FOUNDATIONS AND KETAINING WALLS 201 



Problem 163. A one-ton hammer falls 15 ft. on the head of a pile, and the 

 settlement is observed to be . 1 in. Calculate the safe load for the pile by formulas 

 (104) and (105) and compare the results. 



Problem 164. Under what conditions will the approximate rational formula 

 (104) and the Engineering News formula (105) give substantially the same results ? 



Solution. If the values of R obtained from these two formulas were equal, then 



; whence d = ^ in. For other values of d the rational formula gives the 

 ()d d + 1 



greater value of the bearing power when d < ^ in. , and the empirical formula gives 

 the greater value when d > ^\ in. From this it follows that the empirical formula 

 is only applicable when the settlement at the last blow is small. 



161. Ordinary foundations. Although the foundation of a struc- 

 ture is necessarily the first part to he constructed, it is the last part 

 to be designed, for the weight of the structure determines the nature 

 of the foundation, and this cannot be calculated until the structure 

 has assumed definite proportions. 



The load which a structure is designed to carry consists primarily 

 of three parts. 



1. The dead load, due to the weight of the structure and the per- 

 manent fixtures, such as plumbing and heating apparatus, elevators, 

 water tanks, machinery, etc. 



2. The live load, which depends on the use to which the structure 

 is to be put, and which may vary from 20 lb./ft. 2 to 400 lb./ft. 2 



3. The wind load, due to the overturning action of the wind upon 

 the side of the structure. These three parts of the total load must 

 be calculated separately and then combined so as to give the maxi- 

 mum resultant. The area of the foundation is then found at once 

 by dividing this maximum load by the safe bearing power of the soil. 



The chief concern in designing a foundation, however, is not that 

 its settlement shall be zero, but that it shall be uniform throughout. 

 For if one part of a foundation settles more than another, it is evident 

 that cracks are bound to occur which will seriously weaken the struc- 

 ture and may even destroy its usefulness altogether. Since uniformity 

 of settlement implies uniformity of pressure on the soil, the condition 

 which determines the stability of a foundation and its superstructure 

 is simply uniformity of pressure on the soil. 



The effect of violating this condition is frequently seen, the most 

 common instance being that of ordinary dwelling houses in which 

 several openings, say a door and a number of windows, occur one 



