MECHANICAL PROPERTIES OE .WOODS GROWN IN UNITED STATES. 7 



maximum load in static bending, and height of drop in impact. In 

 flooring, the desirable properties are hardness and slight shrinkage. 



DATA ON GREEN TIMBER. 



Table 1 gives the values obtained from tests on green material. 

 It will be noted that there is a large variation in the moisture content 

 of the various species. All, however, were tested at approximately 

 the moisture content of the living tree and are well above the limit 

 below which differences in moisture content produce differences in 

 strength. Table 1 is more reliable than Table 2, because it is based 

 on a much larger number of tests and on tests which are not in- 

 fluenced by variations hi moisture content. 



DATA ON AIR-DRY TIMBER. 



Table 2, which gives the values obtained in tests of air-dry timber, 

 should be considered as supplementary to Table 1. This table is 

 necessary because the properties of all species are not changed hi the 

 same proportion by drying and all the properties are not equally 

 affected. 



Some of the properties of air-dry wood are subject to rapid change 

 with change in moisture content. For this reason it is necessary in 

 comparing species on the basis of Table 2 to make proper allowance 

 for whatever differences may be shown in the column of moisture 

 content. Table 3 includes figures showing the approximate changes 

 which are made in the various properties of air-dry wood by the 

 addition or subtraction of. 1 per cent of moisture. 



It will be noted from Tables 1 and 2 that in most properties the 

 dry material excels the green. In structural design, however, no 

 allowance should be made for such increase in strength, because in 

 large timbers it is a very indefinite quantity. The increased strength 

 of the wood fibers is usually offset by checks and other defects result- 

 ing from drying. Moreover, many structural timbers are subject to 

 moisture changes, and the outer fibers may at any time become wet 

 enough to reduce the mechanical properties to the level of those of 

 green thnber. For these reasons the strength of green material 

 should be made the basis of stresses to be used in structural design. 



EXPLANATION OF TABLES 1 AND 2. 



(See tables on pp. 27 and 37.) 



NAMES OF SPECIES. 



Many of the species have numerous common names, and not 

 infrequently one common name is applied to several species. This 

 leads to so much confusion that it is necessary to refer to a standard 

 nomenclature. The common and botanical names used in the tables 

 are those given hi Forest Service Bulletin 17, "Check List of the 

 Forest Trees of the United States." 



