
It is, however, highly important that the should, in the first instance, be so arranged as best to fulfil the intended purposes, and especially that such parts should have to support the weight as are able to do it with the least expense of lateral thrust, which is the great evil to be dreaded in a work of these gigantic dimensions, the materials themselves being scarcely ever crushed, when the arch is of a proper form ; and 'the failure of an iron bridge, by the want of ultimate resistance of its parts to a com pressing force, being a thing altogether out of our contemplation ; and it is obvious that the greater the curvature of the resisting parts, the smaller will be • the lateral thrust on the abutments.
We may, therefore, sufficiently answer this ques tion, by saying, that the whole frame of the propo sed bridge, so far as it lies in or near the longitudi nal direction of the arch, may occasionally cooperate in affording a partial resistance if required ; but that the principal part of the force ought to be concen trated in the lower ribs, not far remote from the in trades.
But it is by no means allowable to calculate upon a curve of equilibrium exactly coinciding with the intrados ; since, if this supposition were realized, we should lose more than three-fourths of the strength of our materials, and all the stability of the joints independent of cohesion, so that the slightest exter nal force might throw the curve beyond the limits of the joint, and cause it to open. Nor can we always consider the curve of equilibrium as parallel to the intrados : taking, for example, the case of a bridge like ,Blackfriars, the curve of equilibrium, passing near the middle of the arch-stones, is, and ought to be, nine or ten tht above the intrados at the abut ment, and only two or three feet at the crown; so that the ordinates of this curve are altogether differ. eat from the ordinates which have hitherto been considered by theoretical writers. It may be ima gined that this difference is of no great importance in practice ; but its amount is much greater than the difference between the theoretical curves of equili brium, determined by calculation, and the common est circular or elliptical arches.