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THE FORTH BRIDGE.

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 moinent 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 suggest

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

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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.

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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

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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 but

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ting 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 table gives the principal dimensions and measurements : Two spans each

1,700

685 Fifteen

168 Depth of main girders at piers .

342 centre

50
Width of bridge at piers.

120
centre
Clear headway for navigation at high water 150
Deepest foundation below high water
Highest part of bridge above high water

360
Depth of water in centre of channel

210 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.

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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 band, 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 airchamber 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 seventytwo feet below sea-level. Many persons visited the seventy-feetdiameter 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

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