UNITED STATES DEPARTMENT OF AGRICULTURE 



Contribution from the Forest Service Wi 



HENRY S. GRAVES, Forester *^ 



^, ,^~ FOREST PRODUCTS LABORATORY. Madison, Wisconsin ""j^^.,.^ 

 •^"^"^ "^Wt- jj, Cooperation wilh the University of Wisconsin ' JfUV" \.^U 



Washington, D. C. 



PROFESSIONAL PAPER 



July 16, 1919 



THE RELATION OF THE SHRINKAGE AND STRENGTH 

 PROPERTIES OF WOOD TO ITS SPECIFIC GRAVITY. 



By J. A. Newlin, in charge, Section of Timber Mechanics, and T. H. C. Wilson, 



Engineer in Forest Products. 



CONTENTS. 



Page. 



Purpose 1 



Species-locality averages 6 



Determination of specific gravity 6 



Moisture content of test specimens 6 



Page. 



The equations 7 



Application of the equations 9 



Appendix — Method of deriving equations 10 



PURPOSE. 



It has long been recognized that there are direct relations between 

 the specific gravity, or density, of a wood and its strength properties.^ 

 By the analysis of over 200,000 tests, the Forest Products Laboratory, 

 conducted in cooperation with the University of Wisconsin, Madison, 

 Wis., has now definitely established these relations. It is the pur- 

 pose of this bulletin to state these relations and to put the expression 

 of them in such form as to render them easily useful (1) for estimating 

 the properties of any particular timber; (2) for selecting timber for 

 any given purpose; (3) for comparing the various species; and (4) for 

 determining in what way the species are exceptional and to what uses 

 they are best adapted. 



It has usually been assumed that the strength of wood varies 

 directly with the first power of its density; i.e., that the respective 

 strengths of two sticks would differ in the same proportion as the 

 densities. It was recognized that fiber stress at elastic limit in com- 

 pression perpendicular to the grain, or bearing strength on side 



1 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 4^ 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. 

 65977°— 19— Bull. 676 1 



