The Forth Bridge/The Use of Wire Rope

by Wilhelm Westhofen

The Use of Wire Ropes.

Of all appliances in use during the erection none have given more unmixed satisfaction than steel wire ropes. It may be considered a rash guess, though the writer at any rate has no hesitation in making it, that the list of accidents would have been doubled at least, and a great deal more time and money expended, but for the absolutely reliable character of this article, and for its manifold uses.

In the first instance there were nearly a score of cages or hoists for the raising of men and materials to the different levels at which work was carried on, and these were going continuously day and night for several years, and there is not a single case on record of a rope having given way without having given ample warning. At first the pulleys over which the ropes passed were rather small in diameter, and the result of continuous running was that single wires commenced to give way, but with a crackling noise which soon became known to the attendant at the hoist. With the introduction of pulleys of 3 ft. 6 in. in diameter, and guide pulleys not smaller than 18 in., and having the bottom of the grooved pulleys laid with hard wood, the ropes lasted three to four times as long, that is from nine to twelve months. Even when taken off the hoists, moreover, they were quite good enough to act as guide ropes to the cages, or to support steady weights when used as pennants, or as temporary ties between the various members of the structure.

OIL FURNACES FOR HEATING ANGLE BARS
OIL FURNACES FOR HEATING RIVETS

All cranes used upon the erection had their chains taken off by degrees, and were supplied with wire ropes instead, which did the work more quietly and with far more safety and reliability. Still more useful were the ropes when portions of overhanging girders or other members had to be temporarily suspended from other portions already fixed. In these cases, by means of union screws attached between the girders and the ropes, the former could be raised and lowered at will and held in position until a joint with other portions of the work was made. Thus in the central towers, once they were completed, everything within the area could be suspended, and thus the internal viaduct was built in the easiest manner possible, every length in succession being held up until the girder was complete and able to carry itself. And thus in the same manner while building out the bottom members in cantilevers, the vertical ties and struts, and later on the wind bracings, a few wire ropes attached to the upper members, as far back as necessary to obtain a fixed hold, held up—and if required drew up—the overhanging ends to any desired position.

It is only necessary to call attention to the weight of these ropes in comparison with that of chain cables and hemp ropes of equal strength, to see at once the great advantages which the use of these ropes offered for the particular kind of work which had to be done at the bridge.

Circumference of Wire Rope		Weight per Fathom
		Wire Rope		Cable chain		Hemp Rope.
4	in	12	lb	54	lb	33	lb
3	,,	7	,,	30	,,	19	,,
2¼	,,	3¾	,,	21	,,	11¼	,,

In all cases mentioned here the wire rope had a higher breaking stress than the other two.

Facility of attachment was also a very good feature; for they could be tied like an ordinary rope, or else have the end fixed round a common thimble or deadeye, and be used with shackles. Nearly all the small stages which had been passed up and down or along the members were hung by short pennants made of wire rope.

Finally, they could be slung alongside or across the structure, and by means of running snatch-blocks every place made accessible to the men, or they could be used for raising material by means of single whips worked over a steam-winch barrel. The employment of wire ropes for this purpose is well shown in the large illustration on Plate X. and Plate XV.

The sizes used here mainly were the 2¹⁄₄ in. or ⁷⁄₈ in. diameter, the 2¹⁄₄ in. or ³⁄₄ in. diameter, and the 1⁷⁄₈ in. or ⁵⁄₈ in. diameter were used on cranes or for single whips upon steam winches. For special purposes 3¹⁄₄-in. and 4-in. ropes were used, and for the service of men's cages and hoists the 3 in. or 1 in. diameter. The tests for the latter gave a breaking stress of 26 tons to 28 tons, while nothing heavier than 3 tons in the case of material and 35 cwt. in the case of a cage full of men were ever put upon them.