WOOD AND CAST-IRON COLUMNS 187 



= 5,000 X. 821 =4,105. The ultimate strength of the post 

 is therefore 168X4,105 = 689,640 Ib. With 5 as a factor of 

 safety, the safe strength of the post is 689,640-^5 

 = 137,928 Ib. 



CAST-IRON COLUMNS 



COLUMN FORMULAS 



Cast-iron columns are most frequently used in buildings 

 of moderate height, but in some cases they have been used 

 in buildings of sixteen, and even more, stories. The uncer- 

 tain strength of cast iron has compelled the adoption of a 

 low unit stress per square inch, or, in other words, a high 

 factor of safety. 



The safe loads that cast-iron columns will carry can be 

 obtained by the use of the table on pages 188-189. The 

 first column of this table gives the external diameters of 

 various columns, and the second column, the several thick- 

 nesses that the column of a given diameter is likely to 

 possess. 



EXAMPLE. A hollow, cylindrical column is 18 ft. long, 

 and has an external diameter of 14 in. and an internal one 

 of Hi in. (a) What safe load will it support? (6) What 

 load will it support if the internal diameter is 11 in.? 



SOLUTION. (a) The thickness of the metal is - 



= 1.25 in. Proceeding in the table from 14 in the first 

 column to the value 1 i in the second column, and then con- 

 tinuing to the right until the column headed 18 is reached, 

 the value 386 is found. This shows that the column will 

 support a safe load of 386,000 Ib. 



(b) In a column having an internal diameter of 11 in., 



the thickness of the metal is = H in- Proceeding 



to the right from the value 1$ in the second column of the 

 table, the value 454 is found in the column headed 18; hence, 

 the safe load for this column is 454,000 Ib. 



It is thus evident that a difference in the thickness of the 



