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must be bound down to the stone work; for this purpose let four pieces of iron for each pier be made long enough to reach quite through the pier, and with strong eyes at each end turned up some inches, let two of these pieces be built in each end of the pier, say ten feet down in the stone work, so that the eyes may barely appear on the face of the work, and one brace of the framing can be fastened down to each eye.
If there should be a large space and a number of lesser ones, or should it be necessary to raise the chain at a draw-gate, lay off your plan on some convenient scale as before directed, employ the line and balances, fixing whirls at every bearing, to equalize the tension. In this way the position of the chain will be ascertained at every place, and likewise the length of the suspenders for their respective places; and I venture to say that this plan and this only, of ascertaining the proportions, can be safely depended on.
It is a matter worth knowing, what is the tension of the branches, compared with that of the main chain. It is evident if there were only two branches, and they should open so as to form an angle of 120 degrees, (that is one third of the circle) cach of the branches would then be equally tense with the main chain; but whatever angle the branches form in spreading to receive the stones, the tension can be ascertained by the line and balances.
It may be inquired whether all parts of the chain are equally tense when supporting the bridge? I answer that the tension is about an cleventh less at the middle of the bridge than at the ends. I have ascertained this by taking a line to represent the chain that supports one half the bridge only, and extending it over two pullywhirls, one at the centre of the bridge and the other at the corner where the chain is supported, and loading it horizontally equal as in the case of a bridge. It is evident that the weight at the upper whirl must be greater than that at the lower; and the difference between the two, shows the difference of tension between the middle of the bridge and the two ends.
The spreading of the branches, unless very considerable, increases the tension less than I could have thought. In the length of branches that I have proposed the increase of tension is not worth notice. I have just been trying with a small line and balances, th longest branches two feet three inches, and the shortest fourteen and a half inches, spread to fourteen and a half, and in that case the whole increase of tension in all the branches, appeared to be only one-seventh more than if they had all drawn in a straight line with the main chain. By these experiments it is probable that many of those concerned will be relieved from groundless fears. I have found great difficulty in obtaining permission to let the ends of the chains open each a foot or two off the direct line, so as to make the passage to and from the bridge more free, and remove the chains out of danger.
I know the young mathematician, with mind half matured, would smile at my mode of testing the relative force and effect of the several ties and bracings of any piece of framing: but the well informed, will not so lightly treat any information obtained or supposed to be obtained by actual experiment. If the process is before him, he will carefully ponder all the parts, and discover where the defect lies before he rejects the conclusions drawn therefrom.
SHAPE OF THE LINK. It is plain that the bars in the middle of the link draw in a direct line, and it is easy to tell the strength: but is impossible to get the links fitted into each other as close and full as could be wished ; to remedy which and to be secure in this point, it will be necessary to have those parts of the link made considerably larger. To accomplish this, nine or ten inches of cach end of each bar is left a quarter of an inch larger than the rest, and two such bars make one link. As there is but one link of the chain to cach space between the joists, there will not be much iron expended in this way. It is thought best not to round the inside of the links at the ends where they sit in each other, as there is no friction in the chain when in
Every link will be so wide, that the side of the next one can turn freely in it, and the other side turn round its end, for the workmen will find it convenient to hang up the last made link of the chain, so that the lower end of it may be nearly on a level with the fire and anvil. In this way he will be able to turn up three sides of the one he is closing in, and will find no difficulty in shaping the work to his mind. This wideness of the link must always be filled up with the thickness of the end of the next. A link of inch and half bar will require to be more than two inches and a half wide,
and the end where it is welded must be left just as thick, measuring through the inside of the link; but the outside of the end of the link, may be reduced to one inch and a half; this will give pliancy to the chain. Some have thought that in calculating the strength of the chain, we should not reckon on both sides of the link, because it is single where it passes through the end of the other link. This is a misapprehension: but nothing is more necessary than that this part should be well fortified. Let the quantity of iron in this place be two or three times as much as in any other part of the link.
Although I have taken considerable pains to ascertain and to demonstrate the strength of the chains, it must not be forgot that they may be overloaded. Some books when stating the strength of metals, advise not to load more than half, for fear of the injury Ly a continued tension. There are two other considerations that must be attended to the corrosion at least of some of the parts, and we must not expect the execution of the work as complete as the mind could conceive it. And here I would carnestly recommend all those concerned, to adopt it for a maxim, that the chains in all cases shall be able to support five or six times the weight of the bridge.
It will be prudent in all cases to have the joists and plank as light as can be with safety. In cases where the bridge is twelve or fifteen feet wide, I have put the joists of the lower tier ten feet asunder, they being about ten inches by five in size. Each joist of the upper tier being one continued joist from end to end of the bridge, each space
will bear double what it could were the joists cut into separate pieces for each particular space. Why? Because a joist just long enough to rest on bearings at the ends, can give way under its load by breaking in the middle only; but where it is one continued piece over many bearings, and all loaded, it cannot give way without breaking at both ends and middle. Besides, the whole system is of a yielding texture, much in the nature of network; but they had best be at least one foot deep, and not more than three inches thick in order to stiffen the bridge as much as possible, where the ends pass each other and are bolted will give them thickness enough to stand firm. Plank of two and a half inches thick have always been used, so far as I know.
In regard to the anchoring or fastening of the ends of the chains, there is much diversity of opinion. Many are highly pleased with
the idea of fastening to a rock, when the situation will admit some by letting the end of the chain into the rock with a staple or bolt to fix it to—and others by letting bolts into the rock with the head projecting out for a large piece of iron to rest against. To this large piece of (perhaps cast) iron, the chain is fastened.
This is wandering from our favourite principles. In those methods, the stiffness of the iron is depended on; and suppose the iron fastened in with lead, and the tension so great as nearly to tear the iron to pieces, will not the lead spue out like water? To my mind, all these methods have something in them too precarious and unsafe to be depended on. Give me a sufficient hold of a plat. form of some kind, and let me know the weight of the materials that rest on it, and I shall know on what I depend. And it must also be known in every case what the weight of the bridge is, and thc fastening at each end must be, say one-third more. The fastening is no hard matter, and it ought to be remembered that a drore of cattle may sometinies get on at once.
The bottom of the digging to fasten the chains, had better be sunk say two feet deepest at the side next the bridge. This will give the platform a greater appearance of resisting the drawing of the chain. The shortest branches of the chain must be about eight feet long, and the other two will be about twelve.
It is settled beyond all controversy, that wrapping with canvass and pitch will preserve iron time out of mind, even in sea water, and that good painting is as effectual in an open situation. It must be granted that if any tolerable degree of care is taken, it will be but a very small part of the iron that can receive much injury. But supposing the chain bridge should become a total wreck in five hundred years, what then is the relative value of the ruins of this compared with that of any other bridge? And in point of duration, how will the account stand between this and a wooden bridge, even when covered? Will it be less than ten to one? And when uncovered, will it be less than thirty?
The chain bridge is as favourable to navigation as any other. It is only necessary, as in every other case, to have a pier on each side of the passage. The chains having nothing to support at that place, will
pass in a direct line from the top of the framing on one pier to that of the other, which it is thought will generally give room sufficient for vessels to pass under. I long for a case where a drawbridge is wanted; I should undertake to devise means without any chain crossing above, and cheerfully take all the risk on myself.
Those engaged in such arduous undertakings, will generally pay some attention to the different modes of accomplishing the same purpose. In the Cyclopedia will be found the following notice of iron bridges: “ Bridges of cast iron are much celebrated; in particular that at Coalbrookdale, Shropshire, England, span 100 feet six inches, iron 378 and a half tons. Also the Sunderland bridge over the river Wear, single arch 246 feet span, iron 266 tons, of which 46 are hammered iron. In the years 1795—6, an iron bridge was thrown over the river Teme, in Hartfordshire, England. Its parts were so slender and ill disposed, that no sooner was the centre taken away than the whole tumbled into the river."
In the first of these cases the one hundredth part of the iron would be more than sufficient for such a bridge. And in the second case, less than twenty tons would at a fair calculation, support a bridge of the same extent, that would bear more than four hundred tons burthen. And when these monstrous massess of iron are got together, their bridge is just as far from being completed as ours, when the chains are up and ready to receive the timber; for they too have the flooring to make after the iron is erected,
May I venture to glance at the grand, the majestic arch of solid stone, with
idea of contrast between it and our simple contrivance? Happy for me, utility, economy and despatch, are the ruling passions of the day, and will always take preference of expense, idle elegance and show, until the minds of men become contaminated with vanity or some worse passion.
I confess I have not yet obtained materials for a proper investigation of this subject, but for the present let one case suffice: the Monockasey bridge of about six hundred feet, is nearly completed by the Baltimore turnpike company; the lowest estimate of total expense to finish it, I am told is sixty thousand dollars. Just about one fourth would have answered the same purpose on our plan. I venture to pledge myself, that one third of the money shall erect such a bridge and keep it in repair forever.
Although hundreds of bridges of superior elegance and extent, have gone to ruin in a very short time after they were erected, we