52 



WOOD AND FOREST. 



Woods do not vary nearly so much under compression as under 

 tension, the straight-grained conifers, like hirch and longleaf pine, 

 being near])- as strong under compression as the hard woods, like 

 hickorv and elm, which have entangled fibers, whereas the hard woods 

 are nearly twice as strong as the conifers under tension. 



iloisture has more effect on the strength of wood than any other 

 extrinsic condition. In sound wood under ordinary conditions, it 

 outweighs all other causes which affect strengtlr. When thoroly sea- 

 soned, wood is two or three times stronger, botli under compression 



and in bending, than when 

 green or water soaked." 



The tension or pulling 

 strength of wood is much af- 

 fected by the direction of the 

 grain, a cross-grained piece be- 

 ing only 1/lOth to l/20th as 

 stT'ong as a straight-grained 

 ])iece. But uniler compression 

 there is not much difference; 

 so that if a timber is to be 

 subjected to cross strain, that 

 is the lower half under tension 

 and the u]i]ier half under com- 

 ]>ression, a knot or other cross- 

 grained portion should be in the u|)pcr half. 



Strength also includes the ability to resist sheai'. This is called 

 "shearing strcngtli." It is a measure of the adhesion of one part of 

 the wood to an adjoining jiart. Shearing is what takes place wlien 

 the portion of wood beyond a mortise near the end of a timber. 

 A B C P, Fig. 43, is forced out liy the tenon. In this case it would 

 be shcai'ing along the grain, sometimes calleil detvusion. The resist- 

 ance of the ]W)rtion A B C P, ■/. c. its power of adhesion to the wood 

 adjacent to it on botli sides, is its sbeaiing strength. If the mortised 

 piece were forced d(n\nward until it broke off the tenon at the shoul- 

 der, tliat ^^'ould be shearing across the grain. TJie shearing resistance 

 either with or acioss the grain is small compared with tension and 

 compression, (ireen wood shears much more easilv than dr^^ be- 



Fii.,^. 43. Shearint,^ Slreng-th is Measured 



by the Adhesion of the Portion A, B, C, 



D or to the Wood on both sides of it. 



'Si'P FoiTstry liiillctin Nn. 7(t, pp. 11. \-l, iiinl Fovcstvv Cirrular No. 108. 



