DESIGNING. 211 



alone to resist this stress without considerable additional metal, 

 and in such cases it may be necessary to employ a continuous 

 curved girder in the horizontal plane. 



Overturning Moments at Points of Support. The over- 

 turning moment of the tank at its points of support must be 

 resisted by the connections of the tank to the tower where the 

 weight of the empty structure is insufficient to produce stability 

 of position, but in general it is always considered necessary to 

 rivet the flange of the girder to the top of the posts. 



Where P = total wind pressure ; 



G = distance in feet from top of post to centre of gravity ; 

 M = overturning moment in foot-pounds. 



From the formula given, M = PG\ substituting the values 

 from the diagram (Fig. 58), 



and G = - -=23.4 feet; 



M = 672,516 foot-pounds. 



With the assumption that the direction of the wind is in the 

 direction of a diagonal in the horizontal plane of the figure formed 

 by the posts, the tank would tend to overturning about an axis 

 at the leeward post, and is resisted therefrom by the weight of 

 the tank metal multiplied by its leverage, or in this case, by the 

 radius of the tank; then M==6i, 795X10 = 617,950 foot-pounds. 



Assuming 10,000 Ibs. per square inch of rivet metal as a safe 

 value, each square inch of metal would have a holding-down 

 value of 100,000 foot-pounds; hence 4 rivets, 7/i6-inch diameter, 

 providing an area of 0.6 1 square inch, would prove sufficient 

 to establish equilibrium; however, it is usual to make these 

 connections of considerably larger diameter, usually i-inch rivets 

 being employed and driven at such points as may be convenient 

 structurally. 



