Page:Forth Bridge (1890).djvu/55

Rh stages, gangways, timber stiffeners, floors, and all sorts of things; men attending to the rivet furnaces and trying to keep the machines going which put in their eighty to ninety rivets every hour, electric lightmen shifting lamps, putting in carbons and lixing fresh cables, and finally the hundred and one men who never seem to do anything, and yet cannot somehow be spared.

At last the tops of the platforms have reached their final position; and they can go no further, although the superstructure itself has to rise some 20 ft. more. (See Plate XII. and Fig. 108.) The two platforms by now have come so close together that the inner hoist-frame on one side had to be removed previous to the last lift. The long box girder now projects a long way outside each column, and stands quite clear above them; the whole platform looks as if a heavy gust of wind would lift it up and throw it over the side. The last length of beams and the closing lengths of the plates require to be measured before they can be cut to length. Once more the columns are carefully set to the centre line of the bridge and to north and south, and the levels of all the beams in the columns taken.

While these are being got ready, the gap of about 9 ft. between the two platforms is covered over, and only one platform exists now of more than double width.

The longitudinal platform girders were stiffened and trussed by link chains stretched from support to support. These chains were portions of the Hammersmith Suspension Bridge, bought up by the contractors, and largely made use of here in various ways. But the workmen would not consent to use the whole title due to them, for some of them called them Hammer links, and others called them Smith's links; but it required a Cockney by birth or adoption to get the combination.

These links were passed under double timbers, right under the two girders of each platform, and an initial stress was put upon them by interposing hydraulic cylinders between the timbers and bottom booms of girders, and filling the space made with hardwood blocks. Blocks were now laid down on the top of the platform in the centre line of the top members, and the vertical webs of the bottom booms, the flange angles of the same laid down, and so soon as the top junctions were somewhat advanced, they were connected up at each end. The object was to relieve the platforms as soon as possible of any weight from the top members. The vertical bracings were then put on, and in these the webs of the top booms placed. The girders could now carry themselves, and rivetting was commenced as soon as the top junctions had been fixed and securely joined to the vertical columns. As there was a great deal of work to do upon these top junctions, they were completely surrounded by timber frames covered with boards, and provided with windows and a large number of electric lamps. Thus protected the work could be carried on day and night, and it was necessary, for every beam and every plate had to have its correct measurement and shape taken before it could be made. The ends of the diagonal struts, changing from a flattened round into a square, had to be templated in every plate and beam to make an exact fit. It will give an idea of the magnitude of the work when it is stated that these wooden buildings, of which there are four to a pier, erected at a height of 360 ft. above the water, were about 24 ft. square and fully 35 ft. high, with three floors at different heights.

The top junctions, although not of such importance as the skewbacks, are yet fully worthy of more than a passing remark. (See Figs. 109 to 115.) Here are united two tubular members—the vertical column and the diagonal strut—and five latticed members—the top member in the centre tower, the top member in the cantilever, the first inclined tie in the same, the highest of the four wind-bracings between columns, and the horizontal cross-girder between top junctions (the two latter not shown in the engravings).

The main strength of this junction lies in a number of very large webplates carried on each side of the centre in direct continuation of the webs of the top members. Inside these are the starting-plates, or horns, as they were popularly called, of the diagonal struts, and outside them those of the first inclined tie in bay 1 of (he cantilever. Diaphragms, supported by stiff beams with a gradual change from round to square, connect it with the vertical column. The flanges of the top booms of the top members are carried right through, but the flanges of the bottom booms are replaced by extra plates and heavy angles. Most of the rivetting of the main webs was done by ordinary hydraulic rivetters, but a large number inside the cells, by small rivetters at the 3-ton pressure of similar construction as those used in the cells of the skewbacks. The top junction is shown in Figs. 109 and 110 in elevation and plan, and requires no further explanation. The rivetting of the top member between vertical columns was now pushed on as much as possible, and, as soon as completed, the working platform was transferred to the very top of these girders, and the removal of platform girders and all hydraulic lifting gear, commenced. This was all the more necessary since it will be remembered the platform girders were made out of portions of the first inclined tie of the cantilevers, and was now at once required to fill its proper place in the structure. Many of the plates and illustrations show details in connection with the lifting platforms and the work carried on upon them, but the space here is not sufficient to enter into description of them all.

The removal of these platform girders, many hundred tons in weight, with all the odds and ends of a working platform upon them, was a work which caused no little anxiety; for upon nearly every portion of the structure below, work was carried on by scores of men to whom the fall of a small bolt or nut from that height might cause danger to life or limb. The year in which the central towers were erected shows the greatest number of fatal accidents in any one year, namely, 17, while the average over the seven years is only 9.

The bottom members from their points of junction with the skewbacks are not curved but are carried in a straight line to the first bottom junctions. At the centre of these junctions the angle alters and the members continue straight to the next junction, and so on till the end of the cantilever is reached. At each change the angle becomes more obtuse, until in the middle of the sixth or last bay the members are all but level. Looked at on plan the bottom members are 120 ft. apart, centre to centre at skewbacks, and 32 ft. 2 in. at the end