Forth Bridge

“ We now discard our exploring garments, and having enjoyed a not unneedful wash, we quit the works, and returning homewards, congratulate each other on having trodden the very foundations of the wonderful Forth Bridge, and ponder how little the future traveller, as he lightly skims the estuary at sixty miles an hour, will think of the practical ingenuity and patient labour that wrought, deep down beneath the waters of the Forth, the foundations on which repose the huge structures through which the flying express is whirling him. ”

“ In the atmosphere of enthusiasm that naturally surrounds the successful completion of so vast a structure as the Forth Bridge, one is too apt to forget, in presence of the stupendous work, how much is due to the great army of workers who have laboured—blindly in some cases—since the beginning of the century, and to ascribe some of the credit to its creators which is really due to the general advance in mechanical science characteristic of the age. ”

“ The central girder portion is simply an ordinary double-line railway bridge of 350 ft. span with girders of a type intermediate between the girder of parallel depth and the bowstring. This is an economical type, and many Continental bridges have been so constructed.
When lecturing some years ago, Mr. Baker, with a view of presenting in a form easily understood and popularly remembered, a simple diagram of the manner in which the principal stresses of a cantilever bridge are distributed, devised the following arrangement of a human cantilever, or a living model of the Forth Bridge. ”

“ That this is not so is proved by the fact that large numbers of the Forth Bridge men—carpenters, fitters, and ordinary labourers—were frequently employed below, and many of their number for the first time, when the pressure was already considerable, without experiencing any great inconvenience or harm. ”

“ A quadruple row of timbers, their ends being set in the masonry of the bank, and the pier supports the sides; the centre part is laid from side to side." Making allowance for difference of material the preceding work may fairly be looked upon as the prototype of the proposed Forth Bridge. ”

“ Baker does not labour under the common illusion that a very great undertaking must necessarily be the work of very great men; and, indeed, he told the Mechanical Engineers at Edinburgh that he would be sorry not to believe that there were plenty of engineers and contractors in England, Europe, and America capable of building a Forth Bridge, if called upon to do so ”

“ Messrs Fowler and Baker were the engineers of the Forth railway bridge. It was begun in 1883 and finished in 1890. It is built nearly all of steel, and is one of the most stupendous works of the kind. It crosses two channels formed by the island of Inchgarvie, and each of the channel spans is 1710 feet in the clear and a clear headway of 150 feet under the bridge. Three balanced cantilevers are employed, poised on four gigantic steel tube legs supported on four huge masonry piers. The height of the bridge above the piers is 330 feet. ”

“ Foreign engineers all favor this principle of the Forth Bridge, it is said, since the first publication of the design. Practically every big bridge throughout the world has been built on that principle. ”

“ At each abutment he put up a silly tower pierced with arrow-slits and armed with battlements, advertising a farce of warlike make-believe which scores of foolish engineers would copy and adapt, while leaving their bridges entirely unfortified.
A bridge here and there is supposed to be all right. [Pg 350] Take, for instance, the Forth Bridge, with his 51,000 tons of steel, and his amazing cost, about £3,000,000; he is looked upon as a “safe” bridge, and safe he is if we forget what bombs and shells can do in a few seconds. ”

“ The bridge across the Zambesi is a fit mate in audacity to the falls themselves. It is the highest in the world for it rises 400 feet above the low water level. Its main parabolic arch is a 500 foot span while the total length is 650 feet. Although its construction was fraught with contrast hazard it only cost two lives, despite the fact that seven hundred white men and two thousand natives were employed on it. In the building of the Firth of Forth bridge which was much less dangerous, more than fifty men were killed. ”

“ But, long before 1862, the Warren and other truss-girders had come into use, and I am inclined to say that, so far as novelty in the principle of girder-construction is concerned, I must confine myself to that combination of principles which is represented by the suspended cantilever, of which the Forth Bridge, only now in course of construction, affords the most notable instance. It is difficult to see how a rigid bridge, with 1,700 foot spans, and with the necessity for so much clear headway below, could have been constructed without the application of this principle. ”

“ THE FORTH BRIDGE, CANTILEVER TYPE; ORIGINAL AND FINAL DESIGNS
MESSRS. HARRISON, BARLOW, FOWLER, AND BAKER, ENGINEERS.
On Cantilever Bridges Generally
Some little confusion of thought would appear to exist in many minds as regards this type of construction. Most people are willing to concede the antiquity of the arch and of the suspension system, but are doubtful whether the "continuous girder," if it be rechristened "cantilever and central girder," be not a modern and patentable invention. As a matter of fact, it is a pre-historic arrangement. ”

“ The building of the Forth Bridge Station, and the widening of cutting, bridges, and embankments not at first contemplated, will bring up the total cost to about 20,000l. The contract will probably not be finished until the end of April. ”

“ On March 4, 1890, during a violent gale which blew from the south west with a pressure of 20 lb. per square foot, the Prince of Wales closed the last rivet in the north central girder, and from the south port of the south cantilever end pier declared the bridge open. ”

“ An early estimate has fixed the number of rivets in the Forth Bridge at 5,000,000, but this was evidently insufficient and the figure has risen to 6,500,000. It is, however, doubtful whether even this covers the total amount, for on the central or Inchgarvie pier, where an exact record of rivetting was kept, the number closed there amounts to near upon 2,700,000 alone, and a very large quantity of material was sent across from the shops and the field which had already been rivetted up, and such rivets are not included in the above total. The rivets varied as to diameter from 1+1⁄8 in. ”

“ To enable the contractors to make an early start with the permanent work, that is, the building of the masonry piers, it was necessary that the positions of these should be fixed without delay. A base line about 4000 ft. in length was laid down along the high ground on the south shore, starting from a point in the centre line of the bridge, passing along the North British Railway for some distance and terminating opposite the east breakwater at Port Edgar. Along the breakwater a timber gangway was erected, and near the far end of it, at a distance of about 3000 ft. ”

“ Should the project of making a ship canal between the Forth and Clyde lately brought to the notice of the public ever be carried into execution, and should the Forth thus become an international highway, as well as one of the finest natural harbours of refuge in the world, the largest vessel yet built would have to do no more than strike its topmasts should it happen to pass at the hour of high water of an extra high spring tide. ”

“ The most violent gales which have occurred during the construction of the Forth Bridge are given with the pressures recorded on the wind gauges in the annexed Table, No. I.
* These data are unreliable, owing to faulty registration by the indicator needle, as will be explained. They were altered after this date. The barometer fell to 27.5 in. on that occasion—over 3⁄4 in. within an hour. ”

“ This scheme also proved abortive, and for forty years more the travelling public put up with what they could get, but at last in 1860 the North British Railway fixed upon a site about six miles to the west of South Queensferry as suitable for the construction of a railway bridge. The bridge was to have consisted of a number of large spans of 500 ft. each in the centre, with approaches in shorter spans at either end. ”

“ A revenue of 10 per cent, on the capital expended was considered a very moderate estimate, which proves that the art of writing a highly coloured prospectus is of older date than most people would have thought. To judge by the estimate the designer can hardly have intended to put more than from 2000 to 2500 tons of iron into the bridge, and this quantity distributed over the length would have given the structure a very light and slender appearance, so light indeed that on a dull day it would hardly have been visible, and after a heavy gale probably no longer to be seen on a clear day either. ”

“ Nor must the bridge be looked upon as a thing standing by itself, but rather as a portion—certainly a somewhat expensive portion—of a gigantic system of railway lines converging from all directions upon the capital of Scotland, affording means not only of more speedy and more comfortable travelling, but also giving facilities for the provision of a larger number of those through trains which are constantly becoming more necessary by the yearly addition of many miles of both main lines and branch lines. ”

“ The special character of the structure, and its want of resemblance to a bridge, is set forth in the defendants' answer. [22] Neither man nor beast can cross on it, save in the defendants' cars. The viaduct is for a kind of vehicle which no bridge known in 1790 can carry. It is wholly open. The only roadway is two or more iron rails, separated by a distance of four feet asunder. It is in fact no more a bridge than a sieve is a bucket. ”

“ The first bridge for railway traffic is at Alloa, 20 miles from Queensferry, and the next at Stirling. The Granton-Burntisland passage (see map on the next page) is seriously affected by the weather in the winter months, is often impassable during strong gales, and at the best of times the disembarkation from train to boat and from boat to train is a source of considerable discomfort to passengers, and what is worse a great waste of time. ”

“ Expansion joints are also provided in the approach viaduct girders upon every second pier, two spans being made continuous, the intermediate fixed joints being, however, placed on sliding bedplates identical with the movable ones.
In the two large spans of the cantilever bridge, longitudinal movements are only possible at the Inchgarvie ends of the central girders, the Fife and Queensferry ends of the girders being fixed so far as this movement is concerned. These ends therefore move with, and in the same direction as, the cantilevers upon which they are resting. ”

“ For the sinking of the caissons a number of foreign workmen were employed for a short time, and it is rather a curious coincidence that, as foreign workmen did some of the earliest work in connection with the bridge, so now again a number of foreign men are employed upon some of the last work—namely, the laying of the asphalt pavement along the footpaths of the permanent way, which is done under a sub-contract by the Seyssel Asphalt Company. ”

“ It was, therefore, necessary from time to time, that is every 30 ft. or 40 ft., to secure the members by the interposition of temporary struts or ties either in the shape of lattice girders or of timber balks, and even timber trusses. Thus, after two lifts had been made with the platforms, the diagonal struts in the central towers had not only deflected downwards by their own weight, but had also deflected towards the centre line of the bridge. ”

“ In his case, however, success came early, and within three years he had advanced so far in means and reputation, that he was intrusted with the contract for iron bridges on the Glasgow, Hamilton, and Bothwell Railway, one of them being a very large structure spanning the Clyde. ”

“ The choice of a material for constructing a bridge of novel design, of extraordinary magnitude, and exposed during erection to the effects of powerful atmospheric disturbances, must have been the subject of much anxious thought and reflection to the engineers. ”

“ The whole north approach line, from the abutment of the bridge to Inverkeithing, is on a gradient of 1 in 70, except at one point, where it is level for about 100 yards.
The sum for this contract was 88,678l., increased through extra work by 18,000l., or a total of 106,6782l.—equal to something over 56,000l. per mile. ”

“ In the erection of the bottom members in bay 2 the temporary attachments necessary to hold them up in position were carried partly to the point of intersection of struts 1 and ties 1, partly to the first top junction, and there attached to heads of struts 1.
Hammersmith Bridge links were used for the first and wire ropes for the second, no plate-ties being required. Later on still wire ropes were capable of dealing with most of the lifting required, union screws being used in the place of hydraulic cylinders. ”

“ Fox and Henderson he paid much attention to the system of sinking caissons by compressed air, invented by Mr. Hughes, and first adopted in constructing the foundations of the Rochester Bridge by Messrs. ”