are precisely the same as those on Inchgarvie, while in the Queensferry south-east and south-west, and the Fife north-east and north-west, both longitudinal and lateral movements are provided for. The former is for expansion and contraction between the fixed circular piers and the cantilever end piers, the latter for any lateral deflection in the same length due to wind-pressure acting at right angles, or nearly so. In the cantilever end piers lateral movement is prevented, but longitudinal movement provided for.
It will be noticed that the key-plates, with the exception of those belonging to the fixed cantilevers, are shown in three parts, an oblong centre-piece and two segments of a circle. Of these, the two segments are only 2 in. in thickness, and therefore simply fill up the recess in the lower bedplate, while yet, however, they admit and facilitate, as well as control, a circular movement round the centre. The central part, or oblong key-plate, is 23⁄4 in. in thickness, and enters therefore to the extent of 3⁄4 in. into the upper bedplate, to which it is rivetted. This, however, is somewhat different in the three fixed piers, for here both the two segments and the rectangular centre-plate are all alike 23⁄4 in. in thickness, and the whole key-plate, 12 ft. in diameter, enters into the upper key-plate. The central piece is the only one attached by rivetting, the wings simply are laid loose into the recesses.
The square key-plates in the bedplates of the fixed cantilevers are fixed to the upper bedplate, and enter the lower bedplate to the extent of 1 in. only, the whole bedplate being much lighter than in the case of the free cantilevers.
_Figs._84_and_85,_Page_41.png)
HYDRAULIC CRANES.
_Figs._86_and_87,_Page_41.png)
RIVETTING MACHINE ON PIERS FOR BEDPLATES.
_Fig._88,_Page_41.png)
ARRANGEMENT OF BEDPLATES ON GRANITE PIERS
The upper bedplates form the under part or foot of the main junctions or skewbacks. These plates are, like the lower bedplates, built up of several layers of plates of varying thicknesses, according to position. The lowest course of plates, placed longitudinally, is 3⁄4 in. in thickness, and in this is cut the recess which receives the key-plate of whatever shape it may be. The next course is laid transversely—11⁄4 in. in thickness—and extends, like the first, over the whole area. The third course, laid longitudinally, again is 3⁄4 in. in thickness, but extends only for about two-thirds of the area on the side next to the horizontal connecting bottom member. This plate also takes the attachments for the bottom booms of the horizontal cross bracing and the horizontal diagonal bracing. (See Fig. 91.) A further thickness of plates, as a fourth course, occurs in the case of the fixed piers, to make up for the enlarged recess in these, and this course is formed under the base of the vertical columns only.
These plates are stiffened by twelve girders of I section—about 11 in. deep, and running longitudinally—consisting of web-plates and four angles. The third and fifth girders on each side are omitted, the main webplates of the skewback taking their places. (See Fig. 90 ) Transversely these girders are combined and stiffened by cell-plates, flanged on all four sides in a die. There are eleven rows of these, and upon them are set the main diaphragms, which reach from the bedplates to the crown of the skewbacks. The remaining spaces on each side of the inner webplates, and between these and the outer web-plates, are filled by similar diaphragms, and at the same distances apart.
The sections and plans shown in Figs. 89 to 94 will fully explain these parts, which are of the utmost importance, because through them all the stresses thrown upon or against the structure, must pass on to the supports. The lower half of the skewback is square in form, the upper half for about two-thirds is rounded, the remainder being squared in order to receive the attachments of the top booms of bracing girders mentioned above. The inner main webs are J in. thick on Fife and Queensferry and 1 in. thick on Inchgarvie. and these are carried through from end to end of the skewback at a distance of 3 ft. 6 in. to each side of the centre line. These plates form the main points of attachment for the diagonal struts and for the bottom members, both within the central towers and in the cantilevers. The outer webplates, 1 in. thick on Fife and Queensferry and 11⁄4 in. thick on Inchgarvie, receive the main thrust of the vertical columns, and are carried on each side into the bottom members by a change from the rectangular into the circular form. (See Figs. 92 and 93.) They are considerably stiffened in these parts by horizontal diaphragms and by doubling plates of great strength, and the skeleton work to which they are attached is gradually merged into the main beams of the regular section of the bottom members.
The attachments of the five lattice wind-bracing girders, although most ingeniously contrived, does not call for any special remark.
It will be noticed in connection with the skewbacks that the centre of the vertical columns does not coincide with the centre of the circular granite
