178 TOWERS AND TANKS FOR WATER-WORKS. 



structure, that the record of failures shows that fewer towers 

 have failed as compared with the collapse of stand-pipes. 



While this is true, it is also a fact that in the United States 

 there are very many more stand-pipes in existence than towers 

 and tanks, but on account of the comparatively small increased 

 cost of securing a greater area of bearing surface for the support 

 of the structure, and also from the fact that by the wide spread 

 of the supporting columns of a tower, the stability of the structure 

 can be so increased that the resultant of the overturning moment 

 of the wind and the moment of the weights falls well within the 

 figure limited by the spread of the columns, where the same 

 resultant could only be secured for a stand-pipe by an abnormal 

 area of base. 



The local character of the bearing soil exerts a considerable 

 influence upon the selection of either type of structure, and this 

 factor should be carefully considered in connection with the dis- 

 cussion of foundations, as explained in the succeeding chapter. 



If, after a careful consideration of the conditions from both 

 an engineering and a financial standpoint, it be determined that a 

 tower-and-tank type of reservoir is preferable, the dimensions 

 of the tank being assumed from reasoning analogous to that given 

 in considering the factors in stand-pipe design, a strain-sheet 

 is prepared as explained in the preceding chapter, but which 

 will necessarily be modified, as will be explained hereafter, as 

 far as the thickness of the lower ring and bottom plates are con- 

 cerned; the conditions for their determination being changed. 



In small railway water-supply tanks flat or horizontal bottoms 

 are usually provided, supported upon wooden sills or I beams 

 of iron or steel, attached to the upper deck of the supporting 

 structure. In such cases the thickness of the lower ring is that 

 determined by the formula, but the thickness of the bottom 

 plate will depend upon the spacing of the beams or sills. 



In cities or towns where the tower and tank is to be erected 

 for public supply some other form of bottom is generally specified, 



