| hour, the deflection became 1 inch; that is, more than double the statical deflection. "Since the velocity so greatly increases the effect of a given load in deflecting the bars, it follows, that a much less load will break the bar when it passes over it than when it is placed at rest upon it; and accordingly, in the example above selected, a weight of 4,150 lbs. is required to break the bars, if applied at rest upon their centres; but a weight of 1,778 lbs. is sufficient to produce fracture, if passed over them at the rate of 30 miles an hour. It also appeared that, when motion was given to the load, the points of greatest deflection, and, still more, of the greatest strains, did not remain in the centre of the bars, but were removed nearer to the remote extremity of the bar. The bars, when broken by a travelling load, were always fractured at points beyond their centres, and often broken into four or five pieces, thus indicating the great and unusual strains they had been subjected to." 1

All these experimental trials were conducted by Engineers have generally supposed that the deflechanging a dead weight to the tubes under trial. The tion caused by passing a weight at a high velocity mechanical effect of such a weight is much less in- over a girder is less than the deflection which would jurious than a weight passing over or through it with be produced by the same weight at rest; and even greater or less velocity, as in the case of a railway when they have observed an increase, they have attrain. The commissioners appointed by her Majesty's tributed it solely to the jerks of the engine or train, Commission, of the 27th August, 1848, to inquire produced by passing over inequalities at the junction into the conditions to be observed by engineers in the of the rails, or other similar causes. For the purpose application of iron to structures exposed to violent of examining this question, the commissioners subconcussions and vibration, &c., investigated this mitted two actual bridges to the test of experiment. subject experimentally. They constructed an appa- One was the Ewell Bridge, on the Croydon and ratus by means of which a loaded car was allowed to Epsom line, and the other the Godstone Bridge, run down an inclined plane: the iron bars which were upon the South-Eastern line, both constructed to the subject of the experiment were fixed horizontally carry the railway over a road. "A scaffold was at the bottom of the plane, in such a manner that the constructed, which rested on the road, and was thereloaded car would pass over them with the velocity ac-fore unaffected by the motion of the bridge, and a quired in its descent. Thus, the effects of giving different velocities to the loaded car, in depressing or fracturing the bars, could be observed, and compared with the effects of the same loads, placed at rest upon the bars. Thus, for example, when the carriage, loaded to 1,120 lbs., was placed at rest upon a pair of cast-iron bars, 9 feet long, 4 inches broad, and 1 inch deep, it produced a deflection offths of an inch; but when the carriage was caused to pass over the bars at the rate of 10 miles an hour, the deflection was increased to ths, and went on increasing as the velocity was increased; so that, at 30 miles per application of Iron to Railway Structures." 2 vols. folio. 1850.

VOL. I.

pencil was fixed to the under side of one of the girders of the bridge, so that when the latter was deflected by the weight of the engine or train, either placed at rest or passing over it, the pencil traced the extent of the deflection upon a drawing-board attached to the scaffold. An engine and tender were made to traverse the bridges at different velocities, or to rest upon them, at pleasure. The span of the Ewell Bridge is 48 feet, and the statical deflection due to the above load rather more than th of an inch. This (1) "Report of the Commissioners appointed to inquire into the

R

was slightly but decidedly increased when the engine | one span only, of 400 feet clear width, and two abut-
was made to pass over the bridge; and, at a velocity ments of masonry, of which the design is in harmony
of 50 miles per hour, an increase of 4th was observed. with that of the castle.
As it is known that the strain upon a girder is nearly
proportional to the deflection, it must be inferred,
that, in this case, the velocity of the load enabled it
to exercise the same pressure as if it had been in-
creased by 4th, and placed at rest upon the centre of
the bridge. The weight of the engine and tender was
39 tons, and the velocity enabled it to exercise a
pressure upon the girder equal to a weight of about
45 tons. Similar results were obtained from the
Godstone Bridge."

With respect to the best qualities and mixtures of iron for girders, it appears that engineers have no guarantee that the mixture for which they have stipulated in a contract shall be that used by the founder, and no certain test by which to determine whether a given piece of iron has been manufactured by hot blast or cold blast. Mr. Fox recommends, as a good protection, "that engineers, in contracting for a number of girders, should stipulate that they should not break with less than a certain weight, (leaving the mixture to the founder,) and cause one more than the required number to be cast. The engineer may then select one to be broken, and, if it break with less weight than that agreed upon, the whole may be rejected.”

The details which are now about to be given will refer chiefly to the construction of the Britannia Bridge. The Conway Bridge was erected within a few feet of Telford's suspension bridge, and close beneath the walls of Conway Castle. It consists of

On the

The site selected by Mr. Stephenson for carrying his tubular bridge over the Menai Straits was determined by a mass of rock, occupying the centre of the stream, and of suitable dimensions to serve as the foundation of a central pier, and standing considerably above the level of low water. The distance of this rock, and of the bridge now built over it, from the great suspension bridge of Telford, is one mile lower down the straits, or in a southern direction. Caernarvon side, the shore rises abruptly from the water's edge, and shelves upward with a gentle incli nation; so that a horizontal line passing at an elevation of 100 feet over the water, is, when extended about 400 feet inland from the water-line, only a few feet above the natural surface of the ground. On the Anglesea side, the rocky surface extends for a considerable distance, and, at a length of about 250 feet from the water-line, the surface is from 80 to 90 feet below the horizontal line just described. Hence, the embankment required to continue the railway from the Anglesea end of the bridge is much higher and more extended than that at the Caernarvon end. The Britannia Rock, which rises from near the middle of the bed of the strait, is covered at high water to a depth of 10 feet, and stands at low water about 10 feet above it. A tower of masonry is erected on this rock, and at the clear distance of 460 feet from it, at the limits of the water-way, another tower is built on either side of it, Fig. 340. At the distance of 230 feet

Fig. 341. PLAN OF THE BRITANNIA TUBULAR BRIDGE.

from each of these towers is a continuous abutment of masonry, 176 feet in length. The sides of these five stupendous masses of masonry are tapered with a straight batter. The Britannia tower is 62 feet by 52 feet 5 inches, at the base, and is reduced by the batter to 55 feet 5 inches at the height of 102 feet above high-water line, at which level the tubes pass through it. A plinth extends round the base of this and the other towers; and the height of this tower above high-water level is 200 fect, or nearly 230 feet from the bottom of the foundation on the rock. The stone used for the external parts of all the masonry a hard and durable limestone, known as Anglesea

is

APR 1960

RY

230

marble. The interior of the masonry is a soft red sandstone, quarried at Runcorn, in Cheshire. The Britannia tower is constructed with hollow spaces or chambers within it. The total weight of the masonry in this tower is about 20,000 tons, and about 387 tons of cast-iron, in beams and girders, are built in it. The foundations were laid, and the work constructed up to the level of high-water, during the intervals of the tide. The stones in the whole of the masonry were left with the quarry or rough face, except at the angles, where they were dressed to a square arris, and in the recesses and top entablature, where they were dressed to a fair face all over. The Anglesea and

Caernarvon towers have the same dimensions at the | parallel tubes, each 1,513 feet long. To unite each base as the Britannia tower, but the height is 10 feet of the four sections, short lengths of tube were conless. The abutments are 176 feet in length, and of structed within the towers, which, being united with a width corresponding to the towers, viz. 55 feet at the main lengths, make up each complete and conthe level of the bottom of the tubes. The continua- tinuous tube. For the four shorter, or land tubes, tions of the abutments are surmounted with parapet scaffolds were erected, and the tubes constructed at walls of solid masonry and of considerable height, each once in their final position; but the four main tubes of which terminates at the extremity of the bridge were constructed on timber platforms on the shore, with a projecting pedestal, on which a colossal and conveyed in flat-bottomed vessels, or pontoons, to couchant lion faces the approaching visitor. Each the towers, where they were deposited, and raised to lion is composed of 11 pieces of limestone, and their required elevation of 102 feet above high-water measures 25 feet in length and 12 feet in height, level by means of hydraulic presses. In this way, all weighing about 30 tons. They were executed by scaffolding across the straits was avoided, and the Mr. Thomas. interruption of only half the channel at one time was limited to the brief period occupied in raising each tube from the base of the towers.

The four spaces between the Britannia tower and the other towers, and between these and the abutments, had to be spanned by the iron tubes, and, as each tube serves for only one line of rails, 8 tubes were required, 4 of 460 feet, and 4 of 230 feet, the four longer ones being over the water, and the four shorter ones over the land. Thus it will be understood that each line of way through the bridge is composed of four separate tubes, united together, so that in the double line of railway there are two

The site selected for the construction of the 4 main tubes was on the margin of the Caernarvon shore, to the south of the bridge. An intermediate space was occupied with workshops, &c., and cottages were built for the accommodation of about 500 workmen. Four strong stages were erected upon piles, and a continuous platform laid from end to end of the site, Fig. 342, consisting of timber posts and

of it a small crab, Fig. 343, moving upon wheels, was made to traverse the width of the work, and was thus applied to raise the plates and materials. Portable furnaces accompanied the men employed in riveting.

Fig. 344, is a cross section of one of the tubes, showing the general form of construction: the sides are parallel, but the height is slightly varied. The height externally is 30 feet at the centre in the Britannia tower, and this is reduced to 22 feet at the extremities in the abutments, the bottom line being horizontal, but the top line forming a parabolic curve, the rise of which thus equals the difference in height, or 7 feet 3 inches. The clear internal height is 26 feet at the centre, and 18 fect at the ends. The external width is 14 feet 8 inches, and internally 14 feet, which is further reduced 7 inches by the ribs.

The covering of the tubes consists of malleable iron plates, connected together by rivets with ribs of T and L-iron, besides strips of flat bar-iron over the joints. The tubes are strengthened at top and bottom with internal longitudinal tubes or cells, of which there are 8 in the upper part and 6 in the lower. The plates vary in dimensions and thicknesses. The side plates are reduced in thickness from the ends towards the middle of the tube, and those forming the top and bottom are increased in the same direction. The side plates are alternately 6 feet and 8 feet 8 inches long, and all 2 feet wide. They are arranged vertically, so that the joints occur

Fig. 350 | larger dimensions, or about 5 feet in height and 1 foot in width.