The Forth Bridge/The Steel

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The Steel.

With the exception of a few hundred tons of cast-iron washers and anchor-plates in the piers, and about 2000 tons of deadweight consisting of cast-iron bricks laid in asphalt which are placed in the ends of the two fixed cantilevers terminating in the cantilever end piers the whole superstructure, from holding-down bolts to the ventilators on the extreme top of the vertical columns, and from the granite arches at one end to those at the other end, is built of steel.

Fig. 67. Plan of Drill Roads.
BOTTOM MEMBER ON DRILL ROAD.

The choice of a material for constructing a bridge of novel design, of extraordinary magnitude, and exposed during erection to the effects of powerful atmospheric disturbances, must have been the subject of much anxious thought and reflection to the engineers. But, in whatever way the decision was arrived at, there can be no two opinions that the choice was a happy one. From beginning—and probably a long time before the beginning of this work—to the end, this steel was subjected to every conceivable test, both in a properly scientific manner for purposes of research or investigation, and in an entirely unscientific manner by workmen, whose only excuse can be that they did not know better. But in all cases the steel stood the test, and a more uniform, a more homogeneous, and more satisfactory material could not be wished for. To only quote one instance, the writer has in his possession several pieces of scrap from the shearing-machine, picked up promiscuously and placed under an ordinary diamond-headed drill about 1 in. in diameter. A hole was drilled about ³⁄₄ in deep, and the machine stopped while yet the feed was on, the result being one single corkscrew shaving, about one yard in length, started from the very moment the drill touched the steel and attached yet by the end to the piece of scrap out of which it was bored. It would not, probably, be straining a fact to assume that this behaviour of the steel under severe tests had a great deal to do with the confidence with which the workmen regarded every portion of the structure, and with their belief that no possible load they could pile on the temporary platforms could by any chance bring about a collapse. It is true that, in the early days of plate-bending, some thick plates broke near the edges in a seemingly mysterious manner, but the investigations made and the reasons adduced in connection with these fractures were sufficiently convincing to allay any feeling of distrust.

The Board of Trade stipulations in regard to steel for structures, do not go further than to lay down the rule that the maximum working stress should not exceed one-fourth of the ultimate breaking strain of the steel. No difference is made between the tensile and compressive stresses, nor is any regard paid to the differences between stresses due to dead load or live load alone or in combination nor to the circumstances arising from changes, occurring frequently or rarely, in the nature of the stresses.

The engineers therefore laid down, after consultation with the Board of Trade and with their approval, certain rules in regard to the stresses admissible under varying circumstances.

For tension members the steel was to have an ultimate resistance of not less than 30 nor more than 33 tons per square inch, with an elongation in 8 in. of at least 20 per cent. For compression members a resistance not less than 34 tons nor more than 37 tons per square inch, with 17 per cent. elongation.

With regard to varying stresses for a load varying between nil and a maximum, 20 tons per square

inch of section to be assumed as the ultimate ​

TUBE DRILLING MACHINE ON DRILL ROAD.

strength if the change occurs frequently, and 22+¹⁄₂ tons if occurring rarely.

For stresses alternately tensile and compressive, the ultimate stress to be 10 tons if frequent and 15 tons if seldom, one-third of the ultimate strength to be considered the working stress.

Rivet-steel to have an ultimate strength of 27 tons per square inch, with 30 per cent, of elongation, and shearing resistance to be from 22 tons to 24 tons per square inch.

Cast steel to have an ultimate tensile strength of 30 tons, with 8 to 10 per cent. elongation.

For tension members the sectional area of the rivets in the joints to be 1+¹⁄₂ times the useful section of the boom.

For compression members the area of rivets in butt-joints to be half the useful section of the member.

The original estimate gave the quantity of steel required for the Cantilever Bridge (not including approach viaduct spans) as 42,000 tons. Of this quantity the Landore Works near Swansea in South Wales, of Messrs. Siemens, supplied 12,000 tons, and the Steel Company of Scotland, from their ​Blochairn and Newton Works near Glasgow, the remaining 30,000 tons. For the alterations made subsequently in the design, and the increase of section in various parts, a further quantity of about 16,000 tons was required, about one-half of which was supplied by the Steel Company of Scotland, and the other half by Dalzell's Iron and Steel Works at Motherwell, near Glasgow. The steel for the viaduct spans, about 3200 tons, is not included in the figures given above. The Clyde Rivet Company, Glasgow, supplied about 4200 tons of rivets.

Fig. 75. Tube drilling machine and travelling crane on drill road.
Fig. 76. Plan of No. 1 drill shed.

With regard to tests, one plate out of every fifty, picked out promiscuously, had a strip cut out of it which was planed on all four sides and tested for tensile stress in a 50-ton testing machine. The same proportion was observed in the case of bars. For transverse or bending tests a piece was cut off every plate and every bar, about 2 in. wide, and tested by being bent under the press to a radius of 1+¹⁄₂ in. thickness with the ends of the test-piece closed. As regards the steel supplied by the Steel Company of Scotland, all plates were rolled at the Blochairn Works, and all bars at the Newton Works.

Failures of the steel under test were of the rarest occurrence.

All steel was manufactured by the Siemens-Martin open-hearth process, and all plates, bars, tees, angles, and other pieces, were cut to length and shape as ordered and thus delivered at the works. About 6 per cent. of the total steel delivered was returned as scrap, or between 3000 tons and 4000 tons. A certain proportion of the steel delivered was used for temporary purposes only, and this will account for the difference between the total quantity delivered and that erected.