STORAGE AND DISTRIBUTION. 121 



scale, ^ = 300 sq. ft, and the weight of the 



wall per foot run = 300 x 170= 5 1,000 Ib. The centre 

 of gravity is found by setting out the dotted lines as 

 shown, by extending the base a distance each side 

 equal to the top thickness of the wall, and the top a 

 distance each side likewise equal to the base thick- 

 ness and joining the diagonals whose intersection 

 will be the centre of gravity. 



Water thrust-^' ' (3 



2 



62*5 x 27 x 28*4 ^ 1U 



- = 23,962 Ib. 



Acting as stated J the way up from the base, this 

 thrust always being perpendicular to the face of the 

 wall. Plot the water thrust horizontally and the 

 weight of the wall vertically, as shown, and complete 

 the parallelogram and scale off the resultant. Also 

 very carefully note the distance d. 



Then the maximum pressure on the outer edge 

 will be equal to 



Hr- <k-l- 



W being the vertical component of the resultant. It 

 is always wise to find out when the wall has been 

 so designed, that the resultant falls within the middle 

 third of the base (which it must do if the wall is to be 

 safe), the exact amount of tension or compression on 

 the inner or outer joints of the wall, as the case may 

 be, so as to see if they are within safe limits. Of 

 course, an ordinary mortar joint should not be sub- 

 ject to any tension. A concrete wall may with safety 

 bear a small amount, while a reinforced concrete 



