DESIGNING. 205 



a leverage which for practical purposes may be considered as 

 being equal to the height of the tower and half of the tank, and 

 produces the overturning moment of the structure, which must 

 be resisted by the weight of the metal, acting through a lever 

 arm equal to the horizontal distance from the centre of the figure 

 at its base to an axis about the outer lines of the base. When 

 the forces are not in equilibrium, anchorage must be provided 

 consisting in general of steel rods, two usually to each tower leg, 

 with nuts at each end and provided with a cast-iron washer to 

 be buried in the masonry pier. 



Graphic Diagram. The capacity of the tank, the height of 

 the tower, and other general features having been considered 

 and decided upon, a diagram similar to that shown in Fig. 49 

 may be drawn to any convenient scale and its principal dimensions 

 first indicated. The theoretical thickness of the cylinder plate, 

 web girder, and tank bottom are next to be found from formulas 

 or tables given heretofore, after which the details of the cover 

 and balcony should be determined, illustrations of which have 

 been heretofore shown, and from all of which the gravity loads 

 may be approximately calculated and the resulting stresses to 

 which the circular girder and the supporting columns are sub- 

 jected can be closely determined either analytically or graphically, 

 as may be preferred, and their members properly proportioned. 

 Fig. 59 shows a stress diagram of the loads for a four-post tower 

 of dimensions as indicated in the diagram (Fig. 58). 



Estimating Quantities. Using these dimensions and pro- 

 ceeding according to the formulas given in a previous chapter, 

 capacities, areas, and quantities may be obtained as follows: 



Dimensions and Capacities. Gals. Pounds. 



Cyl. 2oX 39 ft. capacity = $\D*A 91.369 



20 ft. dia. hem. bottom, capacity = I.Q6D 3 15,680 



Total capacity in gallons 107,049 



Cyl. 2oX 39 ft- capacity, 49.1 D 2 A 765,960 



20 ft. dia. hem. bottom capacity, i6.$6D 3 130,880 



Weight of water, total pounds 896,840 



