336 FOUNDATIONS 



foundation of concrete is designed to have each footing project 

 1 ft. beyond the one above. What should be the thickness of 

 each course, assuming the maximum allowable fiber stress of 

 concrete in tension to be 25 Ib. per sq. in.? 



SOLUTION. Since the load is 24,000 Ib. per lin. ft., the 

 intensity on the bottom course is 24 , 000 -r- 8 = 3, 000 Ib. per sq. 

 ft.; on the second course, 24,000-5-6 = 4,000 Ib. per sq. ft.; and 

 on the third, 24,000-^4 = 6,000 Ib. per sq. ft. The respective 



o ffcAAX/ 1 O 



bending moments are, therefore, = 18,000 in.-lb.; 



,.,,. Then, 



b&f 



apply the formula M = - . Here, 6 = 12 in. and/=25 Ib. per 

 6 



sq. in. Solving for d, there results for the bottom course, 

 /18.000 



for the second course, d = 



and for the third course, d = 



50 

 Steel Foundations. In a steel spread foundation. Fig 2, 



W 



TTTTTJ I .. 



\f IIIIlllIIIlIIIIIIl* 



FIG. 2 



the bending moment is considered to be maximum at the center 

 of the beam, and its amount is equal to 



-= 



EXAMPLE. The total load carried by the bottom course of 

 steel I beams. Fig. 2, is 360,000 Ib. ; the length of the beams is 



