STABILITY OF STRUCTURE. 91 



It has been shown that the pressure exerted upon the bot- 

 tom of the vessel is in direct proportion to the head of water, 

 or the area, multiplied by the head of the column in pounds. 



To resist the internal hydrostatic stresses is opposed the 

 thickness and material of the plate and its riveting in a cyl- 

 indrical stand-pipe, and to proportion the opposing plate 

 to safely resist the pressure the following factors must be 

 known or assumed : 1st, The tensile strength of the metal; 

 2d, the percentage of strength of the material; 3d, a reduc- 

 tion of theoretical strength to allow a margin or factor of safe- 

 ty ; and 4th, some unit of length must be adopted repre- 

 senting the surface pressed. The unit of length is usually 

 taken for convenience at 12 in. In designing, 60,000 Ibs. per 

 sq. in. is generally assumed as the unit stress of the material, 

 and allowance for the decreased value of this unit, due to 

 punching and riveting, is made at about 33 per cent, off, or the 

 working value of a 12 in. section is at \ of its original strength ; 

 reducing the ultimate strength by using a factor of safety of 

 4 is considered good practice for such metal structures, not 

 subject to shock, hence the formula for proportioning the 

 thickness of plates intended to resist such hydrostatic pres- 

 sures may be given as 



60,000 X 12" X | 



To proportion the thickness of metal intended to resist the 

 hydrostatic pressure exerted upon the internal surface of any 

 cylinder, divide (i) by (2), therefore the following general ex- 

 pression for the thickness of metal in decimals of an inch for 

 any given diameter of tank and any assumed height : 

 D X //X 62.5 ^ 60.000 X 12" X | 



2 4 



from the above the following original tables have been com- 

 puted : 



