COLUMNS AND STRUCTURES 273 



by having the inner toe deeper than the outer instead of keeping 

 the base level. 



The best remedy is to drive piling to help the earth carry any 

 excessive load. In several of the specifications mentioned, rules 

 are given for the use of piles. A pile acts by the bearing of the 

 lower end of the pile on the soil into which it is driven, plus the 

 friction of the pressed soil on the surface of the pile. The mini- 

 mum distance center to center of piles should not be less than 

 three feet, except under unusual conditions. Driving piles too 

 closely together often results in an actual lessening of the carry- 

 ing capacity. 



To find the center of gravity of a stepped footing such as that 

 shown in Fig. 175 multiply the area of each strip by half the dis- 

 tance from the outer edge. Add the results and divide by the 

 total area. This gives the horizontal distance to the center of 

 gravity. The distance from the bottom is found similarly by 

 dividing the area into strips by vertical lines and multiplying 

 each area by half the depth. It is the method of moments, al- 

 ready explained for irregular sections and for bending moments 

 on rivets. The center of gravity is not essential in the footing 

 problem, but it is essential to obtain the horizontal distance to 

 a vertical line through the center of gravity. 



Eccentric Loads on Column Base 



A column exposed to the force of wind acting to push it to one 

 side will put an eccentric load on the base. There are two cases. 



I. The column hinged to the base. The horizontal force in 

 this case acts at the base of the column. The vertical force acts 

 vertically through the center of the base and the resultant of the 

 horizontal and vertical forces should be kept within the middle 

 third if possible. 



II. The column is fastened to the base sufficiently to make the 

 base practically a part of the column, or at least develop a bend- 

 ing moment - H x ' m which 



H - horizontal thrust, 

 I - length of column. 



In Case I the horizontal force acts at the top of the footing, 

 as shown in I iu. 17<. In Case II it acts halfway up on the 

 column. It is important to remember these distinctions. The 



