A DlXKNXIAI- RlA'OKI) 



/// the Mcchduics of Timber 



While no attempt has been made to arrange the varions subjects 

 in order of size or importance, it seems natural and logical to give first 

 mention to tlie l^asic study of the properties of American woods. This 

 is, without d()ul)t. the most impoiiant series of tests ever conducted on 

 American species, not only in actual size, but also in impoi-tance of 

 results secured. Over 200.000 strength tests and about the same num- 

 ber of specific gravity and moisture determinations, in all well over 

 half a million individual tests, have been made, covering every com- 

 mercial species in the country and many of only minor im])ortance. 

 The tests were made on specimens cut from typical trees and were so 

 planned and executed that j) roper analysis of the results has yielded, 

 l)esides actual and comparative strength values of green and air-dry 

 wood of the various species, much additional information of funda- 

 mental character, such as the relation of strength to weight, of strength 

 to the height in the tree, the effect of distance from the pith upon the 

 strength properties, and the relation between strength and rate of 

 growth. 



The test data are frequently used for special studies, ])eing re- 

 grouped and analyzed to bring out whatever unusual property or 

 relation of properties may be desired. They form the starting point, 

 also, for much of the experimental work upon structin-al material and 

 parts of structures, such as aircraft parts. 



Work of previous experimenters has shown that the amount of 

 moisture present in the wood had a very marked effect u])on its 

 strength, and efforts had been made to deduce the laAvs underlying 

 this relation. It remained for the* timber testing laboratory at Yale 

 to plan and carry out a series of tests which not only yielded specific 

 data on the moisture-strength relation for several species, ])ut also 

 proved the existence of a definite point, now called the "fiber-satura- 

 tion point", l)eyond which the amount of moisture did not affect either 

 strength or shrinkage. This basic information is in constant use in 

 all of the studies of the mechanical and physical properties of wood. 



The requirements for grading rules for structural timbers differ 

 from those for rough and worked lumber since strength is a very im- 

 portant consideration in structural timlier. and it is highly desirable 

 that this class of material be graded on the basis of strength, so tliat the 



