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CHAP. XXII.

PLATE XIV. ON BRIDGES OF IRON


	Although various have been, and are, the opinions relative to the construction of bridges of iron and wood; each artist seems necessitated to resort to something like an arch, but differing in their mode of producing it: they frequently create labor and expense by erecting a complicated fabric.
 
	But on his head, I conceive that first care is to have sufficient butments; after which, let each segment of a circle, composing a rib, be formed of single pieces as long as can conveniently be cast; and it is evident, a circle must be compressed into a straight line, or the butments separate before the bridge can come down.

	It is therefore only necessary to form a segment, so that it may not change its position, by sinking in one part and rising in another, by the various weights which it may have to support, also guard against yielding side-ways; for this purpose, the great quantity of iron or wood is not so material as a judicious arrangement of the parts.

	In iron, or wood, the artist may be furnished with pieces of greater length than possibly can be obtained of stone; consequently, there will not be such numerous joining; and thus the span may further extended extended: on which see Fig. I.

	This represents a segment of iron sixty feet long, eight inches broad by four thick, and may be considered as a single stone of that length; which being placed between butments and the spring, preserved in a perpendicular direction, let five weights be suspended at equal distance; and, in all probability, though each weight amounted to twenty tons, it would support the whole five equal to one hundred tons: yet, let one weight of twenty tons be suspended between the center and end, as in Fig. II. and it is reasonable to suppose, the whole would come to the ground, as the weight would compress one part  and raise the other, destroying the shape of the parts on each other, for want of counter-weights to preserve the equilibrium.  Therefore, after forming segment, the great point is to dispose of the braces, so as to divide the weights equally on the curve.
 
	To effect this, Fig. III. represents a span of hundred and thirty feet, by a scale of one hundred and thirty feet, by a scale of one inch to twenty feet; and is an arrangement of parts which, I conceive, would stand without butments, this may be considered as a bow and string; which string, by keeping the bow bent, answers the purpose of butments; all the other braces being for the purpose of preserving the bow and string in their proper situation, by dividing the weight on the bow.  For instance, a weight over the perpendicular B, will tend to extend 1 and 2; in which cafe, they pass on A and C, and they pull down the bow at F and G; F and G, by the fame system, pull down H and I, and so on, wherever the weight is placed its pressure will be divided along the bow, which consequently cannot vary its position: according to the width of the bridge required, four or more of such ribs must be constructed and placed perhaps ten feet distant from each other; the whole being fastened by cross-bars passing through the stirrups, as in the aqueduct, and prevented from yielding sideways, by the diagonal braces exhibited at A, B, &c. Fig. IV.  After which, the whole may be covered with plates of iron, soiled and graveled, or planked, and covered with earth and gravel in like manner.

	Although, I conceive, there is little doubt but a bridge, as above described, would stand to the length of two or three hundred feet, yet the multiplicity of pieces of which it is composed, in order to preserve the shape of the shape of the segment and relieve the butments, would evidently occasion much labor and nicety of workmanship; therefore it exhibits the importance of simplifying such works, in order to facilitate their formation, and apply every particle of materials, so that they may tend to strengthen the whole, and not be liable to alter the position.
	
	I shall therefore return to the first proposal of adequate butments, to resist the longitudinal pressure of an arch of any dimensions.  In this must be considered, that the butments need not be of the immense size which first strikes the imagination; for whatever dimensions.  an arch of iron or wood may be, the quantity of materials is easily calculated, and the weight which butments will have to refit; for instance, if an arch weighs five hundred tons, and butments opposed to its pressure weighs one thousand tons, they consequently cannot move, not to mention the weight of earth backing, which tends to render them more permanent; therefore, seeing that the foundations are secured, and the springs, if any, well drained, in order to keep them dry, I see no difficulty in constructing butments to support an arch of any dimensions, and that at much less expense than butments and piers could be erected for a stone bridge; in the same situation particuilarily, if such piers were to be built in water of any considerable depth.

	Having premised the butments to be of sufficient strength, I consider the arch, whether it be composed of iron or wood, to be like the segment of a hogshead, and the component parts as near as possible like the staves: for this purpose, in constructing one of iron. Fig. VIII. represents two staves, each of which might be cast in open sand, four feet broad from twelve to fifteen feet long, the pattern being formed to the radius of the spring, a flange on the lower side of the stave should be cast, about on foot broad, with holes to receive the screw pins; across the stave one or more ribs, if thought necessary, should be cast, to give strength to the top plate; these ribs and flanges, in uniting the staves would butt on each other, and ultimately compose a rib to the whole extent of the bridge.

	Having cast the staves as wide and long as experience may hereafter prove expedient, I will suppose it necessary to erect a bridge one hundred feet span, as in Fig. V.; in building the butments, it would be advisable to place two or three segments, of the same radius, as the bridge, in each butment, they being cast with arms, or united to binders, in order to take a firm hold on the masonry, and become a permanent support; the segment thus passing into butments, might be considered as a part of the arch, which, by this means, would be considered as a part of the arch, which, by this means, would butt against the center of the whole weight of the butment, and must push the whole away before the arch could yield; but, without this precaution, the arch would rest so near the top of the butment, as to raise the stone-work and endanger the whole.  Having thus prepared the butments, a center of three or the whole.  Having thus prepared the butments, a center of three or more segments, so that each stave may have two bearings, should be erected;
and the staves being ready, all of one dimension, and the screw-pins all of a size , the arch might be sprung in a few days, breaking joint, as in Fig. VII.  Thus each flange and rib would butt upon its neighbor, and the screw-pins confining the whole, it would be become like one solid segment of a cylinder one foot thick, extending into the piers.

	By this mode, the difficulties which arise in fitting diagonal, perpendicular and lateral braces, are avoided, the top plate performing the office of all such braces; which top-plates, in other compilations, have no tendency to strengthen: thus every particle is applied to resistance, and the materials have but few joining; which junctions have also broad and permanent bearings of one for on each other, the flange and ribs being cast of that depth. Thus, I conceive, a bridge of one hundred feet, or perhaps one hundred and fifty feet span, might be erected at a cheap rate, with a small quantity of materials, yet with the prospect of great durability. If I suppose a bridge one hundred feet span, thirty wide, with the top plate one inch thick, five flanges or ribs, each one foot broad and two inches thick, the whole weight of the arch would not be more than seventy-eight tons, allowing one pound to every four inches of cast-iron.

	So far relates to iron bridges of one hundred or one hundred and fifty feet span,; but should it be necessary to extend them to a greater length, to say three hundred feet, two segments would be requisite, the first, as in FIG. VI. as the principal support; the second, to ease the passage, should be of such a bend as would admit an easy ascent and descent; and, by being part of a curve, it also tends to strengthen; thus. if I say, span three hundred feet, the first spring thirty, the second spring only ten feet, they both must be compressed into straight lines, before they can come down, they will also have twenty perpendicular feet of bearing on the butment, the bearings opposite to the two segments being opposed by segments entering the butments, as before described; the two segments must be kept asunder from each other, as in the Figure. See Plate XV. exhibiting the combination by ribs and braces; also Plate XVI. representing the arch of staves.

	Having exhibited the mode of construction, I conceive it unnecessary to comment on the particular formation, or to draw a comparative view of this and other bridges of iron; as in cases where the may be required; the artist will weigh the various circumstances, investigate the several modes of building, and choose for himself, in which there is a leading deliberation, viz. By what means can a given quantity of materials be arranged, so as to incur the least expense in erecting, and be rendered most permanent; and by which mode will the least materials answer the purpose?

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