JOURNAL

OF

THE FRANKLIN INSTITUTE

OF THE STATE OF PENNSYLVANIA

FOR THE

PROMOTION OF THE MECHANIC ARTS.

JULY, 1848.

CIVIL ENGINEERING.

Strength of Materials for Railway Bridges.

The president of the Royal Scottish Society of Arts, (G. Buchanan, Esq.,) at the request of the council, presented an important communication, at their last meeting, entitled-" An Exposition on the Strength of Materials, particularly, Cast Iron and Malleable Iron, and their Application in the Construction of Railway Bridges."

Mr. Buchanan commenced by stating, that he did not profess to communicate anything original, but would be happy if he could only draw from the stores of information which had of late years been accumulating on this subject under the hands of very eminent, scientific, and practica! men, such leading facts and maxims as might prove a sure guide for our practice; and such truths, when they become known and established on the unerring grounds of experiment and calculation, could not, he thought, be too widely disseminated. The various strains might all be reduced to two kinds, according as the material is either distended or compressed by any force or pressure. From these two all others arise, and either consist or are compounded of them.— The tensile strain is the simplest of all, depending neither on the peculiar form of the materials, nor even on the length, but only on a single element-namely, the section of fracture. This peculiarity of the tensile force was explained and illustrated. In regard to cast iron, the result of the extensive and interesting experiments of Messrs. Hodgkinson and Fairbairn was given; and it was found from the mean. of 16 different trials of English, Welsh, and Scotch iron, both hot and cold blast, that this material will sustain about 7 tons per square inch before breaking, the weakest specimen being 6, and the strongest 94. VOL. XVI, 3RD SERIES.-No. 1.-JULY, 1848

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The limit of fracture, however, can never be approached with safety, not even within a long distance, seeing that this material is liable to unseen imperfections, and, above all, to snap in a moment, without distending itself, or giving any warning of danger. Malleable iron, again, is much superior in tensile strength, and, by its remarkable ductility, inspires confidence in a still higher degree; bears no less, at an average, by various experiments of Telford and Brown, than 27 tons -the weakest 24, and the strongest 29 tons; but, before the half of this load is applied, it begins to stretch, and continues stretching up to the limit of fracture; it is, therefore, not only three times stronger than cast-iron, but may be safely loaded with five times the breaking weight, or about 8 or 9 tons. In regard to the strength of compression, this depends also, as long as the length is limited, on the same elementthe section of fracture; but when a long rod or slender pillar is loaded or compressed, it is liable to bend, not for want of strength, but for want of stability, the least flexure turning it off its centre, and breaking it by lateral force-deranging entirely the simple law applicable to short lengths. In regard to cast-iron, by far the most satisfactory experiments are those by Hodgkinson and Fairbairn. The mean result gives very nearly 50 tons on the square inch-the weakest 36 tons, and the strongest 60 tons. It is thus six times stronger in compression than in distension; and hence it is peculiarly recommended for sustaining any superincumbent weight, as in the case of pillars and of bridges, provided the construction is such as to resolve the strain arising from the load into a longitudinal compression. This is often. in our power by proper arrangements, chiefly giving a sufficient height and curvature to the arch; but in cases where, for the want of headroom, the arch is unduly flattened, or resolved into a straight beam or girder, the danger is that we bring the tensile force into play, and then the use of cast-iron is objectionable, or, at least, requires extreme caution. No direct experiments have been made on malleable iron of short lengths; but from some facts brought out by Mr. Hodgkinson, its strength appears much inferior to cast-iron, chiefly from ductility, whereby it gives way much sooner under a load. It will bear 27 tons, probably much more, without fracture; but with 12 tons it yields to the load, contracts longitudinally, and swells out laterally; and this is another very important fact for our guidance in the use of those different materials. In regard to stone, experiments have been generally made on specimens rather too minute. Like cast-iron, the crushing strength is superior to the tensile, and hence its adaptation for buildings, particularly bridges. Craigleith stone will bear 2 tons on the inch, or upwards of 400 tons on the square foot-Aberdeen granite 600 tons. In regard to bricks, he had occasion to make experiments in relation to the great chimney of the Edinburgh Gas Works. It became matter of consideration, whether the ordinary brick could withstand the pressure of so lofty a coluinn. Trials were, therefore, made with a powerful hydrostatic press, not on small specimens, but on the actual brick. The ordinary stock brick was found to bear 140 tons on the square foot, and the common fire-brick 157 tons; but the brick of which the chimney is constructed, consisting of a mixture of fire

clay and ironstone, bore, a single brick on its bed, no less than 140 tops, equal to 400 tons on the square foot.

The effect of the transverse strain was then considered, and illustrated by various experiments and models. The strain is a compound of the tensile and compressive strain, the one part of a beam loaded in the middle being compressed, and the other distended, and the beam. itself becoming a lever, and acting often with enormous power against its own strength. Hence it became easy to calculate the strength, this being in every case proportional, in the first instance, to the area of the section of fracture, and this original element, modified by the length and depth of the beam, diminishing in exact proportion to the length, and increasing in proportion to the depth.

The transverse strain acting with such severe advantage against our materials, various methods have been contrived for eluding its effects; and for these none is more remarkable than the principle of the arch, the effect of which was illustrated by experiments, and particularly the necessity in flat archies of having secure abutments to resist the horizontal thrust-and this was frequently accomplished, where there is sufficient headroom, by uniting the extremities of the arch by strong malleable iron rods, in the same manner as in the case of the roof; the feet of the rafters are united and prevented from spreading by the tie beams; and this is the principle, the securest of all, on which the great iron bridge at Newcastle, now in progress, is constructed-the object of which is to cross the river and valley of the Tyne, on the highest level of the railways on each side, so as to unite them in one uninterrupted line from London to Berwick, and unite the termini of the different railways, now separated three quarters of a mile or more, into one grand central station, a little to the west of the ancient castle. The distance between this station and the present terminus of the York and Newcastle Railway is 3457 feet, consisting chiefly of the space occupied by the bed of the river Tyne, and the steep bank on each side, well known to travelers in descending from Gateshead Fell on the south, and Dean street on the north, both to be now superseded by the smooth and level surface of the railway, and by a turnpike road running on the same bridge directly under the line of rails. The steep banks on each side are spanned by stone arches of a very substantial character, the river and low banks by six metallic arches, all of the same dimensions and structure, resting on solid piers and lofty columns of masonry. In the bed of the river the piers are laid on very solid foundations of piles and planking, with concrete-many of the piles 40 feet in length, and driven to this depth through hard gravel and sand, till they reach a bed of freestone rock. Nasmyth's celebrated pile driver is in full operation here, and with wonderful effect, and has come most opportunely in aid of the work; driving night and day, at the rate of 60 or 70 strokes a minute, the pile heads often being set on fire by the rapidity and violence of the blows of the ram. Piers laid 2 feet below water mark, and raised about 100 feet to the springing of the arches. The arches consist each of four main ribs of cast-iron, each in five segments, bolted together, and forming one entire arch, 125 feet span, and rising 17 feet 6 inches in the centre, and the level of

the rails on the upper platform 1084 feet above the level of high watermark of the Tyne. Depth of rib 3 feet 9 inches at the springing, and 3 feet 6 inches at the crown, with flanges 12 inches broad, external ribs 2 inches thickness of metal, internal ribs 3 inches. Total sectional area at the crown 644 square inches, which would bear with safety a load of 5000 or 6000 tons, and would form, with proper abutments, a strong arch in itself; but for the fullest security, and to prevent the possibility of inconvenience of risk from deflection or vibration, or otherwise, each rib is united at the springing by strong malleable iron bars, or ties, 7 inches broad and 1 inch deep, of the best scrap iron, and in all 24 in number. The railway is supported above the arch, and the roadway suspended from beneath, by hollow cast-iron pillars 10 feet apart, and each 14 inches square, through which are passed strong malleable iron circular bars, binding the whole into one stiff and solid mass. The sectional area of the horizontal bars is 168 square inches, which would sustain upwards of 4000 tons without breaking, and 1500 tons with perfect safety, but the whole weight of the bridge will not exceed 700 tons, leaving 800 tons of surplus strength. The railway, which is at the summit level, runs on a level 4 feet above the crown of the arched rib, and is supported in the middle by hollow cast-iron trough girders resting on the top of the pillars 10 feet apart, and united by longitudinal timbers laid with strong planking. The roadway runs nearly on a level with the malleable iron ties, leaving a space of about 20 feet clear headroom. In the whole of the work the utmost pains have been bestowed on materials and workmanship, and in making everything complete, the surfaces, which abut together, being regularly planed or turned, as in machinery; and, from all the arrangements, the most successful results may be anticipated from this bridge. The cost of the ironwork and roadway, by the estimates, comes to £112,000, and the contracts for the bridge and viaducts to above £300,000.-Scottish Railway Gazette. Lond. Min. Journ.

The General Railroad Law of the State of New York.

An Act to authorise the formation of Railroad Corporations. [Passed March 27, 1848.]

The People of the State of New York, represented in the Senate and Assembly, do enact as follows:

Section 1. Any number of persons, not less than twenty-five, being subscribers to the stock of any contemplated railroad, may be formed into a corporation for the purpose of constructing, owning, and maintaining such railroad, by complying with the following requirements: When stock to the amount of at least one thousand dollars for every mile of the road so intended to be built, shall be in good faith subscribed, and ten per cent. paid thereon, as hereinafter required, then the said subscribers may elect directors for the said company; and thereupon, they shall severally subscribe articles of association, in which shall be set forth the name of the corporation, the number of years the