CHAPTER IX. 

 DESIGNING CONTINUED. 



IN the general scheme of a water- supply plant, where storage 

 is required and to be obtained only by the erection of a metallic 

 reservoir, it is sometimes deemed expedient to secure a suitable 

 elevation by constructing the tank upon a supporting tower. 

 Such towers are made in many ways and of various materials, 

 brick, wood, and metal being most generally used. The choice 

 of such substructure is determined by the conditions of capacity, 

 cost, and local surroundings. 



As to the question of capacity, the same considerations apply 

 as those explained previously for stand-pipes. 



The height of the tank superstructure may be considered 

 as representing the minimum and maximum desirable or limiting 

 pressures, hence it is argued that a stand-pipe has a large column 

 of water which is useless except to support the effective head 

 of water above the minimum desirable pressure as determined 

 in feet, and that the effective column may be more economically 

 supported by an open substructure, such as a steel tower. Argu- 

 ments are also presented that the lower volume of water in a 

 stand-pipe being useless except for purposes of support, it is 

 objectionable from the fact that it is stagnant and the greater 

 volume of water is more liable to be affected by organic growths. 

 This argument is controverted upon the assumption that the 

 temperature of the water is constantly changing and therefore 

 all sections of the column are equally fresh. It is a fact, however, 

 which is used to the best purposes by builders of this type of 



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