The Forth Bridge/Historical
THE FORTH BRIDGE.
BY W. WESTHOFEN.
Historical.
It has at all times been a subject for controversy and a matter of difficulty to fix the precise boundary line between the river and the sea, that is to say exactly where the sea ends and the river commences. With regard to the Forth and its estuary, the same discussion has been carried on in Parliament and elsewhere with considerable warmth, but does not appear at the present moment to have got any nearer to settlement than in 1882. Taking, however, a point, say Anstruther, in Fifeshire, and another, say Dunbar, in Haddingtonshire, and drawing a line across which, roughly speaking, passes near the May Island and the Bass Rock, we may call it the Forth within and the sea without this imaginary boundary line. Starting westward from it we have 32 miles to Queensferry and 30 miles further to Stirling. On both sides of this great watershed are situated hundreds of square miles of some of the most fertile and best cultivated land in the three kingdoms. There are great coalfields, there is mineral wealth and, besides, an immense supply of food and other commodities which the inhabitants of these districts would wish to exchange or barter. But a serious barrier stood in the way, and stands even to this day, for the only means of intercommunication and of transport between the two shores is afforded by steam ferries or by sailing craft. Of the former there are three the most seaward is that from Granton to Burntisland, five miles long, and 24 miles up the Forth; the next is that between South and North Queensferry, 32 miles from the boundary line; and the third at Kincardine, about 15 miles west of Queensferry. 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. The same holds good, though in a minor degree, at Queensferry and at Kincardine, and the traveller who requires to go from Dunbar to Anstruther, to put an extreme case, and who objects to either of the sea passages, has no choice left him but to go round by Alloa or Stirling, and to pass over about 150 miles of railroad, when the distance between the two towns mentioned is, as the crow flies, less than 18 miles. That under such conditions the commercial and agricultural intercourse and traffic between the eastern counties of Scotland suffered a serious check, and became reduced to a minimum, was but to be expected.
The same disadvantages existed in the case of all those travellers, bent on business or on pleasure, from north to south or vice versâ, who desired to pass through Edinburgh on their way, and who had either to cross by one of the ferries or make the long detour by Stirling, being in either case compelled to submit to a loss of valuable time. Finally, the principal railway companies whose systems are situated in the eastern and midland counties of England and the south of Scotland, could get no access to the northern parts of Scotland except by passing over the lines of a company whose interests were hostile and in opposition to their own. This brought about a most intolerable state of things, and is an easy explanation of the many struggles and attempts made by the East Coast lines to obtain a separate access to the counties north of the Forth. How great the necessity was of having means of communication between the two shores, and how largely even the inadequate provisions made hitherto were taken advantage of, is proved by the fact that in 1805—before a steamboat existed or a railroad was thought of the right of running ferryboats between South and North Queensferry was let at a yearly rental of 2000l., and it is stated in the Parliamentary evidence then taken that the revenue derived by outsiders who run goods and passengers across in yawls and small boats amounted to fully 5000l. per annum in addition.
It must be admitted that the Forth Bridge crosses the river at a point which leaves the eastern counties still somewhat in the same difficulty, but it reduces many of the local distances to be traversed by more than one-half, and the gain in time is considerable. Going by the Forth Bridge and its connecting lines the traveller will now be carried to and safely landed at Perth, Dundee, or Aberdeen, as the case may be, in a comparatively short time after leaving Edinburgh in one and a quarter hours, one and three-quarter hours, and four hours respectively—independent of wind or tide and without difficulty and discomfort. In speaking of the new lines in connection with the bridge this matter will be further referred to.
The justification for the construction of so great a work must, however, be sought in the desire to serve larger interests than those of local traffic merely. In these days of high pressure, of living and working and eating and drinking at top speed, the saving of an hour or two for thousands of struggling men every day is a point of the greatest importance, and every delay, however excusable and unavoidable, is fatal to enterprise. 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. Altogether those concerned in this great undertaking are sanguine that within a very few months the courage, the foresight, and the wisdom of the directors of the Forth Bridge Railway Company, and of the other interested railway systems, will be fully proved by results which it is impossible to estimate even approximately just now.
When and with whom the idea of bridging the Forth first originated is now a matter of pure conjecture. The Roman leader bent on exploration and conquest, probably conjured up in his mind's eye the faint outlines of a bridge as he trudged the weary miles along the south shore and found neither boats to carry him across nor ford to traverse—so must often have the sainted Margaret, the wife of Malcolm Caen-Mohr, on her frequent pious pilgrimages between Edinburgh, Linlithgow, and Dunfermline about the time of the Norman Conquest, and so probably her son Alexander the First of Scotland, who in attempting to cross from South to North Queensferry was overtaken by a gale and beaten down the Firth, and had finally to land on the island of Inchcolm, five miles away. He founded a priory on that island as a thanks offering to Providence for a very narrow escape and in view of a warmer reception and more substantial entertainment should a similar misfortune again befall him. So must also many of the poor wayfarers who got drenched to the skin and suffered the horrors of sea-sickness during the crossing, and so must finally—if there was time for them to do so—the unfortunate party of people who were driven down the Hawes Brae at so rapid a pace that horses, carriage, and passengers went right off the pier into the water and none of them came out alive.
The idea thus floated through many minds until about 150 years ago, when a bridge was first spoken of, but particulars as to design, site, or probable cost do not seem obtainable.
In November, 1805, a proposal was made to construct a double tunnel—15 ft. wide and about the same in height quaintly described as one for comers and one for goers—under the bed of the Forth, at some point to the west of Queensferry. The project, was evidently seriously entertained, for in July, 1806, a prospectus was issued by "a number of noblemen and gentlemen of the first respectability and scientific character," inviting the public to subscribe—the shares being fixed at lOOl. each. Further, in 1807, a pamphlet of about 120 pages was published in Edinburgh, entitled "Observations on the Advantages and Practicability of making Tunnels under Navigable Rivers—applicable to the proposed Tunnel under the Forth. Illustrated with a section and map." Nothing, however, seems to have come of the project, whether owing to difficulties of construction or of financing is not known—most probably both.
Within eleven years another effort was made, and we come upon a pamphlet entitled "Report relative to a Design for a Chain Bridge thrown over the Firth of Forth at Queensferry..... By James Anderson, civil engineer and surveyor, Edinburgh, 1818." There were three elevations, differing as to height and length of clear span, but all equally bold and equally primitive; we give on the next page but one a reproduction on a smaller scale of the diagrams accompanying this report. The site was to have been nearly the same as that of the present bridge, starting from the same point at North Queensferry, passing very nearly over the centre of Inchgarvie, and terminating on the south shore about one-third of a mile east of the Hawes Pier, joining the Edinburgh Road just under Mons Hill. The clear height above high water was to have been either 90 ft. or 110 ft., the main spans 1500 or 2000 ft., the width for carriage road and footpath 33 ft., and the cost 175,000l. or 205,000l.. The time required for completion was stated to be four years. 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.
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. The exact centre line was to have been from a point near Blackness Castle on the south shore to Charleston on the north shore, and connecting lines from near Linlithgow, on the Edinburgh and Glasgow line on the one side, and from Charleston to Dunfermline on the other side, would no doubt have established a very good through line to the north. Borings were taken and other investigations made. A design had been drawn out by Sir Thomas (then Mr.) Bouch, and Parliamentary powers for the construction of the bridge were obtained by an Act in 1865. The river here is about 2½ miles wide, and the greatest depth of water about 60 ft.—but the bottom is loose and uncertain, and it was decided to build up and sink an experimental pier before proceeding further. But troubles intervened, and during the re-arrangement of the North British Railway Company in 1866-1867, the project was abandoned through various causes. For it were substituted improvements in the Queensferry passage by the construction of the railway slips at North Queensferry and Port Edgar. It was at first intended to have swinging landing-stages at each end, rising and falling with the tide—the trains by means of these to be run on and off the ferry steamers. The latter portion of the scheme was not carried out however, owing to the insufficient depth of water and the gradual silting up near the piers, which necessitates the periodical assistance of dredgers to keep a channel clear, although the ferry steamer draws little more than 4½ ft. of water. These conditions are a source of much discomfort to the passengers by this route for the light draught steamer can hardly hold its own against a gale of wind broadside on, and many people become seasick during a passage lasting at the worst of times barely twenty minutes.
In 1873 the Forth Bridge Company was formed for the purpose of carrying out the design by Sir Thomas Bouch of a suspension bridge with two large spans of 1600 ft. each. The capital was raised by the four principal railway companies interested in the East Coast traffic—namely, the Great Northern, the North-Eastern, the Midland, and the North-British, and the companies came to an understanding among themselves that they would between them send so much traffic across the bridge as would suffice to pay a dividend of 6 per cent. per annum on the contract sum. The Act authorising the construction of the bridge was passed in the same year, 1873, and a contract signed, with Messrs. W. Arrol and Co., of Glasgow. An illustration showing elevation and plan of Sir Thomas Bouch's design is shown in Fig. 2. The central towers from which the main chains are suspended were to have been 550 ft. above high water, while the rail level would be at such height as to leave a clear head-room of 150 ft. above high water between the piers. The central tower on Inchgarvie was over 500 ft. long, which brought the foundations upon the sloping rock down to a depth of over 110 ft. below high water. There were two lines of rails carried at a distance of 100 ft. from each other, each line being supported on a pair of strong lattice girders, and these were laterally stiffened by single diagonal bracings reaching from side to side. The piers at Queensferry and on Fife were very nearly in the same position as those of the present bridge, and there were two approach viaducts to reach the high ground upon either side. The bridge was to have been constructed entirely of steel.
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Offices and workshops—which are now standing—were built at Queensferry, and extensive brickworks near Inverkeithing laid out and started. A brick pier—one of eight, which were to form the base of the great Inchgarvie tower—was built at the extreme north-west corner, after a foundation stone had been laid with great ceremony. But the collapse of the ill-fated Tay Bridge in December, 1879, stopped the further progress of the work, and the investigations into the causes of that disaster, and the disclosures made, shook the public confidence in Sir Thomas Bouch's design, and rendered a thorough reconsideration of the whole subject necessary. As a first result of this, the suspension bridge was abandoned, and the four railway companies above named instructed their consulting engineers—Messrs. Barlow, Harrison, and Fowler—to meet and consider the feasibility of building a bridge for railway purposes across the Forth, and assuming the feasibility to be proved, to decide what description of bridge it would be most desirable to adopt. It was fairly well known how many types of bridge there were to select from for such a site; these were (1): Mr. Bouch's original design (Fig. 2); (2) three forms of suspension bridges with stiffening guides and braced chains (Fig. 3); and (3) a cantilever bridge (Fig. 4). The inquiry was most comprehensive. It embraced not only bridges as set forth, but also tunnels, and both of these for different sites.
With regard to tunnels, it was considered that the great depth of water in the two main channels—above 200 ft.—and the high ground upon both shores, would necessitate very steep gradients and long approaches—making the tunnel many miles long, irrespective of the uncertainty of the nature of the ground through which the tunnel would have to be cut.
All things considered, the most suitable site for a bridge was held to be that at Queensferry; and, owing to the great depth of water and the nature of the bottom of the estuary, it was not considered expedient to construct a bridge with shorter spans than those which are indicated by the natural configuration of the ground.
The original design for a continuous girder bridge (see Fig. 4)—on the cantilever and central girder principle which had been submitted by Messrs. Fowler and Baker—was in some particulars modified to suit the conflicting views of the other consulting engineers, and was then submitted to the directors in May, 1881. After consultation with the officers of the Board of Trade, this design (see Fig. 5) was finally adopted, and Messrs. Fowler and Baker were appointed engineers to carry it into execution.
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Fig. 1. Anderson's design for bridge over the forth, 1818.
ALTERNATIVE PRELIMINARY DESIGNS FOR THE FORTH BRIDGE
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permanent way on the bridge, and conduct and manage all traffic, while the Forth Bridge Railway Company undertakes to keep the structure generally in repair and good order.
THE FORTH BRIDGE, CANTILEVER TYPE; ORIGINAL AND FINAL DESIGNS
MESSRS. HARRISON, BARLOW, FOWLER, AND BAKER, ENGINEERS.
_Fig._4,_Page_5.png)
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. In the earliest Egyptian and Indian temples will be found the stone corbel and lintel combination shown in Fig. 6 (see next page), and in the oldest, as in the most modern, wooden bridges will be seen practically the same thing in timber (Fig. 7).
Skeleton bridges on a similar principle have for ages past been thrown by savages across rivers. A sketch of one such on the route of the Canadian Pacific Railway is given in Fig. 8. Perhaps one of the most interesting structures of this kind ever built is a bridge in Thibet, constructed about 220 years ago, and illustrated by Fig. 9.
This sketch is reproduced from a drawing made in 1783 by Lieutenant Davis, R.N., who formed part of the embassy to the Court of the Teshoo Lama in Thibet, an account of which with illustrations was published in London in the year 1800. The book was a popular one at the time, and was translated and republished in Germany, so that both English and German engineers had the opportunity eighty years ago of reading the following, probably the first, description of a "cantilever and central girder" bridge ever published. "The bridge of Wandipore is of singular lightness and beauty in its appearance. The span measures 112 ft.; it consists of three parts, two sides, and a centre nearly equal to each other, the sides having a considerable slope raise the elevation of the centre platform, which is horizontal, some feet above the floor of the galleries. 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.
Descending to more recent times, it will be found that the term "cantilever and central girder" has ever been familiar as a household word to all educated engineers, because in treating on the strains in continuous girders it has almost invariably been the rule of authors to regard the structure as a central girder suspended from two cantilevers at the points of contrary flexure. Thus writing in 1850 on the Britannia Bridge, Mr. Edwin Clark premises severing the beam at the point of contrary flexure, and suspending the central portion from the "semi-beams or cantilevers," and appends the diagram (Fig. 10) in illustration of the resultant strains.
He also gives a sketch (Fig. 11) of "a shorter tube resting on brackets from the pier at either extremity, as below," which had been discussed by Mr. Stephenson in 1846.
In the former year also Sir John Fowler not only talked about severing the beam at the point of contrary flexure and suspending it, but had the experiment tried with a large wooden model, and the result was recorded in the discussion on the Torksey Bridge (Min. Proc. Inst. C.E., vol. ix., page 256).
In 1855 Mr. Barton, in a paper on the Boyne Viaduct (Min. Proc. Inst. C.E., vol. xiv., page 457), pointed out that the points of contrary flexure might be made to coincide with any previously determined points by severing the beam, and he added this most suggestive comment: "In very large spans where it may be a matter of great importance to reduce the weight in the middle of the beam as much as possible, the quantity in material in the top and bottom tables, as well as of the sides, may be reduced to a minimum by throwing the points of contrary flexure towards the middle of the beam, the great weight of material being placed over the piers." This is exactly what has been done in the Forth Bridge girders.
In 1858, Mr. Latham, in his well-known work on wrought-iron bridges (page 222), also speaks of "a girder suspended from the cantilever girders," and in 1859 Mr. W. H. Barlow took out a patent with reference to that and other matters. He preferred making the depth at the pier 11⁄2 times the depth at the centre. In the Forth Bridge the ratio will be 7 to 1 instead of 11⁄2 to 1.
In 1862 Professor Ritter, of Hanover, in his justly popular work "Dach-und-Brucken Constructionen" (Chapter X.) again drew attention to the fact that "hinges can be employed with advantage in girder bridges;" that a "great saving of material is effected by using a continuous girder and breaking the continuity by means of hinges." To enforce his conclusions he works out in full detail the stresses upon all of the members of a continuous girder bridge of 160 metres, or, say, 525 ft. span of the type shown by Fig. 12.
In 1864 Mr. Fowler and Mr. Baker designed a steel bridge of 1000 ft. span on the said system for the proposed South Wales and Great Western Direct Railway Severn Crossing, but the span was subsequently reduced to 600 ft. An Act was obtained for the construction of the bridge, and the contract was let; but, owing to financial difficulties, the work was not proceeded with.
In 1867 Mr. Baker enforced the economical advantages of the continuous girder of varying depths in a series of articles on "Long Span