MECHANICAL PROPERTIES OF 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 in 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 in 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 timber. 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 in Forest Service Bulletin 17, " Check List of the 
Forest Trees of the United States." 
