CROSS-BREAKING STRENGTH. 



Table of stiffness (modulus of elasticity) of dry wood. — General averages 



41 



1 Less than. 

 CROSS-BREAKING OR BENDING STRENGTH. 



When the addition of 100 pounds to the load on our 2 by 2 inch piece 

 begins to add more than one-eighth inch to the deflection, that is, when 

 the stick has been bent beyond its "elastic limit," it still requires an 

 increase of 30 to 50 per cent to the load before the stick breaks. The 

 load which is borne before the limit of elasticity is reached indicates 

 the strength of the wood up to this important point; the load which 

 causes it to break represents its absolute strength, or the "cross-break- 

 ing or bending strength" as it is commonly called. 



In longleaf pine the former (modulus of strength at the elastic limit), 1 

 is commonly about three-fourths of the latter. If left loaded for a con- 

 siderable time, a load but little greater than that which brings the 

 stick to its elastic limit will cause it to break, and this load should there- 

 fore not be exceeded. 



Unlike the stiffness, the strength of a timber varies approximately 

 with the squares of the thickness and decreases directly with increas- 

 ing length and not with the cube of this latter dimension. Thus, if our 

 piece 2 by 2 inches and 4 feet long can bear 1,000 pounds before it 

 breaks, a 2 by 4 inch laid flat will break with about 2,000 pounds, and 

 if set edgewise, it requires about 4,000 pounds to break it, while a 

 piece of the same kind of 2 by 2 inches, and double the length (8 feet), 

 breaks with half the original load, or only 500 pounds. 



All conditions of the material which influence the stiffness also influ- 

 ence the bending strength. Seasoning increases, moisture decreases, 

 the strength; knots and crossgram depress it and both are more 

 dangerous on the lower than on the upper side. But while the conifers 

 with their simple cell structure excel in stiffness, the better hard woods 



1 The elastic limit in this case is somewhat of an arbitrary quantity, namely, the 

 point where 100 pounds produces a deflection 50 per cent greater than the preceding 

 100 pounds. 



