242 TOWERS AND TANKS FOR WATER-WORKS. 



be such as to provide sufficient area to transmit the whole im- 

 posed load to the masonry without stress greater than 100 pounds 

 per square inch, and its depth should not be less than f times 

 its length thus determined. 



The bearing surfaces of all stones should be truly horizontal 

 when set and the depth of each stone should exactly correspond. 



Rod holes for anchorage must be carefully drilled from tem- 

 plets. 



According to Baker, the crushing strength and weights of 

 different stones are as follows. 



Max. Tons Wt. per 



per Sq. Foot. C. F. 



Granite 1510 178 



Limestone 1440 1 74 



Marble 1440 180 



Sandstone 1080 175 



In practice it is safe to assume the bearing value of single 

 stones at from 15 to 30 tons, depending upon their characteristics. 



The dimensions of the masonry pedestals must in each case 

 be determined by the character of the bearing soil of the sub- 

 foundations and the extent of the load to be applied; in other 

 words, the base of the pedestal must be spread so as tc provide 

 a safe bearing and this spread will govern the height. Where 

 the tower design fails to provide for ties connecting adjacent 

 posts, the horizontal thrust of inclined posts must be resisted 

 by the anchorage, and when thus resisted, the thrust produces 

 an overturning moment in the pier which must be considered 

 and provided for, usually by additional spread of the base, which 

 produces a corresponding increased load and resistance. 



