234 TOWERS AAD l^ANKS FOR WATER- WORKS. 



the area, and the maximum pressure to be transferred to the 

 subfoundations will consist both of the normal and variable 

 pressures. From the principles of resistance of materials, 

 previously explained, the *'live load" or variable pressure 

 due to the wind can be found from the formula 



Wind pressure = - ; 



and the maximum pressure will therefore be 



W Ml 

 Max. pressure -^ + ^j ; 



where M = moment of the wind ; 



/ = the leverage at the base ; 



/ = moment of inertia of the shape. 



Where it becomes necessary to extend the base of a 

 foundation in order not to overload the bearing soil, the 

 foundations will extend in regular courses, and the safe pro- 

 jection of the successive courses will depend upon the pres- 

 sure applied as force and the resisting quality of the material 

 of which the courses are composed. 



The theory of this action and resistance is given by Prof. 

 Ira O. Baker, in " A Treatise of Masonry Construction," and 

 is as follows : 



" The area of the foundation having been determined and 

 its centre having been located with reference to the axis of 

 the load, the next step is to determine how much narrower 

 each footing-course may be than the one next below it. 

 The projecting part of the footing rests as a beam fixed at 

 one end and uniformly loaded. The load is the pressure on 

 the earth or on the course below. The set-off of such a 

 course depends upon the amount of the pressure, the trans- 

 verse strength of the material, and the thickness of the 

 course. 



