136 TOWERS AND TANA'S FOR WATER- WORKS. 



The distribution of the vertical forces among the posts is 

 uncertain. To determine it, the position of the axis about 

 which the tank tends to rotate must be known. This depends 

 on the rigidity of the connections, which cannot be determined 

 readily. When the connections give the same resistance to ten- 

 sion as to compression the axis of rotation passes through the 

 centre of the polygon. This condition obtains when the posts 

 are riveted directly to the tank cylinder. When the resistance 

 to tension is very small compared to the resistance to compressicn 

 the axis of rotation is near the extreme leeward post (or posts). 

 This condition obtains when the circular girder rests on top of 

 the posts and is bolted thereto. The former condition gives 

 a maximum tension on the windward side, and the latter a max- 

 mum compression on the leeward side. As the connections 

 of the tank to the post should be rigid, approximating the former 

 condition, that case only will be considered. 



Assume the direction of the wind normal to the side EF and 

 the axis of rotation mm (Fig. 35). Since the stress in each pest 

 is proportional to its distance from the axis of rotation mm, the 

 tension V ' E V F = V B =- V c , and there is no stress at A and D. 

 Taking moments about mm, the moment equation is 



M =4V E X 0.8667?, 



M M 



in which M is the overturning moment previously determined. 

 Next assume the wind in the direction of a diagonal EB 

 and the axis of rotation m'-m f . Then 



M- (V A + V c + V D + V F )o. S R + (V 



