Page:Forth Bridge (1890).djvu/60

Rh in such manner that this tie could be shortened to a certain extent, its upper end at the crossing being now an absolute fixture. By means of an arrangement of cross-girders and hydraulic rams this joint could now be closed to any desired extent, new cover-plates being, of course, required to make good the joint.

All being thus prepared, a theodolite was placed in the centre of each bottom member, and these were lifted, both at the vertical ties and at the end of the inclined ties 1, by means of the hydraulic rams, until the members had risen to the correct angle and a trifle beyond, when hardwood packings were put between the girders, and wedges driven in, so as to secure the maintenance of their position. Thus placed, the holes in the new cover-plates making up the joints in the booms of the vertical ties were drilled at once, and the joints rivetted up.

The closing lengths of the booms of the inclined ties 1 were also measured, and templets made of them and taken to the shops. These lengths were then cut and drilled, and at once put into their places, and the whole of the ties finished up in every part and detail.

The effect of these operations then was: 1. To secure the absolute correctness of the position of the first bottom junctions as regards the height above water, and their distance from the centre line of bridge.

2. To put upon both the inclined ties 1 and the vertical ties that stress which would correspond with their share of weight of the structure put up, and to give them the elongation proportionate to that weight.

With this there were now, outside the central towers, two points established, the position of which was correct, and which were capable of sustaining the full weight which they would have ultimately to sustain in the completed structure.

The next operation was to raise the top members from the temporary vertical supports carried up from the points of crossing, and this also was done by hydraulic rams. The temporary supports raised from the points of intersection and carried to the top members were used at the centre of every bay out, becoming in each case much shorter and lighter. They had to be removed, owing to want of material for making a larger number, frequently before the top members were rivetted up sufficiently to bear their own weight and that of the staging which was attached to them, and considerable deflection in some of the longer sections was the result.

The top member crane could now slide forward another section and build, not only the next section of the top member, but also the upper halves of struts 1. These did not take long. They were built from the bottom upward, in the same way as the inclined ties were built from the top downward. For the struts light square stages fenced on four sides were used, these being 15 ft. square and open in the centre to admit the strut. The stages were accessible by means of wooden ladders laid from bottom upwards or by rope ladders hung from the top. The bars and plates were picked off the viaduct by the top member crane, swung into position, and at once bolted on, while the stage itself, when it required raising, was attached to the crane for the moment and lifted up, when it was attached again by chains or wire ropes to the strut itself.

The tie staging was hung to stout rope tackle and let down by the men working on the ties themselves.

As soon as the struts had been built up to a point close under the junctions with top members the latter had their position checked by theodolites, and, if required, they were upon these points also lifted by hydraulic pressure rams and then drifted up, and when in correct position were at once rivetted. Another fixed point was thus secured, and the first bay in cantilevers practically completed.

The plate girder, reaching transversely from one vertical tie to the other, and upon which are supported the girders of the internal viaduct, had meanwhile been put in place, as also the diagonal bracing below it. The viaduct was then advanced over it and carried forward till it reached the first trestle at end of bay 1, which reaches across the first bottom junctions and gives support to the internal viaduct at this point.

So far completed, all members were at their correct elevation so far as this could be secured, but it was possible, and happened frequently, that, owing to continued strong winds (no permanent wind-bracing was as yet fixed) from one quarter, a certain amount of displacement occurred. Thus,