UNITED STATES DEPARTMENT OF AGRICULTURE 
BULLETIN No. 676 : : 
Contribution from the Forest Service 
HENRY S. GRAVES, Forester 
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. Newtty, in charge, Section of Timber Mechanics,and T. R. C. Wiuson, 
Engineer in Forest Products. 
CONTENTS. 
Page. Page. 
AWE OSO rome aeiarccien alsaisteie me aon asics a cininin s eics ae le ENe CQ UablONS Gases es tee eer Le 7 
Species-locality averages................--..- 6") Application: of the equations 5-222. 2... 2-2... 9 
Determination of specific gravity .........--- 6 | Appendix—Method of deriving equations.... 10 
Moisture content of test specimens........... 6 
PURPCSE. 
Tt 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 madeat the Forest Products Laboratory on seven occ of wood, including 
both hardwood and coniferous species, showed a range of only about 43 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 specifie gravity ofa given piece of wood is a measure 
of the amount of wood substance contained in it. 
65977°—19—Bull, 676——1 
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