Sir Benjamin Baker
(31 Mar 1840 - 19 May 1907)
THE FORTH BRIDGE.
BY BENJAMIN BAKER, C.E.
from Chambers’s Journal (1888)
[p.545]Twenty-three years ago Parliamentary powers were obtained by the North British Railway Company to construct a bridge across the Firth of Tay, and a bridge across the Firth of Forth, for the purpose of ‘securing for the North British, Great Northern, North-Eastern, and Midland Railways, a fair share of the through-traffic between England and the north of Scotland, hitherto practically monopolised by the London and North-Western and Caledonian Railways.’ As all the world knows, the original Tay Bridge was constructed and opened for traffic in May 1878; blown down with the loss of seventy-five lives in December 1879; re-constructed, and again opened for traffic in June of last year. The Forth Bridge authorised by the twenty-three-year-old Act of Parliament was to have crossed the Forth at a point five miles above Queensferry where the water was shallow; but the mud proving to be of practically unlimited depth, the project was of necessity abandoned, and another point of crossing selected. This was found at Queensferry, where the island of Inchgarvie stands as a steppingstone in the middle of the channel, leaving, however, a depth of over 200 feet of water on either side—too deep for intermediate piers—and consequently a bridge having two openings of the unprecedented span of 1700 feet became a necessity, and Parliamentary powers were obtained for its construction fifteen years ago.
Owing to the fall of the Tay Bridge, the original design for the Forth Bridge on the suspension principle was abandoned, and the far more rigid cantilever girder design of Messrs Fowler and Baker was substituted. Operations were commenced in the spring of 1883 by the establishment of large bridge-building works at Queensferry, with special appliances and machines of a novel character adapted to the bending, planing, drilling, and riveting of the 50,000 tons of steel plates and bars required for the superstructure of the great Bridge. The work since then has been continuously pressed on by day and by night, and the termination of the arduous labours of the engineers and contractors is now within measurable distance, for it is anticipated that the autumn of next year will see the completion of the Bridge.
Since the designs of the Forth Bridge were published, many cantilever bridges have been built in America and elsewhere, and the term cantilever has thus become familiar to the public. Such was not the case originally, and one of the first questions asked by visitors to the Forth Bridge was, ‘What is a cantilever bridge?’ The word ‘cantilever’ is, as will be shown in Dr Murray’s new English Dictionary, several hundred years old. It means simply a bracket or projecting arm; and a cantilever bridge consists of two such brackets, and a central beam connecting the two ends. When lecturing recently at the Royal Institution, I exhibited what might be termed a living model of the Forth Bridge, arranged as follows: Two men sitting on chairs extended their arms and supported the same by grasping sticks butting against the chairs. This represented the two double cantilevers. The central beam was represented by a short stick slung from the near hands of the two men, and the anchorages of the cantilevers by ropes extending from the other hands of the men to a couple of piles of bricks. When stresses were brought to bear on this system by a load on the central beam, the men’s arms and the anchorage ropes came into tension, and the sticks and the chair-legs into compression. In the Forth Bridge it is to be imagined that the chairs are placed a third of a mile apart; that the men’s heads are 340 feet above the ground; that the pull on each arm is about 4000 tons, the thrust on each stick over 6000 tons, and the weight on the legs of the chair about 25,000 tons.
The advantages of the cantilever system of construction as regards simplicity and rigidity were appreciated by the Chinese hundreds of years ago, and many timber structures on that [p.546] principle are still to be found. No important metallic structure of the kind was, however, in existence previous to the publishing of the designs of the Forth Bridge. The advantages of the system under the conditions found at the Queensferry crossing are enormous. Thus, as the superstructure can be erected without scaffolding, it is immaterial whether the water be two feet or two hundred feet deep. Again, as the cantilevers are built by commencing with the work over the piers, and adding successive portions of steel-work on each side until the cantilevers project the required distance, there is perfect solidity at all stages of the erection, and there are none of those periods of risk and anxiety which occur when girders are built on temporary staging, or are floated into position on pontoons, or otherwise erected. All of the anticipated advantages of the system have been fully realised in the case of the Forth Bridge, for at the present moment the cantilevers project about half their full length over the sea; upwards of 28,000 tons of steel-work have been erected, and not a single plate or bar has been lost or injured in any way during the wildest gales.
At no period of the operations has the Forth Bridge presented greater features of novelty and interest to its thousands of visitors than at present; nevertheless, there were times in the past when works now hidden and forgotten called for all the vigilance and skill of the engineers and contractors. Such were the pier-works at Inchgarvie and South Queensferry. Each of these piers consists of four columns of concrete and masonry, about seventy feet in diameter, founded on rock or boulder clay at depths up to ninety feet below high-water. The usual way in this country of building such piers is to enclose the site within cofferdams and pump out the water. In such a stormy estuary as the Forth this could not be done, so the piers were founded on enormous diving-bells, seventy feet in diameter, the masonry being built on the top of the bells, and the men working within the same, excavating the earth and passing it through air-locks into the open air, and so, by a process of undercutting, sinking the pier like a huge pile through the soft soil to a solid substratum. Powerful air-pumps kept the diving-bells charged with compressed air, by which means the water was excluded, and the men worked in a brilliantly lighted chamber seventy feet in diameter, at a depth of ninety feet below sea-level, as readily as on dry land. Of course it is not given to every one to work with comfort in a place where the barometer stands as high as one hundred and twenty inches, which it did in the Inchgarvie diving-bell caissons. One of the first sensations in passing from the ordinary atmospheric pressure into compressed air is a painful pressure on the drums of the ears, which is relieved by swallowing. A long continuance in a high pressure leads to paralysis of the nerves, the workmen walk with difficult step and a slight stoop, violent cramps and death often supervene. When Glaisher and Coxwell made their high balloon ascent in 1862, the barometer fell to seven and a half inches, and temporary paralysis of the nerves then occurred; but the matter for surprise is rather that the human organism should sustain at all such wide ranges of atmospheric pressure as from seven and a half inches to one hundred and twenty inches of mercury, than that some amount of personal inconvenience or danger should result from it.
When the masonry piers had been securely founded in the manner described on the rock, or hardly less firm boulder clay forming the bed of the Forth, the erection of the steel superstructure was commenced. Over the piers are lofty steel towers made of four columns 12 feet in diameter and 340 feet high, bound together in all directions to resist wind-storms and the forces resulting from the passage of the heaviest and fastest trains on the East Coast route. An ascent to the top of these towers, at the height of the golden cross on the dome of St Paul’s Cathedral, is an event not soon forgotten. Hundreds of visitors, men of science of all nations, turbaned Indian princes, and even venturesome young ladies have done it, and all alike have been impressed by the sublimity of the scene. Standing on the edge of the top platform and glancing down at the workmen hanging in mid-air by fine wire ropes, at the steam-barges manœuvring below laden with portions of the structure, the vessels of all classes at anchor or sailing, and the whole grand panorama of the Firth of Forth, the scene recalls vividly that passage in King Lear where Edgar leads Gloucester to the edge of the cliff:
And dizzy ‘tis to cast one’s eyes so low!
The crows and choughs that wing the midway air
Show scarce so gross as beetles.
Experience has shown that in a very short time workmen lose all sense of the height at which they are working, and that they can not only glance down, but climb down without any feeling of dizziness.
The steel towers being built, the next step was to commence the erection of the great cantilevers. These each project 680 feet from the towers over the sea, and consist of a curved bottom member, shaped like a fishing-rod, tapering from twelve feet diameter at the butt to five feet diameter at the end, connected by diagonal bracing to an inclined top member of lattice construction. The massive steel-work is erected without scaffolding by powerful steam cranes and winches carried by the Bridge itself.
Everything in connection with the Forth Bridge except the rolling of the steel plates has been done on the spot, and this has necessitated the establishment at the little burgh of Queensferry of one of the largest bridge-building works in the kingdom, capable of turning out 1500 tons of finished girder-work every month. More than half a million sterling has been expended in machinery, buildings, railways, steamboats, and [p.547] other plant. The number of men employed on the works has at times been as high as 4300. Much of the work at the Forth Bridge requires men possessed of great coolness, courage, and hardiness. Nervousness would simply induce an accident, and consequently when crawling along narrow planks or angle bars with a clear drop of three or four hundred feet below them, the men have to dismiss from their minds all ideas of what students of dynamics call the motion of a falling body under the unbalanced action of its own weight. Unfortunately, men have fallen from all heights on to the lower staging, and into the sea; but having reference to the novelty and difficulty of the work, the number of accidents has been singularly small. The works have been carried on under the personal direction of Sir John Fowler, K.C.M.G., and Mr Benjamin Baker, the engineers, and Mr William Arrol, the chief contractor, aided by a large staff of clever and zealous assistants.
It must be admitted on all hands that the great Forth Bridge will be the crowning work of the railway system in this country, and that nothing of the kind of equal importance can reasonably be expected to follow it. It will also be admitted that it would be difficult to exaggerate the benefits which railways have conferred upon this country. At the beginning of the century, when Mr Pitt wanted a few millions, the terms he offered were one hundred and fifty-seven pounds of three per cents. for one hundred sovereigns. We have lately seen Mr Goschen’s successful conversion of the National Debt into a two and three-quarter per cent. stock, and there is little doubt that the altered conditions are largely due to the changes wrought by the development of the railway system. When, therefore, it is asked whether the end will justify the means, and whether the saving in time and distance will pay for the heavy expenditure on the Forth Bridge, the obvious answer is that time is becoming more and more a priceless commodity, and that the quickest route, irrespective of almost all other considerations, will carry the traffic and earn the money. The opening of the Forth Bridge will in all probability lead to a noteworthy acceleration of the already fast running of the northern expresses. That such is practicable is proved by actual experience both in this country and America. A speed of 75 miles is often attained on the Great Northern Railway, and it was also attained two years ago on the New York Central Railway, when an average speed of 65½ miles an hour was maintained for the whole distance of 149 miles between Syracuse and Rochester. Sooner or later, as railway managers have found out to their cost, whatever can be done to improve the train service has to be done, and as the construction of the Forth Bridge has demonstrated the practicability of building railway bridges of great span, New Yorkers have ceased to be content with ferry-boats, and demand the substitution of a bridge across the Hudson. Two such projects are before the public—one a bridge having two spans of 1600 feet each, and another with a single span of 2800 feet. No further evidence is required of the great influence which the Forth Bridge will exercise on the railways of the future; for it is already clearly shown that the Forth Bridge, great work though it be, is the pioneer of still greater works in countries whose physical features and commercial requirements demand the building of railway bridges of great span.