264 PRACTICAL STRUCTURAL DESIGN 



each day, that column is selected on which the live load is from 

 20 to 25 per cent, to allow for the constant machine load. The 

 student can see that there is considerable room for the exercise 

 of judgment in the matter, provided the fact is recognized that 

 to design all footings for the sum of the dead and live loads is 

 wrong. 



The load on walls is assumed to be one foot long. The load on 

 one lineal foot of wall is divided by the allowable soil pressure 

 w per square foot and the width I of the 



footing found, Fig. 170. It is then 

 stepped. An old rule was to draw a line 

 upward, at an angle of 60 degrees from 

 1 I i the end of the footing to the lower 



. I I y 



T ^~ corner of the wall and form steps so the 

 ., "*- line touched the inner corners. By as- 



- ^ 



suming a safe fiber stress for the masonry 



Fig. 170 ^d computing the offsets as projecting 



Steppec 3 ting cantilevers the thickness and projections 



of the steps may be computed. The following formula is used 

 generally by designers; referring to Fig. 170: 

 o = offset in inches, 

 t = thickness in inches, 



p = allowable soil pressure in pounds per sq. in., 

 s = safe unit tensile stress in the material, 

 = 30 Ibs. per sq. in. for 1-3-5 concrete, 

 = 60 Ibs. per sq. in. for 1-2-4 concrete. 



Various authorities give values for stone and for brick laid in 

 cement mortar. The values for stone cannot be used unless the 

 stone projects less than one-half its length beyond the step above. 

 This is to provide for true cantilever action. If built in this way 

 s = 80 to 130 Ibs. per sq. in. for limestone, the same for sandstone 

 and 180 Ibs. per sq. in. for granite. Hard-burned brick laid up in 

 cement mortar in good bond by a first-class mason is considered 

 to be good for 40 Ibs. per sq. in. The author advises the use of 

 concrete. 



o = 



p 



The formula is derived as follows: 



_ 

 X = 



44 2 288 



