CHAPTER II.
A NEW IDEA—THE BRITANNIA TUBULAR.
“Wemust cross the Strait at the Britannia Rock—that is settled.”
“And where is the Britannia Rock?”
“Nearly in mid-channel. It seems placed there for the purpose.”
And the great engineer smiled.
“What are the distances?”
“From coast to coast the span of the Strait is some 1100 feet, with that rock in the centre. Now the problem is, to build a bridge across that gulf of surging water strong enough to bear heavy trains at high speeds, and sufficiently above the water to prevent any interference with navigation.”
“And how will you manage it?”
“First I thought of large cast-iron arches, but they will not do. I doubt if they would stand the strain; and moreover we should impede navigation by raising scaffolding during the building. At length I came to the idea of a tube bridge.”
“What! a tube bridge! I’ve never heard of it!”
“No, it is a new idea. By reconsidering a design I had made for a small bridge over the Lea at Ware in 1841, and thinking over the matter, I came to the idea that a bridge consisting of a hollow beam or tube might solve the difficulty.”
“A huge hollow girder, so to speak!” exclaimed his friend.
“Exactly so. Accordingly,” the engineer continued, “I had drawings prepared and calculations made, by which to ascertain the strength of such a bridge, and they were so satisfactory that I decided on attempting one.”
“It is like constructing one huge hollow beam of iron by rivetting plates together. Can it be done?” remarked his friend.
“The making of the high-level bridge over the Tyne, in which I had a part—the bridge between Newcastle and Gateshead, you know—was a transition between an arched bridge and a girder bridge. A girder of course is a beam, it may be of iron or wood, and the little bridge at Ware has been built of girders made of plates of wrought-iron rivetted together. Therefore, you see, I am not unused to wrought-iron girders, and what they will bear.”
“Why, it is like a huge extension of the primitive log-bridge of our ancestors.”
“If you like,” replied the engineer, laughing.
Robert Stephenson—for he it is whom we suppose to be speaking to his friend on this gigantic engineering enterprise—became satisfied by reflection that the principles involved in constructing an immense tubular beam were but a development of those commonly in use; and Sir William Fairbairn was entrusted with the duty of experimenting as to the strength of tubes, the directors of the Railway Company voting a sum of money for the purpose.
Sir William, then Mr., Fairbairn concluded that rectangular tubes were the strongest, and a model was made of the suggested bridge. It proved successful, and indicated that the tube would be able to stand the strain of a heavy train passing rapidly over it.
In September, 1846, Mr. Fairbairn read a paper on the subject at the meeting of the British Association at Southampton, as also did Professor Hodgkinson, a mathematician, who had verified Fairbairn’s experiments. Not long afterwards Stephenson became satisfied that chains were not needed to assist in supporting the bridge, and that his tubes would be strong enough to support themselves entirely between the piers.
Work therefore went forward. Some 1500 men were engaged on the Britannia Bridge, and the quiet shores of the Menai Straits resounded with the busy hum of hammers and machinery. Cottages of wood were built for the men, and workshops for the punching and rivetting of the plates for the gigantic tubes.
The design included two abutments of masonry on either side of the Strait, and three towers or huge piers, one of which, the centre pier, was to rise from the Britannia Rock, 230 feet high. There are four spans, two over the water of 460 feet each, and two of 230 feet each over the land. Two tubes, quite independent of each other, but lying side by side, form the bridge across. Each tube or beam is 1510 feet long, andweighs 4680 tons. Its weight at one of the long spans is 1587 tons.
Now how could these gigantic tubes be put together and raised to their positions? Here was a problem almost as great as the original one of the bridge itself, and it troubled the engineer sorely.
ROBERT STEPHENSON.
ROBERT STEPHENSON.
“Often at night,” he declared, “I would lie tossing about, seeking sleep in vain. The tubes filled my head. I went to bed with them, and got up with them. In the gray of the morning, when I looked across Gloucester Square, it seemed an immense distance across to the houses on the opposite side. It was nearly the same length as the span of my tubular bridge.”
The principle adopted was to construct the shorter tubes on scaffolds in the places which they were tooccupy. This could be done, for such scaffolding would not impede navigation. But scaffolding could not be built for the large tubes across the great spans of water. What then was to be done?
It was decided to build them on platforms on the shore quite close to the water, and float them when ready on pontoons to their places between the piers, raising them to their position by hydraulic power. Such a task would be hazardous enough. It was first tried at Conway, where a similar bridge was being built by Robert Stephenson, being indeed part of the same railway. The Britannia was, however, a much greater enterprise, though the span of the Conway is 400 feet. The Conway bridge, indeed, is but of one span, and contains two tubes.
The experience at Conway was of great benefit to the gigantic undertaking at the Menai Strait. The floating of the first tube was to take place on the 19th of June, 1849, in the evening; but owing to some of the machinery having given way, the great event was put off to the next night. The shores were crowded with spectators. When the tube was finished it could be transferred to the pontoons; for the tubes had been built at high-water mark. When the pontoons were fairly afloat on this fateful evening, they were held and guided by leading strings of mighty strength. Stephenson himself directed in person, from a point of vantage at the roof of the tube. Thence he gave the signals which had been agreed upon, whilst a crew of sailors, directed by Captain Claxton, manned the strange barque.
A pontoon is a light, buoyant boat, and the tube was supported on sets of these, their speed increasing terribly as they approached their place by the towers. The idea was, as related by Mr. Edwin Clark, Stephenson’s assistant, that they should strike a “butt” properly, underneath the Anglesey Tower, “on which, as upon a centre, the tube was to be veered round into its position across the opening. This position wasdetermined by a twelve-inch line, which was to be paid out to a fixed mark from the Llanfair capstan. The coils of the rope unfortunately over-rode each other upon this capstan, so that it could not be paid out.”
Destruction seemed imminent. The capstan was actually dragged from the platform, and the tube seemed likely to be swept away. Then Mr. Rolfe, the captain of the capstan, shouted to the spectators, and threw out a spare twelve-inch rope. Seizing this, the crowd, with right good-will, rushed it up the field, and clung tightly to it, checking the voyage of the mighty tube. It was brought to the “butt,” and duly turned round.
A recess had been left in the masonry of the tower, and the end near the Britannia pier was drawn into it by means of a chain. The Anglesey end followed. Then the tide gradually sank, the pontoons sank with it, and the tube subsided also to a shelf which had been made at either end. The first stage was accomplished; the mighty tube was in position to be raised.
Shouts of rejoicing burst from the sympathetic crowds, and the boom of cannon joined its congratulatory note at the grand success. But the further stages remained. At midnight the pontoons were all cleared away, and the huge, hollow beam hung silent over the surging water. It rested on the shelves or beds prepared for it at either end. The second great operation, of course, was to haul it up the towers to its permanent position. This was to be performed by hydraulic machinery of great power, and Mr. Stephenson’s instructions were to raise it a short distance at a time, and then build under it.
He took every imaginable precaution against accident or failure; and well was it that he did so, for an accident happened which, but for the careful building under the tube in the towers as it was raised, would have been most calamitous. The accident occurred while Mr. Stephenson was absent in London. One day, suddenly, while the machinery was at workraising the tube, the bottom burst from one of the hydraulic presses, and down fell the tube on to the bed provided for it.
Though the fall was but nine inches, tons weight of metal castings were crushed, and the mighty tube itself was strained and slightly bent. But it was serviceable still, and the fact that it stood the strain so well showed its great strength. It weighed some five thousand tons, and for such an immense weight to fall even three-quarters of a foot was a very severe test.
But for Stephenson’s wise precaution in lifting it slowly, and building underneath it as it was raised, the tube would have crashed to the bottom of the water. As it was, the accident cost £5000; but the tube was soon being hauled upward again. In due course the others followed, and on the 5th of March, 1850, Robert Stephenson inserted the final rivet in the last tube, and the bridge was complete. He crossed over with about a thousand persons, three locomotives whirling them along.
The tubes of the bridge are made of iron plates, and at the top and bottom are a number of small cells or tubes—instead of thick iron plating—which assist in giving strength to the whole gigantic tube. Thus it may be said the floor and roof are tubular, as well as the body. These hollow cells appear to have been Fairbairn’s invention. The size of the tube grows slightly larger at the middle by the Britannia tower, where externally the tubes are 30 feet high, and 26 internally, while they are 22¾ feet and 18¾ feet at the abutments. The width is 14 feet, 8 inches externally, and 13 feet 5 inches inside.
At the Britannia tower the tubes are placed solidly on their bed, but at the abutments, and at the land towers, the tubes rest on roller-beds. This arrangement was adopted to permit of expansion and contraction. Iron, of course, solid and unyielding as it appears, is yet very susceptible to warmth, and the effect of thesun’s rays on this massive iron structure is very marked. A rise of temperature causes it to expand in a comparatively short time, and it is said that the tubes occasionally move two and a-half inches as the sun gleams upon them. Mr. Edwin Clark observed the effect of the sun on the iron, which appears in a small degree to be always moving as the temperature varies. Well, therefore, that the able engineer planned an arrangement allowing for this constant expansion and contraction of the iron mass.
THE BRITANNIA TUBULAR BRIDGE.
THE BRITANNIA TUBULAR BRIDGE.
The Britannia Bridge was a great triumph for Robert Stephenson. He appears first to have seized the idea, and, assisted no doubt by Fairbairn’s experiments and by able coadjutors, he carried it through to a successful completion. He was of course the son of George Stephenson, who had done so much for the locomotive, and according to Smiles, “he almost worshipped hisfather’s memory, and was ever ready to attribute to him the chief merit of his own achievements as an engineer.”
“It was his thorough training,” Mr. Smiles once heard him remark, “his example, and his character, which made me the man I am.” Further, in an address as President of the Institution of Civil Engineers, in January, 1856, he said: “All I know, and all I have done is primarily due to the parent whose memory I cherish and revere.”
That father had died before the Britannia Bridge was completed, though he had been present at the floating of the first tube at Conway. The great engineer passed away on the 12th of August, 1848, at the age of sixty-seven, and his distinguished son Robert, who had no children, only survived him by eleven years.
But before he died he had designed, and Mr. A. M. Ross, who had assisted at the Conway Bridge, had assisted in carrying out the celebrated Victoria Tubular Bridge over the great St. Lawrence River at Montreal.
This bridge was for the Grand Trunk Railway of Canada, and for immense length and vastness of proportions, combined with magnificent strength, is one of the wonders of the world. It is five times as long as the Britannia Bridge, being not far short of two miles. It has a big central span of 330 feet, and twenty-four spans of 242 feet. The iron tubes are suspended sixty feet above the water beneath.
VICTORIA TUBULAR BRIDGE, MONTREAL.
VICTORIA TUBULAR BRIDGE, MONTREAL.
One great difficulty in the problem was the ice.Immense quantities come down in the spring, and to resist this enormous pressure the piers are most massive, containing thousands of tons each of solid masonry. These piers are based on the solid rock, the two central towers being eighteen feet in width and the others fifteen feet. To protect them from the ice, huge guards made of stone blocks clamped with rivets built up in the form of an incline were placed before the piers on the up-stream side. The bridge was begun in July, 1854, and occupied four and a-half years in construction,it being completed in December, 1859, about two months after its designer had died.
Gigantic though this structure is, and great as is the honour which it reflects on Robert Stephenson and the resident and joint engineer Mr. Ross, yet with the exception of the remarkable and massive ice-guards to the piers, it does not differ materially from the Britannia and Conway Tubular Bridges. These were the first famous examples of the new principle.
Why, then, are massive tubular bridges not more generally built? Because they led to another and very natural development in bridge-building, a development whereby great strength for long spans is gained, with, however, a marked saving both in labour and in material. That development was the lattice bridge.