126 



SMALL WATER SUPPLIES. 



be easily computed by assuming it as a loaded and 

 supported beam (not fixed) between the buttresses 

 which would be placed from 6 ft. to 8 ft. apart accord- 

 ing to circumstances. No tension will exist if the 

 inner face between the buttresses is slightly arched 

 in plan. 



Now having designed such a dam, it remains with 

 the designer to calculate its cubic contents, and also 

 the cubical contents of a solid dam to fulfil the same 

 conditions. The saving is generally about 35 per 

 cent, but it must remain to the 

 judgment of the engineer to decide 

 if the saving in actual cubic feet 

 of masonry is going to counter- 

 balance the extra cost entailed in 

 building a wall of this more com- 

 plicated description. The weight 

 of the wall between the buttresses 

 can be found as follows. Referring 

 to fig. 89 



(41) 



Z being as shown and ^ = 62*5 Ib. 



The overturning moment due to the water may be 

 found as follows : 



M = r(r+ i)Bw . . . (42) 

 6 



B being as shown in the figure. 



The foregoing method of construction can be 

 economically carried out in reinforced concrete. 

 Solid masonry walls stand the water thrust by reason 

 of their own weight, but reinforced concrete walls 

 resist the load by means of a lever arm. The main 



