180 



WOOD AND CAST-IRON COLUMNS 



the samples tested. Generally, this percentage amounts 

 to about 18% in ordinary commercial stock. 



The table on page 128 gives the ultimate compressive 

 strengths of the more common kinds of timber, together 

 with their moduli of elasticity. 



SHORT POSTS 



A post, or column, may in its elementary form be con- 

 sidered as a cubical or rectangular block, as shown in Fig. 1. 

 If the post does not exceed in length from six to ten times 

 the smallest dimension of 

 its cross-sectional area, it 

 is designated as a short post, 

 or column. 



The load that a short 

 post may safely carry may 

 be estimated by multiply- 

 ing its sectional area, in 

 square inches, by the safe 

 unit compression of the 

 material parallel to the 

 grain. The ultimate unit 

 values for compression are 

 given in the table on page 

 128. A factor of safety 

 of 5 is generally used, but 

 in some instances it may 

 be good practice to use 6. 

 The proper factor of safety to choose is usually governed 

 by the conditions to be met. 



EXAMPLE. A short post of Georgia yellow pine is 12 in. 

 square and 6 ft. long. What safe load will it support while 

 standing on end? The factor of safety is 5. 



SOLUTION. The ultimate strength of Georgia yellow pine 

 is 5,000 Ib. ; hence, the safe unit compressive stress is 5,000 * 5 

 = 1,000 Ib. The area of the post is 12 in. X 12 in. = 144 sq. in. 

 Therefore, the safe load is 144X1,000 = 144,000 Ib. 



Posts under compression develop more strength if the end 

 surfaces are true and level. The tendency then is to resist 



FIG. 1 



