682 MODERN GASWORKS PRACTICE
here it should be pointed out that in the riveting of tanks it is the vertical seams which require particular forethought, the horizontal joints give little trouble—are double butt-joints designed so that a portion of the rivets are acting in single shear and some in double shear, and so arranged that the efficiency of the joint is 93 per cent. All the horizontal seams are lap joints single-riveted with rivets varying in size from lf inch at the base to 1 inch for the top row. Both plates and butt-straps are bevelled-edged, this or ordinary planing being commonly carried out to facilitate caulking. The tank bottom is formed of | inch plates, 20 feet long, and 7 feet 6 inches wide, connected together by f inch rivets. The curb row of plates is f inch thick. The bottom curb consists of an external angle, 8 inches by 8 inches by 1| inch thick, put together in twenty-six sections joined by angle connection. plates. Theupper curb of tlie tank consists of a horizontal plate | inch thick attached to the tank
Fig. 423.—Top Curb for. Steel Tank (Spibal Holder).
shell by a 5 inch by 5 inch by | inch angle, and is stifEened round its outer edge by a 12 inch cliannel.
A point of importance in the design of holders with. steel tanks is that of ensuring that the tank structure shall itself be as independent as possible of the stresses set up in. the guide-framing. Any such, stresses should be carried down, not to the top curb of the tank, but to tlie foundation. This precept is often overlooked, with the result that the tank plates are subjected to stresses for which they were not designed. A frequent error in design is that of carrying the diagonal cross-bracing of the panel down to the top curb of the tank, thus leaving the tank-panels unbraced. As regards the guiding of the bell in the tank it is customary to provide intermediate rails