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With wooden trusses, fastened with iron

fur timber, labor, paint and profit, $550 6 2,000 lbs. of iron fastenings,........

.... 150. Whole first cost, ...........

$700. (Some have cost $1000, or $12,000, and taken

3 to 4 thousand pounds of iron).......... To renow $550 worth of perishable material

once in 9 years, will require, at 5 per
cent, compound interest, .................... $1,000.

Total for perpetual maintenance, ...... $1,700. The reason of the apparent difference between this result, and that arrived at from the general comparison of the cost, &c., of wood and iron, is, that the bridges here referred to, have been constructed with a very large amount of iron fastenings, and with large quantities of casing and painting for protection and appeai. ance. Were the comparison confined strictly to the expense of timber work, in the sustaining parts of the trusses, the result would he found not to differ so essentially from that of the general comparison.

The above estimate of $700, for the first cost of a 72 foot wooden bridge, though considerably below the average cost of canal bridges of that description, is nevertheless believed to be greatly above the minimum for which bridges may be built, dispensing with the rarts which are not essential to strength.

It is probable that bridges may be built for $500, as chout the minimum, of equal strength and convenience, 'n, nearly the same durability, as those hitherto built af w. ins Erie Canal Enlargement at a cost of from 39: to 1,000 dollars. Upon this supposition, which Lay be regarded as an extreme case in favor of wood, too coraparison will stand thus :

$500

First cost of wooden structure, .........
Capital invested at 5 per cent to produce $500

once in 9 years for renewal,.......... .........

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Total for perpetual maintenance, ................. $1409 The same for iron structure, as above, ......... 1260

Balance in favor of the iron bridge...... ........ $149

Finally, since theoretical calculation and general comparison show a probable advantage, for a long term of time, and experience, as far as it has gone, shows a decided advantage in favor of iron, it would seem very unwise to discard the latter, without at least a fair trial of its merits. If in the first essays at iron bridge building, the iron bridge has competed so successfully with wooden bridges, improved by the experience of ages, may not the most satisfactory results be anticipated from an equal degree of experience in the construction and use of iron bridges ?

LXXXIV. Presuning the affirmative to be the only rational answer to the above question, I have arranged the details of plans for carrying into practice the preceding principles and suggestions in the construction of rail road bridges of iron.

I have also made careful detailed estimates of the expense of bridges of different dimensions and in different circumstances, some of the more general results of which I will here state.

In proportioning the parts of a rail road bridge, I have assumed that it may be exposed to a load of 2,000lbs. per foot run, for the whole, or any part of its length, in addition to its own weight; and in case of tension, have allowed one square inch cross section of wrought iron for every 10,000 ibs. of the maximum strain produced upon every part by such weights, acting by dead presbure. In case of thrust, or crushing force, I have allowed one square inch cross section of cast iron, for every 12,000lbs. acting on pieces (mostly in the form of hollow cylinders), of a length equal to 18 diameters, and a greater amount of material, where the ratio of leugth to diameter is greater; always having regard to practicability, as well as theoretical proportions, in adjusting the dimensions of the part.

My estimates, made upon these bases, have fully satisfied me that a bridge of 100 feet span, with track upon the top (with wooden cross-beams), will cost about $2,000, or $20 per foot, assuming the present prices of iron (1846), in ordinary ciri umstances. If the track pass near the bottom of the trusses, the expense will be increased by two or three dollars a foot.

For a span of 140 feet, by a liberal detailed estimate I make, in round numbers, a cost of $4,000. For 70 feet, I estimate a cost of 9 to 10 hundred dollars, according to circumstances.

Thus it will be seen that actual estimate makes the cost of a single stretch of any length, very nearly as the square of the length, as should be expected from the nature of the case. Hence, knowing the cost of a span of any given length, we readily deduce that of a span of any other length, in similar circumstances, with reliable certainty.

Now, although my investigations have forced the conviction upon me, that where strong and durable bridges are required, iron should be preferred in their construction, still there is a multitude of cases where wooden structures should be preferred; especially in sections of country comparatively new, where timber is plenty and capital scarce; and where improvements must necessarily be of a more temporary character.

With this view of the subject, I have given considerable attention to the details of wooden bridges; and, with a good deal of investigation and experiment, have arranged plans which are confidently believed to pos. 8e88 important advantages over the plans generally in use.

The preceding few pages have been transcribed froni the author's original and first essay upon bridge building; and are introduced here, not on account of any practical value they may possess in the present state of progress in the science of bridge construction. But 'they may possess some little interest as marking about the starting point of the construction and use of Iron Truss Bridges.

If the estimates above exhibited, of the cost of iron bridges, appear small and inadequate, under the lights furnished by the experience of a quarter of a century, much allowance may be claimed on account of the change of times and circumstances within the period in question. And, when it is borne in mind that the author actually contracted for, and built iron railroad bridges of 40 and 50 feet span, for $10, and of 146 feet for $30 per foot, the estimates above given may not seem entirely preposterous, although much higher prices are obtained for bridges of like dimensions at the present day.

PRACTICAL DETAILS.

LXXXV. In preceding pages I have endeavored to give a short and comprehensive general view of the subject, and to ascertain and point out the best general plans and proportions, for the main longitudinal trusses, or side frames of bridges, and the relative stresses of their several parts.

The side trusses may be regarded as vastly the most important parts of the structure, and the strength and sufficiency of these being secured, there is much less difficulty in arranging the remaining parts, the forces to which they are exposed being much less than those acting upon the trusses. I propose now to enter more into details, and give such practical explanations and specifications as to the strength of materials, the methods of connecting the several parts or pieces, both in the main trusses, and other parts of the structure, illustrated by the necessary plans and diagrams, as, it is hoped, will enable the young engineer and practical builder to proceed with judgment and confidence in this important branch of the profession.

IRON BRIDGES.

STRENGTH OF IRON. LXXXVI. Iron has the power of resisting mechani. cal forces in several different ways. It may resist forces that tend to stretch it asunder, or forces which tend to compress and crush it; the former producing what is sometimes called a positive, and the latter, a negative strain. It may also be exposed to, and resist forces tending to produce rupture by extending one side of the piece, and compressing the opposite side; as where a bar of iron supported at the ends, is made to sustain a weight in the middle, which tends to stretch the

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