Lives of the Engineers / The Locomotive. George and Robert Stephenson

by Samuel Smiles

Horizontal girders are of three kinds. The Tubular is constructed of riveted rectangular boiler plates. Where the span is large, the road passes within the tube; where the span is comparatively small, the roadway is supported by two or more rectangular beams. Next there is the Lattice girder, borrowed from the loose rough timber bridges of the American engineers, consisting of a top and bottom flange connected by a number of flat iron bars, riveted across each other at a certain angle, the roadway resting on the top, or being suspended at the bottom between the lattice on either side. Bridges on the same construction are now extensively used for crossing the broad rivers of India, and are especially designed with a view to their easy transport and erection. The Trellis or Warren girder is a modification of the same plan, consisting of a top and bottom flange, with a connecting web of diagonal flat bars, forming a complete system of triangulation—hence the name of “Triangular girder,” by which it is generally known. The merit of this form consists in its comparative rigidity, strength, lightness, and economy of material These bridges are also extensively employed in spanning the rivers of India. One of the best specimens is the Crumlin viaduct, 200 feet high at one point, which spans the river and valley of the Ebbw near the village of Crumlin in South Wales. This viaduct is about a third of a mile

long, divided into two parts by a ridge of hills which runs through the centre of the valley—each part forming a separate viaduct, the one of seven equal spans of 150 feet, the other of three spans of the same diameter. The bridge has been very skilfully designed and constructed, and, by reason of its great dimensions and novel arrangements, is entitled to be regarded as one of the most remarkable engineering works of the day.

“In calculating the strength of these different classes of girders,” Mr. Stephenson observed, “one ruling principle appertains, and is common to all of them. Primarily and essentially, the ultimate strength is considered to exist in the top and bottom,—the former being exposed to a compression force by the action of the load, and the latter to a force of tension; therefore, whatever be the class or denomination of girders, they must all be alike in amount of effective material in these members, if their spans and depths are the same, and they have to sustain the same amount of load. Hence, the question of comparative merit amongst the different classes of construction of beams or girders is really narrowed to the method of connecting the top and bottom webs, so called.” In the tubular system the connexion is effected by continuous boiler plates riveted together; and in the lattice and trellis bridges by flat iron bars, more or less numerous, forming a series of struts and ties. Those engineers who advocate the employment of the latter form of construction, set forth as its principal advantage the saving of material which is effected by employing bars instead of iron plates; whereas Mr. Stephenson and his followers urge, that in point of economy the boiler plate side is equal to the bars, whilst in point of effective strength and rigidity it is decidedly superior. To show the comparative economy of material, he contrasted the lattice girder bridge over the river Trent, on the Great Northern Railway near Newark, with the tubes of the Victoria Bridge. In the former case, where the span is 240½ feet, and the bridge 13 feet wide, the weight including bearings is 292 tons;

in the latter, where the span is 242 feet, the width of the tube 16 feet, the weight including bearings is 275 tons, showing a balance in favour of the Victoria Tube of 17 tons. The comparison between the Newark Dyke Bridge and the Tubular Bridge over the river Aire is equally favourable to the latter; and no one can have travelled over the Great Northern line to York without noting that, as respects rigidity under the passing train, the Tubular Bridge is decidedly superior. It is ascertained that the deflection caused by a passing load is considerably greater in the former case; and Mr. Stephenson was also of opinion that the sides of all trellis or lattice girders are useless, except for the purpose of connecting the top and bottom, and keeping them in their position. They depend upon their connexion with the top and bottom webs for their own support; and since they could not sustain their shape, but would collapse immediately on their being disconnected from their top and bottom members, it is evident that they add to the strain upon them, and consequently to that extent reduce the ultimate strength of the beams. “I admit,” he added, “that there is no formula for valuing the solid sides for strains, and that at present we only ascribe to them the value or use of connecting the top and bottom; yet we are aware that, from their continuity and solidity, they are of value to resist horizontal and many other strains, independently of the top and bottom, by which they add very much to the stiffness of the beam; and the fact of their containing more material than is necessary to connect the top and bottom webs, has by no means been fairly established.” Another important advantage of the Tubular bridge over the Trellis or Lattice structure, consists in its greater safety in event of a train running off the line,—a contingency which has more than once occurred on a tubular bridge without detriment, whereas in event of such an accident occurring on a Trellis or Lattice bridge, it must infallibly be destroyed. Where the proposed bridge is of the unusual length of a mile and a quarter, it is obvious

that this consideration must have had no small weight with the directors, who eventually decided on proceeding with the Tubular Bridge according to Mr. Stephenson’s original design.

From the first projection of the Victoria Bridge, the difficulties of executing such a work across a wide river, down which an avalanche of ice rushes to the sea every spring, were pronounced almost insurmountable by those best acquainted with the locality. The ice of two thousand miles of inland lakes and upper rivers, besides their tributaries, is then poured down stream, and, in the neighbourhood of Montreal especially, it is often piled up to the height of from forty to fifty feet, placing the surrounding country under water, and doing severe damage to the massive stone buildings along the noble river front of the city. To resist so prodigious a pressure, it was necessary that the piers of the proposed bridge should be of the most solid and massive description. Their foundations are placed in the solid rock; for none of the artificial methods of obtaining foundations, suggested by some engineers for cheapness’ sake, were found practicable in this case. Where the force exercised against the piers was likely to be so great, it was felt that timber ice-breakers, timber or cast-iron piling, or even rubble-work, would have proved but temporary expedients. The two centre piers are eighteen feet wide, and the remaining twenty-two piers fifteen feet; to arrest and break the ice, an inclined plane, composed of great blocks of stone, was added to the up-river side of each pier—each block weighing from seven to ten tons, and the whole were firmly clamped together with iron rivets.

To convey some idea of the immense force which these piers are required to resist, we may briefly describe the breaking up of the ice in March, 1858, while the bridge was under construction. Fourteen out of the twenty-four piers were then finished, together with the formidable abutments and approaches to the bridge. The ice in the

river began to show signs of weakness on the 29th March, but it was not until the 31st that a general movement became observable, which continued for an hour, when it suddenly stopped, and the water rose rapidly. On the following day, at noon, a grand movement commenced; the waters rose about four feet in two minutes, up to a level with many of the Montreal streets. The fields of ice at the same time were suddenly elevated to an incredible height; and so overwhelming were they in appearance, that crowds of the townspeople, who had assembled on the quay to watch the progress of the flood, ran for their lives. This movement lasted about twenty minutes, during which the jammed ice destroyed several portions of the quay-wall, grinding the hardest blocks to atoms. The embanked approaches to the Victoria Bridge had tremendous forces to resist. In the full channel of the stream, the ice in its passage between the piers was broken up by the force of the blow immediately on its coming in contact with the cutwaters. Sometimes thick sheets of ice were seen to rise up and rear on end against the piers, but by the force of the current they were speedily made to roll over into the stream, and in a moment after were out of sight. For the two next days the river was still high, until on the 4th April the waters seemed suddenly to give way, and by the following day the river was flowing clear and smooth as a millpond, nothing of winter remaining except the masses of bordage ice which were strewn along the shores of the stream. On examination of the piers of the bridge, it was found that they had admirably resisted the tremendous pressure; and though the timber “cribwork” erected to facilitate the placing of floating pontoons to form the dams, was found considerably disturbed and in some places seriously damaged, the piers, with the exception of one or two heavy stone blocks, which were still unfinished, escaped uninjured. One heavy block of many tons’ weight was carried to a considerable distance, and must have been torn out of its

place by sheer force, as several of the broken fragments were found left in the pier.

The works in connection with the Victoria Bridge were begun on the 22nd July, 1854, when the first stone was laid, and continued uninterruptedly during a period of 5½ years, until the 17th December, 1859, when the bridge was finished and taken off the contractor’s hands. It was formally opened for traffic early in 1860; though Robert Stephenson did not live to see its completion.