The Forth Bridge/Workshops

by Wilhelm Westhofen

Workshops.

Two principal divisions were made in regard to the construction, namely, between the tubular members and the latticed girders. The former were done in the upper or No. 2 shed and on the drill-roads adjoining it; the latter, in the lower or No. 1 shed. Considering that as much of the work as could possibly be done was to be drilled with all parts put together, it will be at once understood that not only did the drilling form a most important part of the work, but also that a great deal of special plant designed for each particular purpose would be required. As a broad rule, therefore, all tubular members were put together and drilled through the various thicknesses at once, the only parts done separately being the portions of beams and diaphragms not in contact with the outer shell. In the lattice-girders the booms were drilled singly, on specially prepared beds, made up of timber blocks; or, if worth while, on cast-iron frames. The lattice-bars were drilled by other machines.

In the building of the tubular members the first work to be done was to bend the plates to the required curve. For this purpose the plates were put into a gas furnace (sec Figs. 60 and 61), heated up to a bright red heat, and then passed into a hydraulic press, where, between two dies or saddles, they received the necessary curvatures. (See Figs. 62 and 63.) The plates were then placed aside in piles, covered with ashes, and gradually cooled down. However carefully this is done, some alteration takes place in the shape of the plates; and to remedy this the plates, when quite cold, were put into the hydraulic press once more and received a final setting. They were then placed into a planing machine (see Fig. 66), where the two long side edges were planed down to the required dimensions; while in another machine of simple construction the end edges were dealt with in the same manner. (See Figs. 64 and 65.) They were now ready to be built up into tubes on the drill-roads. (See Fig. 67.) Here a long cylinder, or mandrel with wings attached, was supported on a number of trestles, and round this mandrel the tube was built up, the diaphragms being bolted to the wings of the mandrel. (Figs. 68, 69, 70, and 71.) Meanwhile, the beams had been prepared and drilled with the angles attached by ordinary radial drilling machines in the shop. (See Fig. 74.) The beams were now bolted with their inner ends to the angle-iron rings of the diaphragms; and on the beams again, the inside, and later the outside, plates were fixed by means of ordinary clamps and by draw-washers. The underside of the tube was also supported by timber blocks built from the ground. The centre of the mandrel was now checked by means of a theodolite, and, if necessary, corrected. All was now ready for drilling, except, perhaps, that all holes in the lap-joints of the plates and those for the circular girders required to be marked. The drilling machine, specially designed for this work, was now brought forward. (Figs. 72, 73, and 75.) It consists of a wrought-iron carriage A running on a double line of rails on each side of the tube, and to it are fixed in the centre line of the tube two circular frames C C¹ about 10 ft. apart, and completely embracing the tube with about 6 in. of play all round. Upon the frames are placed five cross-girders D D, so arranged that by means of circular rack and worm B B they can work right round the tube, each girder carrying two spindles H H, which can run from end to end. On one side of the carriage is placed an engine E E and boiler, and the driving of the spindles is accomplished by an endless cotton rope. Thus arranged, the ten spindles point radially to the centre of the tube, no matter where placed. This machine was now pushed forward to the tube, timber packings being removed in front and replaced behind as the machine advanced, and drilling was carried on until all holes (about 100 per foot run) in the 8 ft. section were drilled. The machine was then moved forward to the next section. On the side to which the machine was moving new beams and plates were always added, while on the other side they were removed and taken to the stack-yard. Before being removed, however, every plate, every beam, angle, cover or strap was typed with distinguishing letters, which made it possible, at a moment's notice, to fix its position in the structure. In tubular members other than circular, the side frames in which the cross-girders which carried the boring spindles moved, instead of being circular, were of the same shape as the members themselves; and on flat surfaces, for instance, the spindles were thereby placed at right angles instead of radially.

multiple drilling machine; No. 1 shed.

Besides the machines here described there were a number of others, all for special purposes, each of which carried out its appointed work in a most efficient manner. Many different types of hydraulic presses for the shaping of bars or corner plates for the square-ended struts were used, and had constantly to be altered as circumstances required.

Drilling machines, similar only in respect of the fact that they drilled holes, and that they moved alongside the work which was made a fixture, were in use in the lower or No. 1 shed. (See Fig. 76.) These machines had at times ten boring spindles going at one and the same time, and there were on several of the machines as many as thirteen spindles available for use. These multiple drilling machines are shown in Figs. 77, 78, 79, and 80.

Here also an ingenious tool was in operation which cut the 3-in. round holes out of the lower bedplates, and which also cut the larger round holes 7 in. in diameter, and the oblong holes 11 in. by 7 in., with rounded corners (see Fig. 96), out of the upper bedplates. This machine consisted of a spindle turned by worm and wheel, and upon this spindle was a saddle in which could move forward and backward a horizontal slide which carried a roller on top, and a cutting tool at bottom. The roller at top moved between two frames, which could be removed at will, and which had the same shape which it was desired the hole below cut by the tool should have. It followed therefore that whatever shape these frames had, the tool was obliged to follow and cut a hole the same shape. By placing the tool at different distances from the revolving centre while leaving the roller at the same distance, the size of the hole could be diminished or increased to any desired scale. A machine for planing the very long plates required in the lattice girders is shown in Figs. 81, 82, and 83. It will be noticed that it has an end saddle and tool as well, thus allowing two edges of the plate to be planed at one time. A further feature of the machine is the use of hydraulic rams for holding down the plate in lieu of the usual screws.

Here all the tension members, wind-bracing girders, bedplates, and the top junctions with flat sides were put together and drilled, as also a mass of small detail work, and all the temporary girders used in the erection and in the stagings.

Outside these workshops and the drill-roads, many acres of ground were taken up with the building up and fitting together of the various girders, of which the single booms had been drilled in No. 1 shed. Thus a great portion of the internal viaduct was put together and a number of bracings which had to be templated in the first instance in situ, were done and multiplied. These portions of the internal viaduct were erected for one pair only, however, and in the others it was simply repeated.

The bottom junctions, or junctions between bottom members, struts and ties and wind-bracings in cantilevers, were laid down and finished on the drill-roads, while those of the top members with struts and ties were laid down and built in tho field.

For the handling of the plates, beams, and other parts of the tubular members and the skewbacks and bottom junctions, powerful travelling cranes (see Fig. 75), some with curved jibs 40 ft. high, were used, and these could move about all over the drill-roads, being shifted from one line of rails to another by means of traversers. Ordinary derrick cranes worked by steam or by hand were also largely used.

multiple drilling machine; No. 1 shed.

In the shops, however, most of this work was done by hydraulic cranes of very simple construction, yet eminently adapted for their work. (See Figs. 84 and 85.) In many places they were so disposed that material could be lifted and swung round from one to the other so as to traverse the whole length and breadth of the shop. (See Fig. 76.)

A large amount of rivetting was done in the yards and the field in the case of such portions of the girders as the booms of the tension members, and later on, whole portions of the rectangular wind-bracing girders, of which then the joints only required rivetting up after they were erected. Much rivetting was also done upon the longitudinal beams and the diaphragms in the tubular members. All this rivetting was done by hydraulic machines, and could be finished at an extremely cheap rate.

The work in both No. 1 and No. 2 sheds, and on the drill-roads, was carried on day and night, though the number of men was much greater during the day-time. The working hours were from 6 a.m. till 5.15 p.m., and from 5.45 p.m. until 5.45 a.m. for night shift. No deduction was made for meal hours during the night, the full twelve hours being paid for. When necessary the work in the field was also carried on during the night.

The total output of work from all these places has amounted to as much as 1800 tons in a month, which is a very large quantity when it is considered through how many hands every piece was required to pass before it could be called finished.