FO UN DA TIONZ. 239 



transverse strength of such a plate or washer depends upon 

 its thickness, and an exact formula is difficult to arrive at, 

 but that used by Kidder is probably upon the safe side, and 

 is as follows : 



I W X P~ 



Thickness of plate in inches = \ / 



y 1600 



where W is the unit load per square inch in the present case 

 140 Ibs. ; P, the projection of the edge of the plate beyond 

 the rod, in this case say 6.5 in. Substituting these values in 

 the formula, the thickness of the cast-iron plate or washer is 

 a little less than 2 in. at its thickest part next the rod. 



As a rule, the bearing-value of the soil will seldom be 

 considered safe for a load as great as that considered above, 

 and the bearing-value of the soil must be increased by 

 spreading the foundations over a greater area. 



In order to consider such a condition, assume that the 

 bearing-value of the soil is not over 2 tons per sq. ft. of sur- 

 face, and that the same conditions exist as were considered in 

 the preceding example. Let the safe bearing 2 tons be rep- 



W W 



resented by B, and B = -j- ; then A = -=- . Let A be the 



A J) 



total area and Wthe total load. 



The total constant weight of the tank and water was 

 found to be 1776 tons; the wind pressure, approximately I 

 ton per sq. ft., exerted over an area of 452 sq. ft., adds 452 

 tons; while the weight of the masonry was estimated at about 

 27 tons per vertical foot, and for 6 feet amounts to 162 tons, 

 or a total, W, of 2390 tons. Substituting this value for \V 



W 

 in the formula A = --, the required area of base is about 



39 feet ; but spreading the base increases the weight of the 

 foundations, therefore some greater diameter must be selected 



