286 



CONCRETE DESIGN 



allowable load. Thus, if it is decided to allow an intensity 

 of stress of 300 Ib. per sq. in., and the column is of square 

 section 10 in. on a side, the allowable load will be 10 X 10 X 300 

 = 30,000 Ib. The breaking load' on columns between two 

 and twelve times as high as the least dimension of their 

 cross-section seems to be independent of their height. A 

 column between these two limits, however, cannot with- 

 stand as high an intensity of stress as a cube, for it is more 



ULTIMATE UNIT CRUSHING STRENGTH OF STONE CON- 

 CRETE WITH PORTLAND-CEMENT MORTAR 



NOTE. Fc 

 table. 



75% of the figures given in the 



likely to break by shearing. For this reason, when employ- 

 ing values taken from the tables on pages 286 and 287, for 

 column calculations, a larger factor of safety should be used 

 than with other work. This factor is usually taken as at 

 least 6, and 10 or higher for masonry. 



EXAMPLE. What is the allowable working load on a 

 concrete column that is 10 ft. high and 12 in. in diameter and 

 made of 1-2-4 stone concrete 6 mo. old with a factor of 

 safety of 6. 



SOLUTION. The cross-sectional area of the column is 

 .7854 X12 2 = 113.1 sq. in. From the table the ultimate 



