Page:Forth Bridge (1890).djvu/63

Rh the deck up to the tops of the towers, and then to each side down the cantilevers, not only the pressure pipes for the water working the hydraulic rivetters, but also for the supply of compressed air and oil to the furnaces. The oil was brought in pipes connected with a tank on the top of the central tower, and run down to the furnace tank by gravity.

With the last intersections built in bay 6, and struts and ties, top members and bottom members carried past them, it only remained to put up the end posts to complete the cantilevers.

The weight which had now been raised upon the supports may be shortly stated as follows:

or, on the three points, a grand total of 49,050 tons, not counting the approach viaduct spans at either end.

But, apart from the permanent work, many hundred tons of weight in the shape of cranes, temporary girders, winches, steam boilers, rivet furnaces, rivetting machines, miles of steel wire ropes and of gangways, and acres of solid timber staging were suspended from these cantilevers. A heavy shower of rain would in a few minutes put an extra weight of a hundred tons, and the storm would try its worst against these immense surfaces, but with no result. Only divided by a gap of some 350 ft. it was now possible to take up a position in a gale of wind, and by fixing a point on the opposite cantilever end and another point fully half a mile further back on the shore, try to see what the lateral deflection might be. But, whatever its amount, it was too small to be noticed by the naked eye; nor could any movement be felt except a slight vibration whenever an extra heavy gust of wind would hurl itself against the solid face of steel plates.

In setting out the centre lines in the vertical sense of the bottom members while building out plate by plate, allowance had been made at every junction for the natural and unavoidable deflection in the whole cantilever as a mass. This was, of course, also done in setting out the internal viaduct.

It was intended that there should be in the free cantilevers a camber of 10 in. at the end posts when the cantilever was completed—that is, the line of rails at the end post would be 10 in. higher than in the central towers when no load was on the bridge. But to get this it was necessary to set each section higher by so much as it would deflect by the addition of the remaining sections further forward. The point aimed at was, therefore, set another 10 in. up, or 1 ft. 8 in. altogether. This was, of course, entirely a matter of calculation and of judgment, and in the end the cantilevers arrived at the position in which they were desired to be.

It remains now to tell how the central girders were erected and the final connections made.

The end posts are hollow boxes about $4 1/2$ ft. deep and 3 ft. wide, by 40 ft. in height, and are closed on three sides, the fourth side towards the central girder being open. The bottom members project right to the end of the post, while the top members stop short at the closed or inner side, except the webplates, which, in the shape of large gussets, are also carried full to the open end. So far the four free cantilevers are exactly alike, but in the further arrangements they differ considerably, as will presently be described.

The end posts of the two fixed cantilevers where they rest in the cantilever end piers are different from the above. Here the posts are replaced by a large box about 8 ft. long and extending over the whole width and height of the end of the cantilever, out of which an arched way has been cut to allow the passage of the trains. An end elevation of this box is shown in Fig. 23 on a preceding page. The object of these boxes has been already explained, and will be again referred to in connection with the expansion movements provided for at this point. They are filled with cast-iron bricks and punchings and other scrap all laid in asphalt poured in hot and firmly set, thus preventing shifting and at the same time making the box water-tight. About 1000 tons of dead weight is placed at these points over and above the weight of the steelwork.

The central girders have already been described as having a slightly raised or curved top member of polygonal form; that is to say, it is straight from one support to another, a kink taking place at the point of support, in the same manner as the bottom member in the cantilevers. The bottom member is straight. The two are connected by eight pairs of cross-bracings on each side, intersecting each other at the centre and consisting of struts and ties. The girder is 350 ft. in length, 40 ft. high at the ends, and 50 ft. high in the centre. It is divided into eight bays of slightly unequal length. The lop members are braced together by 16 sets of diagonal lattice bracings, while the bottom members are connected by solid plate girders acting as cross-bearers, one at the centre and one at the end of every bay, in addition to those forming the ends of the girders. Vertical ties are attached to each intersection of struts and ties and carry the bottom members between the bottom junctions. The bottom members are trough-shaped and about 3 ft. high by 2 ft. 6 in. wide;