70 



beyond its safe compressive strength and should be modi- 

 fied to suit this strength when the latter does not con- 

 form to the value of 35 tons. Substituting this value of 

 stress in the above formula and reducing we have D equals 



the square root of 



Having obtained D from this formula the total stress 

 In the bars in tons % D. The bars may be placed as 

 shown in plan. The size of the bars should be so taken 

 that the bars will not be spaced more than 12 in. apart. 

 The total height H of the footing should be at least 3 in. 

 greater than the depth D. 



Let W=2 ft., load = 20 tons per lin. ft., and safe bearing 

 power of soil 2 tons. Then Wi = 10 ft. and P = 4 tt. D 

 equals the square root of 



8X20X48 in. 



~~2\ = 19 inches 



And stress % x 19 rr 14.25 tons, requiring % In. square 

 bars 4% in. on centers. Their length would be 



W/---=8 ft. 



II. FORMULA FOR PIER FOOTINGS. 



As in the case for wall footings we have given the di- 

 mensions of the supported pier and footing W and U'i and 

 the total load carried L. 



Our formula for obtaining the stress in the tension 



bars running in each direction 



In which we have as before the two unknown quantities 

 stress and D. In order that the concrete may not be 

 compressed beyond its safe working strength we impose 



the condition 4 X Stress =~X ( 



from which stress = 



Substituting this value of stress in the above formula 

 and reducing we have D equals tue square root of 



