August 1 1, 1887] 



NATURE 



353 



THE STRUCTURE AND PROGRESS OF THE 

 FORTH BRIDGE} 



AS a visit to the works of the Forth Bridge is included in the 

 programme of the present meeting of this Institution, the 

 I author trusts that a short sketch of the preliminary proceedings, 

 » with a description of the structure and progress, from one en- 

 gaged on the work from its outset, will prove of interest in 

 explaining the reasons and means adopted for connecting the 

 railways on opposite shores of the Firth of Forth, at the site of 

 the historic ferry and still existing Hawes Inn, whose time-table 

 for the departures of the ferry-boat is so quaintly alluded to in 

 The Antiquary." 



Previous Proposal.— Y ox many years, suggestions for establish- 

 ing direct communication between the southern railways running 

 into Edinburgh and the Fifeshire lines, with the object of more 

 direct access to Perth and the north, had been frequently con- 

 sidered by the companies interested in that route ; but until an 

 Act of Parliament was obtained in 1873 for the construction of a 

 suspension bridge, designed by the late Sir Thomas Bouch, for 

 crossing the Forth at the site of the present works, no proposal 

 gave prospect of successful issue. Although the type of bridge 

 then proposed was not one generally considered applicable for 

 the passage of railway trains, yet no positive objection seems to 

 have been taken to it, inasmuch as a contract was entered into 

 for its construction, workshops were erected at the site, and 

 foundations were started. But after the severe gale at the close 

 of the year 1879, so destructive to a viaduct in an equally exposed 

 position, it was deemed prudent to suspend operations ; and the 

 directors of the North-Eastern, Midland, and Great Northern 

 Railway Companies, which each have an interest in obtaining 

 direct access to the eastern and northern districts of Scotland, I 

 requested their respective consulting engineers, Mr. T. E. 

 Harrison, Mr. W. H, Barlow, and Mr. (now Sir John) Fowler, ' 

 to confer together and report upon the possibility of some other 

 plan for making through communication between the existing 

 lines at the point already selected. Tunnelling was out of the 

 question on account of the depth of the water ; the proposals 

 therefore took the form of bridges. 



Present Plan.—On. May 4, 1881, the engineers submitted their 

 joint report, unanimously agreeing that the steel bridge on the 

 cantilever and central-girder system, designed by Sir John 

 Fowler and Mr. Benjamin Baker, was not only the least expen- 

 sive, but the best suited for the situation. The soundness of this 

 decision has since received confirmation in the fact that seven 

 long-span bridges have been or are now under construction in 

 different parts of the world, and many more are proposed on 

 the principle adopted for the Forth Bridge. For the substitution 

 of this design in place of the suspension bridge contemplated in 

 1873, the Forth Bridge Railway Company appointed Sir John 

 Fowler and Mr. Baker their engineers, and obtained an Act of 

 Parliament in July 1882. The financial obligations for the con- 

 struction of the bridge having been undertaken by the railway 

 companies interested in the through route — namely, the North- 

 Eastern, Midland, Great Northern, and North British— tenders 

 were invited for the work, and from the applications received 

 two offers were selected ; and with the combined firm of Messrs. 

 Tancred Arrol and Co. a contract was made in December 1882 

 for the entire execution of the work. 



General Dimensions. — The total length cf the bridge will be 

 8300 feet, or 380 feet over one mile and a half. There are two 

 nnain spans of 1700 feet each, two side spans of 675 feet each, 

 with the ends counterbalanced and anchored to the masonry, 

 and three intervening piers ; these together make up about a 

 mile of the total length, and the remainder is composed of 

 fifteen approach spans of 168 feet each, and of masonry arches 

 and abutments. For a length of 500 feet in the centre of each 

 of the two 1700-feet spans there is a clear headway for naviga- 

 tion of 150 feet above high water; the rails being placed at a 

 level 6 feet higher. From the base of the deepest pier to the 

 top of the cantilevers the total height is 450 feet, or only 10 feet 

 less than the Great Pyramid of Egypt. 



The cross sections of the main spans are of trapezoidal form, 

 330 feet in height from centre to centre of the members over the 

 piers, and 33 and 120 feet in width across top and bottom re- 

 spectively, and tapering tow ards the ends of the cantilevers, thus 

 giving a form which is eminently suitable for withstanding lateral 

 pressure. The girders carrying the railway are supported at 



' Paper read by Mr. E. Malcolm Wood before the Institution of Mechanical 

 Engineers, on Tuesday, August 2. 



intervals inside the cantilevers, &c., by trestles or cross frames, 

 and a continuous lattice-work parapet 4^ feet above the rails 

 extends the whole length of the bridge. 



Load, and Wind Pressure.— In addition to its own weight the 

 bridge IS being constructed to support, without exceeding in any 

 member the unit stresses permitted by the Board of Trade, a 

 load equivalent to trains of unlimited length equal to i ton per 

 foot run on each line of railway, or passing trains consisting 

 each of two engines and tenders at the head of sixty coal trucks 

 weighing 15 tons each ; and also to withstand a lateral wind 

 pressure of 56 lbs. per square foot of exposed surface of train and 

 structure. The magnitude of the lateral pressure may be judged 

 from the fact that over the mile length of main spans the esti- 

 mated surface exposed to a point blank wind at right angles to 

 the bridge amounts to a little more than 7^ acres ; the pressure 

 of 56 lbs. per square foot on this surface would therefore be 

 equivalent to a total of more than 8000 tons. In addition to 

 lateral winds, the direction from any point of the compass has 

 been provided for, even including the imaginary condition of 

 each group of main piers becoming the centre of a whirlwind. 

 Effects of temperature will be provided for in the rails, and at 

 the junctions of the central girders with the cantilevers ; and the 

 bearings on both the main piers and under the weighted ends of 

 the cantilevers have provision made for allowing movements due 

 to changes of temperature and to the elasticity of the cantilevers 

 under lateral pressure. The lateral play allowed is limited, so 

 that the whole of the piers may act in concert to resist combined 

 actions of all forces tending to disturb the normal state of rest of 

 the 50,<x)0 tons of permanent load. As a further provision 48 

 steel bolts of 2J inches diameter, secured 24 feet down in the 

 masonry by anchor plates, hold down the bed plates with an 

 initial tension of 2000 tons ; the nuts and saddle-plates are so 

 arranged as to allow freedom of lateral movement to the skew 

 backs ; but any lifting would at once be prevented by the 

 anchorage coming into action, which however could only happen 

 under the assumed circumstances of a wind pressure more than 

 double that already mentioned, acting over the whole estimated 

 surfaces. The maximum pressure on the base of the piers will 

 be a little over 6 tons per square foot. 



Forms of Paris.— The enormous forces to be resisted have 

 been met by adopting the most suitable forms of parts for with- 

 standing the stresses. Tubular members are used for compres- 

 sion, and open-braced box-forms for tension. These parts vary 

 in size as required. Though the tubular form has scarcely been 

 used in this country for bridge members since its employment by 

 the late Mr. Brunei, no difficulty has arisen in connexion with 

 its use; even the junctions are dealt with as readily as the 

 generality of the work. 



Masonry. — The masonry for the main piers, above the whin- 

 stone concrete filling of the caissons, consists of a casing of 

 Aberdeen granite, inclosing and bonded into a hearting of 

 Arbroath stone set in cement, and strengthened by three massive 

 wrought-iron belts built into the stone-work. The deepest pier 

 weighs about 20,000 tons. The remainder of the masonry of 

 the piers and abutments is of a similar class, whinstone being 

 largely used in the interiors. 



Steel. — For the principal members of the superstructure subject 

 exclusively to compression, the steel used has a tensile strength 

 of from 34 to 37 tons per square inch, with at least 17 per cent, 

 of elongation in a length of 8 inches ; for the other parts 

 20 per cent, of elongation, with 30 to 33 tons tensile strength. 

 The rivet steel has 25 per cent, elongation, and 26 to 28 tons 

 tensile strength per square inch. The whole of the steel is 

 manufactured by the Siemens process. No sheared edges or 

 punched holes are permitted. 



Work started. — No time was lost by the contractors in start- 

 ing the work. The land was at once entered on ; the old work- 

 shops were put in order, and the extensive range of offices, 

 stores, workshops, and yards was commenced, which now cover 

 fifty acres. Meanwhile the centre line of the bridge was fixed, 

 and the position of the piers determined. The foundations of 

 those on land were begun simultaneously with the building of 

 temporary jetties for gaining access to the piers that had to be 

 sunk below water-level. These jetties, which are still used for 

 conveying the material, are in themselves no small work ; the 

 southern or Queensferry jetty extends 2200 feet from the shore, 

 and is connected with the workshops by an incline worked by a 

 rope driven by a stationary engine. In order that the operations 

 might be carried on continuously day and night when needful, 

 electric light installations, supplemented by lucigens, were laid 



