WOOD AND CAST-IRON COLUMNS 179 



WOOD AND CAST-IRON COLUMNS 



WOODEN POSTS 



MATERIALS 



The kinds of timber usually employed for columns are the 

 long-leaf and short-leaf yellow pines, red pine, white and 

 red oak, spruce, hemlock, cypress, fir, and redwood. The 

 timber generally preferred is the yellow pine, the long-leaf 

 variety being stronger and more durable than the short- 

 leaf. The disadvantage of this wood is that the resinous sap 

 makes it very inflammable. 



The compressive strength of timber varies greatly, accord- 

 ing to the amount of moisture it contains, a decrease in 

 moisture resulting in increased strength. The percentage 

 of moisture in wood is usually reckoned from the dry weight. 

 Thus, if a certain piece of timber that weighed 165 Ib. when 

 green weighs 100 Ib. when kiln dried, it would be said that 

 in this instance the wood when green contained 65% of 

 moisture. 



The drying of green wood does not effect an increase in 

 strength until the moisture is decreased to a value amounting 

 to about 20 to 30% of the weight of the dry material; that 

 is, the strength of a piece of green wood when being dried 

 remains constant until the moisture remaining in the piece 

 is reduced to from about 20 to 30% of the weight of the dry 

 wood, and from this stage the strength starts to increase. 



Experiments made by Tieman, the results of which are 

 given in the Proceedings of the American Society for Testing 

 Materials, show, for instance, the following variations in 

 strength of long -leaf pine: If the strength of the green 

 wood is taken as 1 , then the strength of air-dried wood con- 

 taining 12% moisture is 2.4, and that of kiln-dried wood 

 containing 3% moisture is 2.9. 



Thus, it is important when consulting tables of strength 

 of timber to know the percentage of moisture contained in 



