Profile of Sir Benjamin Baker, Designer of the Forth Bridge
Sir Benjamin Baker, Designer of the Forth Bridge
While Sir Benjamin Baker has designed and advised upon many engineering works of great importance, including, among others, the ship railway across the Bay of Fundy, several subaqueous tunnels, the barrage of the Nile, in brief, most of the great engineering projects of recent years, his name will probably always recall more particularly the Forth Bridge which has been classed as the greatest engineering structure of modern times, exceeding not only all other bridges in reference to length, dimensions, weight, and other standards of greatness, but also excelling all other structures in its remarkable adaptation of means to ends, and in the unique character of the operations necessary to its accomplishment.
Sir Benjamin Baker was trained first in a South Wales ironworks, and then in the engineering offices of Sir John Fowler, with whom he was associated afterwards in carrying out the Forth Bridge enterprise. The varied character of the work that came to be discharged in the office of Sir John Fowler afforded him a large and valuable experience in almost every branch of engineering. He had, of course, done a great deal of excel- lent work before ever he had any knowledge of the Forth Bridge; he had written a book on hanging span bridges, which was published in England, America, Germany, and Holland, and had prepared and read before one or the other of the various engineering societies papers on different subjects connected with engineering, which secured for him the full confidence of the profession. More especially had he acquired a reputation as a bold designer of novel constructions and as a patient and cautious experimenter on the properties of materials of construction, with the view of ascertaining and determining their fitness for different purposes.
About twenty years ago, in order to determine whether engineers and manufacturers had arrived at a certain definite quality for the use of steel in tyres and axles, Sir Benjamin obtained half-a-dozen pairs of tyres and axles from as many of the leading British and other makers and submitted them to a series of tests which gave startling and unsatisfactory results. Not only was there no uniformity in the quality of the steel supplied by the several makers, but even the two tyres made by the same maker gave, as a rule, widely different qualities. The conclusion drawn by Sir Benjamin from his experiments was that it would be imprudent on the part of an engineer to leave the quality of steel supplied for either tyres or axles to the discretion of a manufacturer, or to forego the most rigid system of inspection. He found that a sample of steel may be cut from a tyre or axle, and be tested with perfectly satisfactory results, as regards tensile strength and elongation, and yet that the tyre, as a whole, may be frail under moderate shocks, either on account of the steel being inferior, or from its want of uniformity.
The attention which Sir Benjamin Baker had given to the principles of bridge construction, long before he had any thought of the Forth Bridge, is illustrated by some of his remarks at the Institution of Civil Engineers, during a discussion on the erection of iron bridges. In this case he showed that bridges had been erected by building out at least two centuries before it was attempted in modern Europe, Turner, in his history of his embassy to Thibet, having illustrated and described an ancient bridge on the cantilever and central girder system which had been so erected. Sir Benjamin also found that in 1810 a Mr. Pope proposed to construct a cantilever bridge of 1800 feet span across the East River, at New York, which was to be erected by building out. A more practical undertaking, however, was the railway bridge across the River Dal, in Sweden, built by Mr. E. Hutchinson, of Darlington, having a centre span of 208 feet, which was built out, without staging or auxiliary girders. In designing the Forth Bridge, Sir Benjamin Baker arranged to build the projecting cantilevers on each side of the central towers, and many novel and ingenious features were introduced by himself and Sir W. Arrol for the safe erection of the structure.
The work of bridging the Forth, although not actually commenced until the month of February, 1883, and not completed until seven years afterwards, had often been proposed. Plans for the purpose are said to have been actually drawn up as long ago as 1810. The next practical step appears to have been taken in 1879, when Sir Thomas Bouch prepared a plan for a suspension bridge, which had virtually been adopted when the disaster to the Tay Bridge caused the proposal to be given up. As early as 1870, however, Sir Benjamin (then Mr. Baker had given his estimate of the weight of steel required for a double line railway bridge of 1700 feet span, and advocated the system of construction subsequently adopted at the Forth on the grounds of economy, rigidity, and facility of erection. The design of Sir John Fowler and Mr. Benjamin Baker was ultimately adopted, under powers obtained in 1882.
For the purposes of the determination of the qualities of the materials to be used in the construction of the Forth Bridge, Sir Benjamin Baker carried out many experiments on steel, resulting in the decision to use material of a higher tensile strength than was considered admissible either for ships or for boilers.
As at least one-half of the steel employed on the Forth Bridge is in compression, it was important to gain an increase of resistance without any sacrifice in the facility of working and safety. Hence it was urged by Sir Benjamin that, if practicable, the material most suitable for the compression members of such a structure as the Forth Bridge would be 34 to 37 -ton steel, which had been previously squeezed endwise in the direction of the stress. Experiments were also carried out under Sir Benjamin’s direction on the resisting power of different classes of iron and steel to repeated bendings, on the effect of sheared edges on fracture, and many other problems that had to be solved in so serious an undertaking. During the seven years that the Forth Bridge was under construction there were probably no more laborious or anxious men in the United Kingdom than its engineers and contractors. Sir Benjamin was on the ground late and early; he had to decide and solve every new difficulty as it came up; he had practically no experience of similar structures to guide him, which rendered his work not less scientifically accurate, perhaps, but certainly more practically difficult; and he had to work out for himself many matters that were settled for the first time in the drawing-office at Westminster or at South Queensferry while the bridge was in course of erection.
On the occasion of the opening of the Forth Bridge, on the 4th of March, 1890, the Prince of Wales stated some facts that are worth recalling. The extreme length of the bridge, including the approach viaduct, is 1^ mile, and the actual length of the cantilever portion of it is a mile and twenty yards.
The extreme height of the steel structure above high-water mark, and above the bottom of the deepest foundation, is 452 feet. The wind pressure provided for is 56 pounds on each square foot of area, amounting in the aggregate to about 7700 tons of lateral pressure on the cantilever portion of the bridge. The surface of the bridge requiring to be painted is about 135 acres. About eight million rivets have been employed, and some 42 miles of bent plates have been used in the tubes. The total weight of steel in the bridge is about 51,000 tons. The total expenditure on the bridge and approach railways up to the date of its opening was £3,177,206.
In recognition of his distinguished work as an engineer, at home and in the colonies, Mr. Baker received the honour of K. C. M. G. after the opening of the Forth Bridge in 1890. He was also elected to Fellowship in the Royal Society. He was made a member of the Council of the Institution of Civil Engineers before his connection with the Forth Bridge, and later filled the office of president. In 1885 he was chairman of the Mechanical Section of the British Association, and subsequently received the Poncelet prize of 2000 francs from the French Academy of Sciences. On the occasion of the tercentenary of Dublin University he (with Lord Armstrong) was made Honorary Master of Engineering. He is also an honorary member of the American Society of Mechanical Engineers, and of many other institutions.