flattened before rivetting as much as 3 in., and had to be constantly strutted with timbers to keep it in form.
The plates forming the shell of the vertical columns varied in thickness in the case of the Fife and Queensferry central towers, from 1⁄2 in. at bottom to 3⁄8 in. at top; and, in the case of Inchgarvie, from 5⁄8 in. at bottom to 7⁄16 in. at top. The beams decreased in strength proportionately.
The diagonal columns or struts in central tower: are of different shape, namely, circular top and bottom, but flattened on both sides in order to facilitate their intersection with each other and their entering into junctions top and bottom, when they are gradually changed into rectangular section. It has been pointed out in the general description of the bridge, that the effects of the live load upon the Inchgarvie cantilever are different from those in the Fife and Queensferry piers. The diagonal struts in the former case are therefore very much stronger than in the latter. The bottom and top plates are 11⁄4 in. in thickness, while the corner and side plates are 1 in. thick. The intersection of these two struts at centre of the tower is very heavy, some 80 tons of metal being massed therein. The inside beams are of corresponding strength. These diagonal columns were rivetted in the case of Inchgarvie from top to bottom by a special machine of similar design as that for the vertical columns, but more difficult to work on account of the angle at which the machine was placed.
Meanwhile, steam derrick cranes were erected on platforms raised some 30 ft. to 40 ft. above deck, commanding the whole of the skewbacks, and these were now built with portions of the diagonal struts, the vertical columns and the struts 1 in cantilevers, to as great a height as could be reached by means of these cranes. All this work was put together as carefully as possible, the larger number of holes being drifted up, but it was only bolted together pending the necessary checks and corrections being made by means of theodolites. This setting out was a work of no small difficulty, seeing that the members had to be set, not only to an inclination towards the point of intersection, but had at the same time to follow the uniform batter of the vertical columns, and had therefore a strong tendency to lean to the centre of the tower. So soon as the vertical columns and struts had been built to a height of about 50 ft. above deck, to which, in the case of Inchgarvie, had to be added the central ties, preparations were made for the construction of the lifting platforms, by means of which the central towers were raised to their full height.
PORTABLE HYDRAULIC RIVETTERS_Figs._97_to_100,_Page_44.png)
A staging, reaching on each side, north and south, from column to column, about 25 ft. in width, was raised from the deck to a height of about 38 ft. above the level of the staging, and upon this a pair of longitudinal girders were built. These girders were about 190 ft. long in the case of Fife and Queensferry, and 350 ft. long in the case of Inchgarvie. (See Figs. 101 and 102.) There were four girders—one to each side of the vertical columns—placed 18 ft. 6 in. apart, with angle cross bracings top and bottom, and diagonal bracing between top booms on one side, and bottom booms on the other side, alternately. The main booms, D D, Fig. 101, and most of the vertical angle bracing, were permanent work, being, in fact, the booms belonging to a portion of ties 1 in the cantilevers borrowed for the occasion. This platform and all its details are shown in a large number of the text illustrations as well as plates.
During the time these girders were being put together, a strong box girder C C was built on a staging between vertical columns east and west, which girder passed through openings left in both columns, and projecting outside of each, to the extent of about 4 ft. or 5 ft. For this purpose one plate was left out on each side of the vertical columns during the time the girder passed aloft. This girder in its turn was supported upon a slide-block inside the vertical column, which block was attached to a cross-girder B B. This cross-girder was of double box section, all in plate, and in its turn was supported by a number of pins about 13⁄4 in. in diameter, which passed through girder and through holes drilled for the purpose in four of the H beams of the vertical columns. Immediately below this box girder, and at a distance of about 1 ft. from it, was a second box girder of the same dimen- sions A A, and held up in the same manner by a number of pins. Upon the lower girder was placed, on a rocking underside, a hydraulic ram G G 14 in.
