weight of the iron; that is, if a square piece whose length equals 15 diameters bear m pounds, and the crushing weight for pieces of 2 diameters be n pounds to obtain the resistance (R), of a piece of (15—a), diameters in length, take m + 2/3 (n—m)=R.

a

13

XC. It has already been remarked that in practice, materials should be exposed to much less strain than their absolute strength is capable of sustaining for a short time. This fact is universally recognized, and the reasons for it, are perhaps, sufficiently obvious; still it may be proper to mention a few of them in this place.

First, there is a great want of uniformity in the quality and strength of materials of the same kind, and no degree of precaution can always guard against the employment of those containing defective portions possessing less than the average strength.

Again, when materials are exposed to a strain, although it be but a small part of what they can ultimately bear, a change is produced in the arrangement of their particles, from which they are frequently unable fully to recover; and whence they generally become weakened, especially if they be repeatedly exposed to such process. Hence, it often happens that a piece is broken with a smaller strain, than it has previously borne without apparent injury.

Now, there is no means of estimating exactly the allowance necessary to be made on account either of these facts, as well as, probably, many others. Consequently, we can not determine with certainty, how much of a given material may be relied on to sustain with safety a given force. We should therefore, incline toward the side of safety, the more strongly, in pro

portion as the consequences of a failure would be the more disastrous. The breaking of a bridge is liable, in most cases, to be a serious affair, involving hazard to life and limb, as well as destruction of property Hence, they should be constructed of such strength asto render failure quite out of the range of probability, if not absolutely impossible.

XCI. Good wrought iron bars, will not undergo permanent change of form under a tensile strain of less than from 20,000 to 30,000 pounds to the square inch; and though they will not actually be torn asunder with a stress below 50 or 60 thousand, and often more, to the inch, any elongation would certainly be deleterious to the work containing them, even if not dangerous from liability to fracture. Hence, it is certainly not advisable to expose the material to a stress beyond the lowest limit of complete elasticity.

In the original predecessor of this work, the traditional allowance of 15,000lbs. to the square inch, was adopted as the tensile stress to which wrought iron might safely be exposed, and beyond which it was deemed improper to rely upon it. No evidences or arguments since that time, have induced a change of opinion in this respect. But in the case of a bridge, there is variety and uncertainty as to the exact amount of load, as well as in relation to the limit of safe strain for the material; and while it seemed probable that the load of a single track rail road bridge would never exceed 2,000lbs. to the lineal foot upon any part of its length, still, seeing that rail roads were comparatively a new institution, and iron bridges for rail roads almost unheard of, especially in this country, it was deemed wise, in recommending their introduction, to so adjust

their proportions as to meet almost any possible contingencies.

This could be accomplished either by assuming a greater possible load for the bridge, or a lower limit. to the stress of materials with the smaller load, with the same ultimate result. And, perhaps the former would have been the more consistent course, as avoiding the seeming absurdity of the assumption that iron could safely stand a strain of 15,000lbs. in a common bridge, but only 10,000lb in a rail road bridge; and the no less seeming absurdity of assuming that the same material could stand 50 per cent more strain in a bridge composed partly of wood, than in one entirely constructed of iron. Now, instances in great numbers could be pointed out, of rail road bridges of wood and iron, where 2,000lbs. to the lineal foot would produce a stress considerably exceeding 15,000 to the inch upon certain bolts of wrought iron.*

*The author had occasion several years ago to refer to the following instances in corroboration of the statement above made, in this wise "The best evidence that exists as to the capacity of a material to bear a strain with safety, is derived from experience as to the strain it has been exposed to in works, and conditions similar to those in which it is proposed to employ it, and where it has by long usage, proved itself adequate to the labor required of it. If wrought iron, for example, has been used in railroad bridges for a great number of years, in numerous and repeated instances, where a given load, in addition to the weight of structure, would produce upon it a tension of 15,000lbs. to the square inch, and has withstood such usage without cases of failure not caused by manifest defects in the quality of material, or by casualties which such structures are not expected to be proof against; it may be fairly assumed to be reasonably safe and reliable in other railroad bridges where a similar gross load can not produce a greater stress; and much more so, where a like load can only produce a stress one-half, or two-thirds as great.

Now, it is provided in the plan herewith presented, that a lead of 2,000lbs. to the lineal foot upon each pair of rails, on the whole, or any part of the length of the bridge, can not produce upon any part of the wrought iron work in the trusses, a tension exceeding 10,000lbs. to the square inch; and, to show that such provision is eminently safe and liberal, I proceed to give some examples of what the same material is liable to with the same load in other structures, where long and severe usage has fully proved its sufficiency.

And yet, it was deemed expedient by the author of this work, in the outset of the introduction of iron rail road bridges, to provide that 2,000lbs. to the foot upon each pair of tracks, should not give a stress exceeding 10,000lb to the square inch upon any part of the wrought iron work, not from a conviction that the material was unsafe under a stress of 15,000lbs. but to provide against the possible contingency of its being sometimes exposed to greater stress than that produced by a dead weight of 2,000lb. to the lineal foot.

XCII. The use of cast iron to sustain a tensile strain, should undoubtedly be avoided, as a general

To begin with an instance near at hand; the bridge from the island to the main shore on the Hudson River rail road at East Albany, has, in one of its stretches, trusses 48 feet long, in 8 panels. It is a double track bridge with three trusses, of which the middle one sustains onehalf of the two pairs of tracks, and of the loads passing over them.

The truss is composed of top and bottom chords, and thrust braces of timber, and vertical suspension bolts of wrought iron, in pairs; and it is at once obvious that of the weight of the tracks and their loads (or, of the half bearing upon the centre truss), is concentrated on the two pairs of suspension rods located 6 feet from each end. [See diagram.] The weight of middle truss, and other parts of the structure sustained by it, probably exceeds 16,000 lbs., of which 7, or 14,000 lbs. bear

FIG. 25A.

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upon the endmost suspension bolts. Add 2,000 lbs. per foot for of one pair of tracks, or rails, and it makes 56,000lbs. upon the suspension bolts in question, with only one track loaded. These bolts are 4 in number, and 18" in diameter; and, allowing 3 // to be cut away by screw thread, the aggregate net, available cross section of the four, is equal to 4.43 square inches; whence the tension, with only one track loaded, is 12,641 lbs. to the square inch, and 22,120 lbs. to the inch with both tracks loaded.

2. The bridge leading into the freight house of the Boston rail road, at East Albany, is a Howe bridge," and acts upon the same principle as the one just spoken of. It is a double track bridge with two trusses, having 8 panels of 10'8'', and is a heavy covered bridge. Allowing 64 tons for weight of superstructure, or 56,000 lbs. for the portion sustained by the endmost bolts of each truss, and 2,000 lbs per foot upon one track, of which at least, bears on one truss, giving

rule; and, if on certain occasions it should be liable to that kind of action to a small extent, the stress should probably not be allowed to exceed 3,000 to 4,000 pounds to the square inch.

When exposed to compression, in pieces of such length as to break by lateral deflection, it is believed it may be safely loaded to one-third of its absolute capacity. If a long piece exposed to a negative strain. have a defective part, it does not diminish its power of resistance to the same extent as when it acts by tension. The power of negative resistance being, in a measure, inversely as the deflection produced by a

100,000 lbs. on the end bolts, we have 156,000lbs. sustained by 6 bolts of 14" diameter, containing 8.1 square inches, besides screw thread. This is a strain of 19,259 lbs. to the square inch with one track, and 25,452 lbs with both tracks loaded with 2,000 lbs. to the lineal foot.

3. The East bridge over the creek in the south part of Troy, is a double track covered bridge with three trusses, having 8 panels of 12'8'' each, or 88.66 ft sustained by the endmost suspension bolts. Say, of weight of structure bearing on end bolts of middle truss, 35,00 · lbs. and of load upon one track 88,666, making 123,666 lbs. on 4 bolts of 17 diameter and two of 13" diameter, having a net cross-section of about 7.65 square inches. Hence the stress must be 16,156 lbs. to the inch, with one track loaded, and 27,750 lbs., with 2,000 lbs. to the foot up on each track.

4. The West bridge over the same stream, a few rods below the last mentioned, has three trusses containing 9 panels of 10 ft. each in length. It is a high truss bridge with roof and siding.

For weight of superstructure on endmost bolts of middle truss, say 28,000 lbs and for load on one track, 84,000, making 112,000 lbs. on 4 bolts of 13 containing a net section of 5.41 square inches, giving & tension of 29 702 lbs. to the inch for one track, and 36,229 lbs. for both tracks loaded with 2,000 lbs. to the lineal foot.

5. The bridge across the Erie canal near Canastota, on the N. C. RR., is a double tack bridge with 2 trusses, which have 9 par els of 10 feet. If the superstructure be estimated to weigh 40 tons, it gives a little over 35,000 Ibs, on the end bolts of each truss Add of 80 tous for 2,000 lbs. per lineal foot upon one track, and it gives 141,666 lbs. on 4 bolts of 14" diameter, and 5.41 square inches of net cross-section; equal to 26,173 lbs. to the inch, with one track, and 36 044 lbs. with both tracks loaded.”

All these cases are stated from personal examination by the author, except the last, which was reported to him from authority considered reliable. The cases were not selected, but taken as the most accessible and conveniemt for the author's observation. And still, he can not help regarding them as remarkable, and somewhat exceptional cases