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Fig, 332. Rectio* Of Thb Iron Bridge At Sunderland, 20
FEET FROM THE NORTHERN ABUTMENT.
on each side for the vessels. The centre for supporting the areh was fixed on this scaffolding. Some time after the centre was removed, the areh was found to have moved in a horizontal direction eastward, forming a curve of 12 to 18 inches versed sine. This dangerous and unexpected occurrence was skilfully counteracted by introducing transverse and diagonal tie-bars and braces, assisted by screws and wedges, by which the whole was restored to its original position.
Several iron bridges were erected by Telford, the first of which was that across the Severn, at Buildwas, in Shropshire, consisting of a single areh, 130 feet in span, with a versed sine or rise of 27 feet. The areh consists of 3 ribs, 9 feet apart, or I8 feet wide from out to out. These ribs are 3 feet 10 inches in depth, and connected transversely by tie-bars. The spandrils for supporting the roadway are formed of vertical bars of cast-iron, and the abutments are of stone.
The largest iron-areh bridge is the Southwark Brulge over the Thames, at London. It was designed and erected by Itennie. It consists of 3 arehes, all segments of the same cirele; the centre areh being 240 feet in span, with a rise of 24 feet, and the two side arehes being each 210 feet in span, with a rise of 18 feet 10 inches. The piers are 24 feet thick; the width of the roadway over the bridge is 28 feet, and the footways on cither side are each 7 feet in width. Each areh consists of 8 ribs, and each rib of
15 pieces, of such a depth that the rib is 6 feet deep at the crown, and 8 feet deep at the springing. The metal is 2J inches thick in the middle, and i\ inches at the top and bottom of the ribs. The ribs are con nected transversely by cast-iron tie-braces, of the same depth as the ribs, but open in the centre of each; and in the diagonal direction the ribs arc connected by another series of ribs, so that each areh consists of a series of hollow masses or voussoirs, similar to those of stone bridges. All the segmental castings forming each areh, as well as the transverse and diagonal tie-braces, are kept in their places by dovetailed sockets, and long cast-iron wedges, thus obviating the necessity for bolts. The spaudrils are composed of cast-iron diagonal framing, and the roadway is formed upon cast-iron plates, resting upon the spandrils, and joined with iron cement. The abutments and piers are of stone, built upon platforms of timber, and surrounded by guard or sheathing piles, driven into the bed of the river. In the erection of the bridge, the ribs were commenced in the centre of the span, and continued regularly on both sides towards the piers and abutments. Upon these, connecting and bed plates were secured in the masonry, and when the last segment of each rib was fixed, 3 wedges of cast-iron, each 9 feet long and 9 inches wide, were introduced behind each rib, and nicely fitted and adjusted to them. These wedges were formed with a very slight taper, and were driven simultaneously with heavy hammers, so that the arehes were nearly lifted from the centres, which were thus readily removed. The whole of the ironwork had been so carefully prepared, that, when the work was completed, scareely any sinking of the arehes could be detected. It was found, from experiments made during the progress of the works, that the average effect of the expansion caused by the summer increase of temperature was a rise of the arehes, to the extent of about lj inch at the crown, the abutments being fixed points. The weight of metal in the centre areh was 1,665 tons; in the two side arehes, 2,920 tons; making a total of 4,585 tons. The first casting for this bridge was run on the 1st January, 1815, and the bridge was opened 25th Mareh, 1819.
Iron-areh bridges are the same in principle as archbridges of stone and other materials, which derive their strength and stability by transferring the effect of the loads placed upon them to the abutments. But "when the peculiar properties of cast-iron had been studied, with a view to its extended application in buildings, and the proportions had been correctly determined for beams of this material, intended to supersede horizontal beams of wood, their employment in the formation of bridges of limited span soon followed; and in the railway works executed during the last twenty years, we have numberless examples of cast-iron girder-bridges, as we have also of cast-iron areh-bridges, of considerable dimensions, and great ingenuity of design and arrangement. The cast-iron girder-bridge, depending for its strength upon the sectional area of the girder, at that point in its length over which the weight or load acts, requires abutments to resist vertical pressure only; while the abutments of areh-bridges have to resist the lateral thrust of the areh. In the cast-iron girder-bridge, moreover, the depth of the structure is reduced to that of the section of material due to the maximum load; and hence the peculiar applicability of this form for railway bridges, in which it is desirable to preserve a minimum distance from the under side or soffit of the girder to the level of the roadway above. But the limitation of span to which girders are safely applicable has always restricted their employment in bridges, and 40 feet has commonly been considered the maximum length of bearing to which single castiron girders can be safely applied, liable to be loaded with railway trains, or other heavy weights."'
The desire to retain this convenient form of structure, and to extend its use to larger spans, has led to attempts to combine wrought-iron with cast metal; malleable-iron bars or rods being fitted to cast-iron girders, so as to form a kind of metal trussing, the depth of the truss being limited to that of the girder. But the great defect in these compound constructions consists in the difficulty of making the two kinds of iron act fully together in bearing the load. "The strength of cast-iron depends upon its rigidity; for, although it possesses the property of elasticity, this cannot be tasked with safety; and it is well known that repeated deflections will often destroy a casting which has withstood previous pressures with apparent impunity. Malleable-iron, on the other hand, applied in the form of truss-bars to cast-iron girders, is intended to act by the application of its tensile strength; but the effect of this can only be secured when it becomes active before the cast-iron girder has suffered any dangerous deflection. It is therefore indispensable that the adjustment of the length of the bars during all changes of temperature shall be strictly preserved,—a condition which is physically impracticable by any known form of construction or arrangement of parts." This defect was fatally illustrated by the failure of the largest bridge of this kind, erected over the Dee, near Chester, on the line of the Chester and Holyhead Railway. On the 24th May, 1847, one of the girders gave way, and the cause of the failure was variously ascribed to a passing train having got off the rails, and to an undue loading of the bridge with additional ballasting.
The first attempts to substitute wrought-iron for cast-iron in the construction of girders were made by joining plates of rolled-iron vertically with rivets, and attaching a strip of angle-iron on each side, both at top and bottom, so as to form artificial flanges, to give the required strength at those parts. Girders thus formed are said to have been applied by Messrs. Fairbairn in the construction of floors, in 1832, and they have been used as deck-beams in ships. Such girders were found liable to yield by twisting or bending laterally;
(1) "Tubular and other Iron Girder Bridges," &'c. By G. Drysdale Dempsey, C.E. This valuable little volume, forminone of the numbers of Mr. Weale's Rudimentary Series, has been of great assistance in the preparation of this Section.
and to obviate this defect, as well as to obtain the great strength and rigidity required in the employment of wrought-iron girders for railway constructions, the tubular form was designed, and T-iron used in forming vertical ribs, so that the side plates might be arranged vertically. Wrought-iron thus applied having been proved by experiment to have less power to resist compression than extension, it was desirable to increase the strength of the upper part of girders constructed of this material, and a separate compartment or cell was formed to obtain this object. These improvements were made by Mr. W. Fairbairn, and patented, October 8, 1846. The girders are formed of plates of metal, united by rivets and ribs of rolled-iron: the side plates are put together with but-joints, covered on the outside with stiles, or covering-plates, and on the inside with vertical ribs of angle or T-iron, the side plates, stiles, and ribs being riveted together. The top of this hollow beam is formed with two or more rectangular cells, composed of plates arranged vertically, and connected by strips of angle-iron and rivets with the top and side plates. The bottom is formed of iron plates, connected together by covering-plates over the cross-joints, and attached to the side plates by angle-iron and rivets. The top may be constructed either of cast or of malleable iron, and cellular, rectangular, or of an elliptical or any other form, to prevent the top puckering from compression; or other methods may be employed, such as thick metallic castings, or lighter iron plates, arranged so as to form hollow cells. The bottom may also be constructed of a series of plates, either of single or double thickness, riveted together. The joints of the plates alternate or break with each other, and are riveted by a peculiar method which the inventor calls chain-riceting, as it forms a chain of plates throughout; and the structure so unites the covering-plates as not to weaken the plates by rows of transverse rivet-holes, but to form a connecting link to each joint by a series of longitudinal rivets or pins. "This useful invention," says Mr. Dempsey, "which comprises the best methods yet devised for uniting the several parts of structures of plate and bar iron, contains also the essential principles upon which tubular girders may be, and have been, constructed, of a size adequate to form bridges within themselves, and admit the interior passage of railway trains or other traffic."
The first wrought-iron tubular girder bridge built under this patent, was over the Leeds and Liverpool Canal, for the purpose of carrying the Blackburn and Bolton Railway. Fig. 333 is an elevation, and Fig. 334 a transverse section of this bridge: Fig. 335 is an enlarged transverse section of one of the outer girders, and Fig. 336 an enlarged longitudinal view of part of one of the girders, showing a section of one of the cross timbers on which the railway is supported. The span of this bridge is 60 feet, and each girder is 66 feet in total length, the bearings of the masonry being cach 3 feet long. The two lines of rails are carried between three parallel girders, G, each of which consists of a
Fig. 335. Fig. 336.
angle-iron; of side plates T"ff inch thick, joined vertically by rivets to T-iron ribs, and also riveted to the bottom plate of the top compartment, and its internal angle-irons, through longitndinal ribs of angle-iron placed externally; and of double bottom plates, each J inch thick, joined by rivets to external longitndinal strips of angle-iron. The rails are laid upon longitndinal timbers, which, with intermediate planking, arc supported upon transverse beams of wood suspended by double straps of wrought-iron, which pass upwards through the bottom plates of the girders, and are secured by screw nuts. A vertical bolt of wrought-iron also passes through a cast-iron socket in the top compartment of the girder, and downward through each cross-beam, below which it is fixed with a washer-plate and screwed nut. This structure on being tried by severe tests was found equal to any weight to which it could be subjected. Three locomotives, each weighing 20 tons, occupying the entire span of 60 feet, were run together as a train, at rates varying from 5 to 25 miles an hour, and produced a deflection in the centre of the bridge of only .025 of a foot. Two wedges of the height of 1 inch were then placed on the rails in the middle of the span, and the dropping of the engines from this height, when at a speed of 8 or 10 miles per hour, caused a deflection of only .035 of a foot, which was increased to .045 of a foot, or nearly half an inch only, when wedges l£ inch in thickness were substituted. A bridge of this kind of 60 feet span, containing 30 tons weight of iron, cost only 900/.; whereas a cast-iron trussed girder bridge of the same, required 76 tons of castron, and 14 tons of wrought-iron, and cost 1,432/. 16$. Malleable-iron has also been applied in various other forms of combination to the construction of bridges.
The Patent Iron Bridge suggested by Mr. George Smart in 1824, consisting of a combination of wroughtiron bars, arranged in a diagonal form, is the parent of the numerous lattice-bridges so common in America. These present a vertical framing perfectly horizontal on its upper and lower lines, and composed of iron bars crossing each other in a diagonal direction, forming angles of about 18° with the horizon. (Fig. 337.) The framing also comprises vertical or hanging-bars, and 4«se-bars forming the lower horizontal lines of the framing, and also passing horizontally over each alternate row of intersections of the diagonal bars. Each bar is forged of enlarged width at the points of intersection, through which bolts are fixed to connect the whole together. Two of these trussed frames erected vertically and parallel to each other form the supports of the roadway to be formed between them, the two frames being tied together by transverse connecting rods, the roadway or flooring being situated near the top of the frames, and never on the lower bars, which Mr. Smart regarded as an erroneous practice in wooden bridges. Between the frames, crossbraces, consisting of two light bars, are fixed, bolted together and fitted to the connecting rods. A lattice bridge of wrought-iron erected across the line of the Dublin and Drogheda Railway is 84 feet in clear span. Fig. 337 is a portion of a lattice bridge erected by Mr. Osborne on the Waterford and Limerick Railway.
Mr. Dempsey points out an important distinction between the simple lattice or diagonal framing, and the roof framing. In the former, the strength is obtained by the connexion of the bars at each intersection, while the abutting principle of the roof is disregarded. The strain is therefore home wholly by the rivets or pins which pass through the crossing bars, and the effect of this strain is shown in the gradual loosening of the pins. The bars are also considerably weakened by the holes through the middle of them, and in wooden lattice bridges, fracturc and failure of the material have often resulted. The lattice principle has been considerably improved upon by Mr. R. B. Osborne, C. E., by the introduction of a top and bottom chord of malleable-iron with intermediate braces of cast-iron in the form of rectangular tubes.
For other forms and applications of tubular beams and girders, we must refer to Mr. Dempsey's work, and pass on to notice the grand application of the hollow girder or lube in the Conway and Britannia Bridges, on the line of railway from Ohester to Holyhead. The first of these tubular bridges was required for carrying the railway over the river Conway, and at about 18 miles further on, the separation of the Island of Anglesea from the main land of Caernarvonshire by the straits of Menai, gave occasion for the bolder structure of which the central pier is skilfully based on a rock, termed the Britannia Rock, from which the bridge derives its name.
It was the original intention of Mr. Robert Stephenson, the engineer of the works, to have crossed the straits with a cast-iron arched bridge, in two spans of 450 feet each, the height of the arches to be 100 feet from the level of the water to the crown of the arch, and the springing 50 feet from the level. As it was necessary that the water-way should not be interrupted by scaffolding or centering, it was proposed to fix the half-arches in each side of the central Dier in portions simultaneously, and connect them with tie-rods, so that the weight on either side should balance that on the other.
This design for an arched bridge was frustrated by a condition insisted on by the Lords Commissioners of the Admiralty, as conservators of the navigation: viz. that the clear height of water-way under the lowest part of the arches, or their springing, should not be less than 105 feet. To have complied with this condition, the whole structure must have been raised 50 feet above the proposed position, thereby involving an immense additional cost in the piers and aoutments, besides being irreconcilable to the adjoining levels of the railway. The engineer, therefore, abandoned the arched form altogether, and "conceived the original idea of a huge tubular bridge, to be constructed of riveted plates of wrought-iron, and supported by chains, and of such dimensions as to allow of the passage of locomotive engines and railway trains through the interior of it." In April, 1845, Mr. Stephenson obtained the assistance of Mr. William Fairbairn in carrying out his scheme. The first idea seemed to be, that the tube should be either of a circular or an egg-shaped sectional form, and that it should act by it3 rigidity and weight as a stiffener, and prevent, or, at least to some extent, counteract, the undulations due to the catenary principle of construction. Mr. Fairbairn appears from the first to have been opposed to the application of chains, even as an auxiliary. "I always felt that, in a combination of two bodies, the one of a perfectly rigid, and the other of a flexible nature, there was a principle of weakness; for the vibrations to which the one would be subjected would call into operation forces whose constant action upon the rivets and fastenings of the other could not but tend to loosen them, and thus, by a slow, but sure agency, to break up the bridge."l
But Mr. Stephenson wisely determined that these and all other points should be submitted to the test of an elaborate experimental inquiry, to determine, first, the peculiar sectional form which should be
(1) "Conway and Britannia Tubular Bridges." By William Fairbairn, C.E. London. 181g.
given to the tubes, and, secondly, the distribution and dimensions of the material which would ensure the required strength and rigidity of the entire structure. These experiments were conducted by Mr. William Fairbairn, assisted at a later period, dating from the 19th September, 1845, by Mr. Eaton Hodgkinson; the details and results of which were reported to Mr. Stephenson, who appended them to his own report, presented to the directors of the railway company in February 1846. An account of these experiments is given in Mr. Fairbairn's work, and an abstract of them is given in Mr. Dempsey's little volume. Experiments were made on the strength of iron tubes of various forms: Mr. Fairbairn's object being first directed to test the principle, originally suggested by Mr. Stephenson, of a structure every part of which, although rigid, should be brought into a state of tension, and whose strength should consist not as that of a beam or girder does in its resistance to extension on the one side and to compression on the other, but in a resistance to extension on both sides. It was found in the early experiments that the weakness of the tube was in its upper surface, which yielded to compression before the under side was upon the point of yielding to extension. Experiments were then made with a view to strengthen the upper surface, so that it should not be on the point of yielding to compression until the under surface was about to yield by extension. "This state of the tube was a necessary condition to the greatest economy of its material, for in any state in which it was not on the point of yielding on the one side at the instant when it was on the point of yielding on the other, some of the material might be taken from the stronger side without causing that to yield and added to the weaker so as to prevent that side from yielding, and thus the tube would be rendered stronger by a new distribution of its material. It was with reference to this principle that the rectangular form of section had suggested itself to me, in the place of the circular or the elliptical forms proposed by Mr. Stephenson, and that I had ordered the top of the tube to be thickened. It now occurred to me that the top might be strengthened more effectually by other means than by thickening it, and I directed two additional tubes to be constructed, the one rectangular, and the other elliptical, with hollow triangular cells ox fins to prevent crushing. These experiments led to the trial of the rectangular form of tube with a corrugated top, the superior strength of which decided me to adopt that cellular structure of the top of the tube which ultimately merged in a single row of rectangular cells. It is this cellular structure which gives to the bridges now standing across the Conway Straits their principal element of strength." On the 9th February, 1846, Mr. Stephenson made his Report to the Directors of the Chester and Holyhead Railway, in which he gives a summary of Messrs. Fairbairn and Hodgkinson's experiments. "The first serics of experiments," he says, "was made with plain circular tubes, the second with elliptical, and the third with rectangular. In the whole of these, this remarkable and unexpected fact was brought to light,—viz. that in such tubes the power of wrought-iron to resist compression was much less than its power to resist tension,—being exactly the reverse of that which holds with castiron: for example, in cast-iron beams for sustaining weight, the proper form is to dispose of the greater portion of the material at the bottom side of the beam,—whereas with wrought-iron these experiments demonstrate beyond any doubt, that the greater portion of the material should be distributed on the upper side of the beam. We have arrived therefore at a fact having a most important bearing upon the construction of the tube; viz. that rigidity and strength are best obtained by throwing the greatest thickness of material into the upper side. Another instructive lesson which the experiments have disclosed is, that the rectangular tube is by far the strongest, and that the circular and elliptical should be discarded altogether. This result is extremely fortunate, as it greatly facilitates the mechanical arrangements for not merely the construction, but the permanent maintenance of the bridge."
Appended to Mr. Stephenson's Report are separate Reports, the one by Mr. Fairbairn and the other by Mr. Hodgkinson. Mr. Fairbairn remarks that "with tubes of a rectangular shape, having the top side about double the thickness of the bottom, and the sides only half the thickness of the bottom, or one fourth the thickuess of the top, nearly double the strength was obtained. In experiment 14, a tube of the rectangular form, 9j inches square, with top and bottom plates of equal thickness, the breaking
weight was 3,738lbs.
^ . <J$kcting a stronger plate on the top
lie, the strength was increased to 8,273lbs.
The difference being . . 4,535lbs.;
onsiderably more than double the strength sustained by the tube when the top and bottom sides were equal. The experiments given in No. 15 are of the same character, where the top plate is as near as possible double the thickness of the bottom. In these experiments the tube was first crippled by doubling up the thin plate on the top side, which was done with a weight of .... 3,788lbs. It was then reversed with the thick- side upwards, and by this change the breaking weight was increased to 7.148 lbs.
Making a difference of . 3,360 lbs.; or an increase of nearly double the strength, by the simple operation of reversing the tube, and turning it upside down.
"The same degree of importance is attached to a similar form when the depth in the middle is double the width of the tube. From the experiment (No. 16) we deduce the same results in a tube where the depth is I8 J and the breadth 9£ inches. Loading this tube with 6,812 lbs. (the thin plate being
uppermost,) it follows precisely the same law as before, and becomes wrinkled with a hummock rising on the top side, so as to render it no longer safe to sustain the load. Take, however, the same tube and reverse it with the thick plate upwards, and you not only straighten the part previously injured, but you increase the resisting power from 6,812 lbs. to 12,188 lbs. Let us now examine the tube in the 29th experiment, where the top is composed of corrugated iron, forming two tubular cavities extending longitndinally along its upper side. This, it will be observed, presents the best form for resisting the puckering or crushing force, which on almost every occasion was present in the previous experiments. Having loaded the tube with increasing weights, it ultimately gave way by tearing the sides from the top and bottom plates, at nearly one and the same instant after the last weight, 22,469 lbs. was laid on. The greatly increased strength indicated by this form of tube is highly satisfactory; and provided these facts be duly appreciated in the construction of the bridge, they will, I have no doubt, lead to the balance of the two resisting forces of tension and compression."
Referring to the suspension-chains which had been proposed as an additional element of security in the bridge, Mr. Fairbairn remarks, that "although suspension-chains may be useful in the construction, in the first instance, they would nevertheless be highly improper to depend upon as the principal support of the bridge. Under every circumstance, I am of opinion that the tubes should be made sufficiently strong to sustain, not only their own weight, but, in addition to that load, 2,000 tons equally distributed over the surface of the platform; a load ten times greater than they will ever be called upon to support In fact, it should be a huge sheet-iron hollow girder, of sufneient strength and stiffness to sustain these weights; and, provided the parts are well proportioned, and the plates properly riveted, you may strip off the chains, and leave it as a useful monument of the enterprise and energy of the age in which it was constructed."
In the Table at the top of the next page, some of the data and the results of experiments upon rectangular tubes are brought together.
When the course of these experiments had indicated the form of tube which must actually be employed for the bridge, a model tube was constructed, as nearly as possible one-sixth the dimensions of the proposed tubes across the Menai Straits. The apparatus used for experimenting upon it is shown in Figs. 338 and 339. Fig. 339 represents at A a sideview, 75 feet between the supports; B is the platform supporting the weights, suspended from two cross-bars passing through the sides, and resting upon the outside angle-iron and the bottom of the tube, as shown in Fig. 338 at a a. cc is a balk of timber, passing under the cross-links ib, which, on being raised by two powerful screw-jacks ee, lifted the load from off the tube, in order to ascertain the effects of the successive loads upon the elastic powers of the tube. The temporary supports D D were only used