MECHANICAL PEOPERTIES OF WOODS GROWN" IN UNITED STATES. 5 



Also, in making comparisons, it is important that the data should 

 really be representative of the classes of material which it is proposed 

 to compare. For example, it is not just to take the figures derived 

 from Rocky Mountain Douglas fir, which is known to "be inferior to 

 the Pacific coast type, 1 as representative of the coast fir. Nor in 

 general can a comparison of species properly be made from results of 

 tests on large timbers alone; for in practically all cases the large 

 timbers tested have not been selected as representative of the species, 

 but have been chosen to determine the effect of defects, the effect of 

 preservative treatment, or for the solution of other and similar 

 problems. 



Comparisons should not be made with greater refinement than the 

 data justify. The change which additional tests would probably 

 make in the average values and the probable variation of a given 

 stick or lot of material from these average values should be considered. 

 Numerical measures of these probable variations are given in Table 3. 



CAUSES OF VARIATIONS IN STRENGTH. 



Variations in strength of timber can be accounted for more accu- 

 rately than is usually supposed. In some species there is a difference 

 in strength in wood from different positions in the tree, different 

 localities of growth, etc. But such variations have been overesti- 

 mated, and a knowledge of them is not essential in order to estimate 

 "with a fair degree of accuracy the properties of a piece of timber. 

 Differences in strength are usually due to differences in defects, 

 moisture content, or density, or to combinations of these. 



Defects are not considered in this publication. Their effects on 

 structural timbers are discussed in Forest Service Bulletin 108; and 

 limitations on their size, character, and location are given in the 

 grading rules for structural timber which have been recommended 

 by the Forest Service. 2 



Differences of moisture content cause considerable variation in the 

 strength values of air-dry or partially air-dry material, but have no 

 effect as long as all material is thoroughly green. 



One of the principal factors causing differences in strength is 

 variable density. As might be expected, the greater the density of a 

 given stick or the more wood it has 3 per unit volume, the stronger is 

 the stick. 



1 See also "Localities AVhere Grown," p. 8. 



3 See "Discussion of the Proposed Forest Service Rules for Grading the Strength of Southern Pine 

 Structural Timbers," by II. S. Betts, Proceedings of Am. Soc. for Test. Materials, Vol. XV, 1915, p. 368. 



3 Accurate determinations made at the Forest Products Laboratory on seven species of wood, including 

 both hardwood and coniferous species, showed a range of only about 4J per cent in the density of the wood 

 substance, or material of which the cell walls are composed. Since the density of wood substance is so 

 nearly constant, it may be said that the density or specific gravity of a given piece of wood is a measure 

 of the amount of wood substance contained in it. 



