FOUNDATIONS 

 V 6Vd 



335 



and minimum at B and equal to 

 V 



When the center of pressure is at t, d = and P& = O, while 



6 



; that is, twice the average intensity. 



If the center 



of pressure falls between t and A , Pb becomes negative, which 

 means that the foundation at B is then subjected to an uplifting 

 force; in order, therefore, that this should not occur, the 

 foundation must be so designed that the center of pressure will 

 fall within i, the middle third of the line AB. This principle 

 is known as the rule of the middle third. 



SPREAD FOUNDATIONS 



Spread foundations are used in order to enlarge the base of a 

 structure until it covers an area of subfoundation that can 

 safely carry the weight of the structure. This is ordinarily 

 accomplished by means of offsets called footings, as shown in 

 Fig. 1. 



Masonry Foundations. In masonry 

 construction, the footings may be treated 

 as cantilevers uniformly loaded. The force 

 acting on mn, for instance, is the upward 

 pressure on the part ab of the subfounda- 

 tion. This pressure is assumed to be uni- 

 formly distributed, its intensity being equal 

 to the total load on the structure divided 

 by the area of the subfoundation. Like- 

 wise, the force acting on pq is part of the 

 upward pressure, or reaction, on nn. The intensity in this 

 case is the total load of the structure divided by the area at nn. 



EXAMPLE. Fig. 1 shows a wall A 2 ft. thick carrying a load 

 of 12 T. per lin. ft. of wall, including its own weight. The 



p IG 





