Originally published in July 1889

To ENGINEERs the interest of an engineering work has little relation to size or appearance. A difficult piece of tunnelling hidden beneath some mountain, or the removal of a dangerous shoal not visible through the muddy waters of a great river, may be of as keen an interest to the engineer as a colossal work like the Forth Bridge. But to the general public it cannot be denied that in all ages a great bridge-or what for the time was considered a great bridge—has always proved to excite greater attention than any other class of engineering work. Thus, hundreds who know that Telford built the Menai Suspension Bridge would be puzzled to enumerate his other engineering triumphs; and the same may be said of Rennie’s London Bridge and Stephenson’s Britannia Bridge.

The interest shown by the public in the Forth Bridge is the same in kind, and only different in degree, to that evinced in earlier days about other bridges. It has been suggested that, as the Forth Bridge is now approaching completion, the proper moment has arrived for conveying to the readers of this Review a general notion of the size and principle of construction of that great structure, and the present brief paper is the outcome of the suggestion.

That there is nothing new under the sun is a perfectly safe statement to make at all times. Neither the idea of a bridge across the Forth at Queensferry, nor the principle of construction upon which the present structure is based, is novel. In 1804 an Edinburgh surveyor published designs for a bridge across the Forth at the same spot, and with spans of the same magnitude as the present bridge. The designs showed a suspension bridge with chains like the cable of a fifty-ton yacht, and the total weight of iron was estimated at 200 tons, as compared with the 50,000 tons of steel in the present structure.

So far as the audacity of the conception of a bridge 1,700 feet in span is concerned, we can, therefore, make no claim for originality. As regards the cantilever principle of construction there is even less novelty. A daily paper recently stated in authoritative style that the Forth Bridge cantilever system was ‘ borrowed from the United States,’ which statement could only be paralleled in absurdity by an allegation that the English language was borrowed from the United States. As a matter of fact, the cantilever principle of construction is much older than the English language, for we find it in the stone corbel and lintel combination of the earliest Egyptian and Indian temples which preceded the introduction of the arch. It was in all probability ‘invented’ by some intelligent savage, who, wanting to get across a stream too deep to ford and too wide to jump, utilised the projecting branches of two opposite trees as cantilevers, or brackets, and connected them by a short independent piece of timber, and so formed a cantilever and central girder structure, identical in main principles with the great Forth Bridge. Perhaps one of the most interesting of comparatively modern works of this kind is a bridge in Thibet constructed about 230 years ago, and illustrated by fig 1.

The 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 ninety 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 feet; 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 true prototype of the present Forth Bridge.

The adaptability of the cantilever system of construction for railway bridges of large span became obvious to ourselves, and no doubt to others, soon after the invention of Bessemer made cheap steel a possibility. In 1865 we designed a steel cantilever bridge of 1,000 feet span for a proposed viaduct across the Severn, near the site of the present tunnel; but it was not until 1881 that the Forth Bridge designs were published in the English and American technical journals. These designs naturally attracted much attention, and with characteristic promptness American engineers realised the advantages of the system, and designed and built the following year a steel cantilever railway bridge on the Canadian Pacific Railway, and have

since followed on with more than half a dozen others of the same type of construction.

Owing to the arched form of the under side of the Forth Bridge cantilevers, many persons visiting the works or seeing the drawings entertain the mistaken notion that the principle of construction is analogous to that of the arch, and that the insertion of a keystone will be required to complete the work. This assumption is entirely wrong. The 1,710 feet spans are traversed by two cantilevers or brackets, each projecting 680 feet from the piers, and connected by a central girder 350 feet in length.

The true principle of construction and the nature of the stresses are well illustrated by a ‘living model’ of the bridge photographed at the works some time ago (fig. 2). 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 girder was represented by a short stick slung from one arm of each man, and the anchorages by ropes extending from the other arms to a couple of piles of bricks. When stresses are brought on this system by a load on the central girder, the men’s arms and the anchorage ropes come into tension, and the sticks and chair legs into compression. In the Forth Bridge one must imagine the chairs placed a third of a mile apart and the men’s heads to be 360 feet above the ground, and further understand that this pull on the men’s arms approaches 10,000 tons and the pressure of the legs of the chair on the ground 100,000 tons. It is hardly necessary to add that, as regards size and weight, no existing bridge at all approaches the Forth Bridge. Each span of the latter would cross the Green Park at one bound from Piccadilly to Buckingham Palace, and over 50,000 tons of steel are used in the complete structure.

The following gives the principal measurements and dimensions

In 1883 a commencement was made with the works of the Forth Bridge on the present design. Simultaneously with the erection of shops and machinery for the manufacture of the superstructure, a start was made with the pier-work. Each main pier consists of a group of four cylindrical masonry piers about 70 feet diameter, founded on rock or hard boulder clay at depths ranging up to 90 feet below high water. Six of the cylindrical piers were put in place by what is known as the compressed-air system. That is to say, they were built as hollow cylinders in the first place, then floated into position, sunk to the proper level, and afterwards filled up solid with masonry. An airtight roof was formed seven feet above the bottom edge of the hollow cylinder, so that a chamber like a huge diving-bell 70 feet in diameter and seven feet high constituted the bottom of each pier. When in position the water was driven out of the chambers by forcing in compressed air, and men then entered them through air-locks and carried on the excavation 90 feet below the waves of the Forth as easily as on dry land. At times the height of the barometer in the working chamber attained 80 inches, but the men suffered little inconvenience beyond the usual pain in the joints which results from too long a stay in compressed air.

Where boulder clay formed the foundation the labour of excavating that extremely hard and tenacious material in the compressed air-chamber proved too exhausting; pickaxes were of little avail, and the trained Italian labourers who were chiefly employed lost heart over the work. There were, however, plenty of hydraulic appliances at hand, and Mr. Arrol quickly got over the difficulty. Spades with hydraulic rams in the hollow handles were made, and, with the roof of the compressed air-chamber to thrust against, the workmen had merely to hold the handle vertically, turn a little tap, and down went the spade with a force of three tons into the hitherto intractable clay.

At Inchgarvie-the island in the middle of the Forth without which the Bridge could never have been built, as the depth of water on either side is over 200 feet-the foundation was of rock, and a different method of excavation became necessary. A very strong and costly iron staging was erected, and the floating caisson or hollow masonry pier was moored alongside. Divers and labourers had previously levelled up the sloping rock bottom with sand-bags to form a bed for the caisson. Workmen then entered the compressed air-chamber through the air-locks and shafts of access, and the high ledge of rock was blasted away, holes being driven by rock drills worked by compressed air and otherwise under the cutting edge of the caisson to allow the latter to quietly and gradually sink into its final position on a level bed of whinstone rock seventy-two feet below sea-level. Many persons visited the seventy-feet-diameter electrically-lighted chamber lying deep below the waves of the Forth. On several occasions salmon found their way in, deluded no doubt by the temptingly aerated condition of the water due to the rush of compressed air under the cutting edge of the caisson.

Although the pier-work of the Forth Bridge presented many points of novelty, the chief interest of the work undoubtedly centres in the manufacture and erection of the steel superstructure. To manufacture the many miles of twelve-feet diameter and smaller tubes forming the compression members, and the still greater length of lattice girders forming the tension members, numberless machines of all kinds, many of them of special design, by Mr. Arrol, the contractor, were required, and the working of these machines was for several years carried on uninterruptedly day and night. At times 1,800 tons of finished steelwork has been turned out of the shops each month. All of the steel has proved of admirable quality, trustworthy in every respect. The average strength is one-half greater than that of the best wrought iron, and the ductility of the steel plates is fully three times that of corresponding iron plates. However, the dimensions of the parts are such that, even if made of iron, the Forth Bridge would be stronger than many existing railway bridges, and trains could traverse it with perfect safety.

Owing to the unprecedented span and the weight of the structure itself the dead load is far in excess of any number of railway trains which could be brought upon the bridge. Thus the weight of one of the 1,700-feet spans is about 16,000 tons, and the heaviest rolling load would in practice be not more than a couple of coal trains weighing together, say, 800 tons, or only five per cent. of the dead weight. Wind is a more important element than train weight, and with the assumed pressure of 56 lbs. per square foot the estimated lateral pressure on each 1,700 feet span is 2,000 tons, or two and a half times as much as the rolling load. To resist wind the structure is ‘straddle-legged,’ that is, the lofty columns over the piers are 120 feet apart at the base and 33 feet at the top. Similarly the cantilever bottom members widen out at the piers. To convey an idea of the enormous forces which the cantilevers are capable of resisting it may be stated that a pull of 45,000 tons would be required to tear asunder the top ties, whilst the greatest pull from passing trains would be less than 2,000 tons.

Expansion and contraction of the huge mass of metal have been carefully provided for. The steel superstructure is not rigidly bolted to the masonry, but is free to slide within certain limits under heavy wind pressure or large variations of temperature. Perfect freedom of movement in the 1,700-feet span is attained by resting one end of the central girder on a rocking column, and the rails here are free to slide to the extent of eighteen inches under changes of temperature.

One of the chief advantages of the cantilever system is facility and safety of erection, as such bridges can be erected by commencing at the piers and adding successive bays of the cantilever right and left until the whole is complete. There is thus no moment of insecurity when the safety of the whole structure is dependent upon the integrity of some temporary staging springing from the bed of the river, and liable to be carried away by storm or flood. As a result of experience at the Forth Bridge, not a single bolt has been disturbed by the heaviest storms which have occurred during the erection of the bridge. Although at times the idea has been prevalent that the accidents to workmen at the Forth Bridge were notably great, the fact is that the reverse is the case. The total loss of life in the past six years amongst the two thousand to four thousand workmen has been less than two-thirds of that resulting from the single act of oversight or carelessness at Armagh the other day. Hundreds of the Forth Bridge workmen have proved themselves to be as hardy and plucky a set of men as could ever have existed in past times. Only a fine line divides praiseworthy daring from reprehensible recklessness, and it is certainly not for the general public to bring a charge of recklessness against the Forth Bridge workmen, as witness the loss of life every year at level crossings on railways. It takes just about five seconds to walk across a railway, and yet last year over one hundred people selected the very five seconds when trains were passing to attempt the feat, and failed-so the less said about recklessness the better, so far as the general public is concerned.

The present state of the works is shown by fig. 3, and it is anticipated that the superstructure will be sufficiently complete to admit of the passage of a train across the Forth next October. It may be asked, Will the resultant shortening of the distance between North and South justify so much labour and cost? and the answer must be that, in the present day, time is pre-eminently the equivalent of money. A glance at any map will show that the Forth Bridge is the missing link in the great East Coast chain of communication, along which gallop the fastest trains in the world. Between London and Edinburgh the greater part of the journey is done at a speed exceeding 50 miles an hour; the 105 miles between Grantham and King’s Cross averages 54 miles for the whole journey, and some time ago the 4.18 P.M. train from Grantham was timed to run 24 miles in 22 minutes, one mile being done in 46 seconds, or at the rate of 74 miles an hour. Compare this with the anticipations of the last generation. In 1825 the Quarterly Review, in an appreciative article on the proposed Woolwich Railway, deprecated any wild estimates as to speed. “We will back,’ it said, ‘old Father Thames against the Woolwich Railway for any sum. We trust that Parliament will in all railways it may sanction limit the speed to eight or nine miles an hour, which is as great as can be ventured on with safety? When the prophets have failed so egregiously in the past, it would be rash for any one to venture to predict that even larger and more costly works than the Forth Bridge will not be considered a necessity of the railway system of the future. There is, indeed, exhibited at the Paris Exhibition a set of plans for a bridge across the English Channel by no less distinguished firms than Messrs. Schneider of Creusot and M. Hirsent, contractor, having seventy cantilever spans similar to those at the Forth Bridge; and last year a charter was granted by the American Legislature for a bridge across the Hudson River of 2,800 feet span; so, in the opinion of our French and American brethren at least, finality is by no means attained in the Forth Bridge.

JOHN FOWLER & B BAKER