Page:Forth Bridge (1890).djvu/54

Rh have towards each other, it is easy to understand that, independent of any weight which was imposed on them, they would be liable to deviate from their true position. It was, therefore, necessary from time to time, that is every 30 ft. or 40 ft., to secure the members by the interposition of temporary struts or ties either in the shape of lattice girders or of timber balks, and even timber trusses. Thus, after two lifts had been made with the platforms, the diagonal struts in the central towers had not only deflected downwards by their own weight, but had also deflected towards the centre line of the bridge. It was not possible to go any further without putting them right, and this was done by girders placed both longitudinally and transversely between the struts, and using hydraulic rams to push them apart. In the case of the vertical columns the same difficulties arose, and here strong timber struts were interposed. For the same reason, the first diagonal bracing between these columns was kept up as close as possible to the platform. (See Plate X.)

Nor was this the only matter requiring care, for although these columns might be kept the proper distance apart from each other, yet might they also both deflect in one direction, be this east or west, north or south. With the heavy platforms carried on these comparatively unconnected members, a strong wind from one side or the other could produce serious distortions and deflections, and the rivetting up of the vertical columns following so close upon the rise of the platforms, made them so stiff that it was not easy to deal with them. In checking the columns, reference was always made to the centre line of the bridge, which was thrown upon the cross girder which carried the platforms, and measurement from this centre was taken to each side. That side which deviated least was first dealt with and pulled or pushed to the right position; there it was held by wire rope ties or timber struts, and the other corrected subsequently. This cross-girder itself was used for pushing the columns apart or in, one side being fixed for the time and the other left loose.

The first thorough correction was made at the top of the first vertical wind-bracing between the columns, where a solid plate girder passes right across, which carries the internal viaduct girders. This is shown in Plate X. The large gussets were fixed to the columns, and the booms of the latticed girders brought up to them. But the gussets were not yet drilled, and only after the position of one column had been ascertained and corrected, these holes were drilled, and the corner booms fixed. A fixed point being thus obtained, the other column was either pushed out or drawn in as the case might be, and the booms fixed in the same manner.

By this time the platforms had already approached each other to half the original distance, and all the temporary girders became much shorter and more easy to handle. The men also had become so familiar with the work that they knew exactly what was required, and made little account of the height at which the platforms had now arrived, namely, some 200 ft.

At the next halt a great deal of work required to be done, namely, to build in the crossing of the diagonal struts in the centre of the pier. This for obvious reasons could not very well be done above the platform, but some 20 ft. below. In the case of Inchgarvie this crossing contains 80 tons of steel on each side, and at the crossing of the vertical tie at centre, and the horizontal bracing occurs at the same point, a very intricate piece of work required to be done.

Here the vertical columns could be corrected for position north and south by means of the horizontal bracing which runs longitudinally from one vertical column to another, intersecting the crossing of the diagonal struts. A pair of horizontal bracings are also placed here between the four columns as shown in Fig. 4, Plate III. A strong framework, similar to the one immediately above the circular granite piers, thus closed the lower half of the central tower, and a new start upwards could be made from a fresh fixed base. At this stage a prodigious amount of work was done, immense quantities of material were drawn up the hoist and distributed on the top of the platforms where the platers, working on tall ladders, bolted up beams and plates often 25 ft. above the platforms. Between the top of the platform and the bottom, men were at work removing and replacing temporary bracing, which came in the way of the diagonal struts as they were built up. Inside the columns, the hydraulic men were busy preparing for the next lift and examining the caps, leathers, and pipe joints. Below these in the cages and inside the tubes, the gangs of machine rivettors vied with each other which could get done the quickest, a premium being paid to the squad that had done its section first. Again, below the cages, men were replacing diaphragms and other details of the structure which had been removed to allow the lifting girders to pass, and still lower down, squads of hand-rivetters were rivetting up beams and diaphragms, and putting in such rivets as the machine had not been able to do. Besides these a host of other men carpenters to put up