368 



STEEL STAND-PIPES AND ELEVATED TANKS ON TOWERS. CHAP. XL 



FIG. 2. CIRCULAR GIRDER. 

 



Now in the author's "Design of Walls, Bins and Grain Elevators" it is proved that the 

 bending moment at the supports is 



it W-r ( l I \ , . 



M i = I cot- I (20) 



n \a 2 2) 



and the maximum moment midway between the posts is 



,, 

 = Mi -cos- 



2 



W-r 

 2n 



2 sin 2 

 4 



(21) 



The torsional moment is zero at the supports and midway between the columns, and is a 

 maximum at the points of zero bending moment at points between the columns. 

 The torsional moment is 



,, . , W-r . .. . W-a'-rf sin'\ 



T b = Mi'sm a.' (i cos a') H ( I r - ) 



2n 4 \ a' J 



Values of M and T are given in Table la. 



(22) 



TABLE la. 

 STRESSES IN CIRCULAR GIRDERS. 



Stresses in Columns. The stresses in the columns will be due to the dead load and to the 

 wind moment. The vertical components of the dead load stress will be equal to W divided by 

 the number of columns, where W is the total weight of tank and the water. To calculate the 

 stresses due to wind moment in the columns proceed as follows: Calculate the wind force by 

 multiplying the exposed surface by the wind pressure, and assume the wind force as acting through 

 the center of gravity of the exposed surface. The pressure on circular tanks may be taken at 

 two-thirds of 30 Ib. per sq. ft. of the surface at right angles to the direction of the wind. To 

 calculate the stresses in the columns at any point pass a horizontal section through the columns 



