WATER CONDUCTORS 133 



the bands. When the pipe is filled with water the staves swell 

 sufficiently to bed the bands slightly into the wood and make 

 the longitudinal joints water-tight. 



The size of the bands and the spacing are naturally related, 

 and when properly designed they should be strained to their safe 

 resisting value, and the bearing pressure on the stave must not be 

 greater than the safe bearing value of the wood. It has been 

 found from actual experience that the width of contact between 

 the band and pipe is equal to about the radius of the band before 

 the fibers of the wood are crushed beyond safety. The safe 

 crushing stress for wood is generally taken as 650 pounds per square 

 inch, and putting the safe stress in the band equal to the safe- 

 bearing pressure, we get 



or 



ITS 



Where 



r = radius of band in inches; 

 R = internal radius of pipe in inches; 

 t = thickness of stave in inches; 



s = safe tensile strength of band. Taking the ultimate strength 

 of steel as 60,000 pounds, and assuming a factor of 

 safety of 4, the safe strength is 15,000 pounds per 

 square inch. 



The number and thus the spacing of the bands depends on the 

 stresses due to the water pressure and to the swelling of the wood. 

 The sum of these two stresses should be equal to the safe strength 

 of the band, as determined by the previous formula. 

 Thus 



and 



pR+tE' 

 where 



d= spacing of bands in inches; 

 p = water pressure in pounds per square inch; 

 E = swelling force of wood per square inch. This is usually 

 assumed to be approximately equal to 100. 



