134 TOWERS AND TANKS FOR WATER-WORKS. 



likely to occur. Since wind stresses do not control the design of 

 the heavier parts of the structure, there is but little excess ma- 

 terial used if the assumed wind pressure is greater than the actual 

 wind pressure. The writer considers it good practice to design 

 water-towers to withstand a wind pressure of 30 pounds per 

 square foot of projected area of the tank and the members of the 

 tower. 



Stresses in the Cylinder. Under the assumption made 

 above, the forces acting on the cylinder will be as shown in the 

 diagram (Fig. 34). The cylinder may be treated as a cantilever 

 beam. Then the bending moment at the fixed end in inch- 

 pounds is 



The extreme fibre stress resulting is determined from the formula 



_M L 

 " 



in which S is the extreme fibre stress per square inch, M is the 

 bending moment just determined, r is the distance in inches of 

 the extreme fibre from the centre, which in this case is the radius 

 of the cylinder, and 7 is the moment of inertia. The moment 

 of inertia of a thin cylinder about a diameter is 



t being the thickness of the shell. Then 



iSoA 2 DXr A 2 



In water-towers of usual dimensions this stress is small, but in 

 stand-pipes and chimneys it may become large. The local bend- 

 ing stresses are indeterminate. 



