204 STBENGTH OF MATERIALS 



What has been said in the preceding article in regard to the cal- 

 culation of the loads carried by the foundation also applies to the 

 calculation of column loads, and the method of designing a column 

 footing is essentially the same as for a masonry footing, explained 

 above. Thus let P denote the total column load in tons, c the length 

 of one side of the base plate in inches, and / the length in inches of 

 the beams supporting it (Fig. 141). Then, if the base plate is assumed 

 to be stiff enough to carry the load on its perimeter, the maximum 

 moment M will occur at one edge of the base plate. Since the reac- 



tion on one side of the base plate is 2000 P > the amount of 

 this moment is 



2000 P(l - c) I - c _ 250 P(l - c)" 

 ~TT ~T~ ~T 



Consequently, if n is the number of beams supporting the base plate, 

 the maximum moment for one beam is 



250P(/-,f . 

 . = - - in. Ib. 



If the base plate is assumed to be only still' enough to distribute 

 the load uniformly, the maximum moment will occur at the renter 

 of the beams, and its value will be (cf. Article ."._' A', 



2000 ?(l-\ 

 M= ^ U. = 250 P(2 / - r) in. Ib. 



In this case the maximum moment for one beam is 



250 P(11 



M l = - in. Ib. 



n 



Now let^? denote the allowable fiber stress per square inch. / the 

 moment of inertia of a cross section of one beam, and > half tin- 

 depth of the beam. Then the moment of resistance of one beam is 



e 



For foundation work p is usually taken to be 20,000 Ib./inA Substi- 

 tuting this value, the moment of resistance becomes 



M= 20,000 - = 20,000$ 



6 



