TESTS OF WESTERN YELLOW PINE CAR SILLS, ETC. 15 



CONCLUSIONS. 



The comparatively small number of experiments made on western 

 yellow pine limits the conclusions to be drawn regarding this species 

 to the following : 



(a) The strength values of structural timbers are influenced con- 

 siderably by the defects found in them. These values vary accord- 

 ing to the grades in the green material ; but the increase in strength 

 from air seasoning is not uniform and does not vary with the grades. 



(h) Seasoning greatly increases the strength of the wood, the in- 

 crease being greater and more uniform in small, clear sticks than 

 in structural timbers, owing to the development of defects in the 

 latter. Lowering the moisture content of yellow pine causes it to 

 become more brittle. 



(c) Western yellow pine is a lighter wood than the other western 

 lumber species, weighing approximately 26.9 pounds per cubic foot, 

 oven dry, in structural sizes. The dry weight of clear wood readily 

 suggests its strength or weakness, but this factor alone can not be 

 depended upon to indicate comparative strength when structural 

 forms of various grades are taken into consideration, owing to the 

 presence of defects which have an important influence on their 

 strength. 



(d) The table of comparison of the strengths of various western 

 species (Table 12) is based on tests of small, clear specimens. In 

 addition to the results of tests on western yellow pine, as previously 

 described, there are included average values derived from similar 

 tests on material from five trees from each of four other localities. 

 In comparing species from data in this table it is well to base com- 

 parisons on results of tests of green material, since differences in 

 moisture content of green material do not produce differences in 

 strength. It must also be remembered that the figures given are 

 averages and that the variability of timber is such that individual 

 specimens of a species may excel the average for another species 

 which averages considerably higher, or they may fall below the 

 average for a species which averages considerably lower. "When 

 values from tests of air-dry materials are used for comparison careful 

 attention should be given to the moisture content of the material 

 compared (whether it is of two or more species or of one species 

 from two or more localities) and the effect of differences of moisture 

 considered. The effect of moisture and the methods for adapting 

 strength values from one moisture content to another are gone into 

 in Forest Service Bulletin 70, " Effect of Moisture on the Strength 

 and Stiffness of Wood." 



