18 BULLETIN 556, U. S. DEPARTMENT OF AGRICULTURE. 
shear in beams. For small details, in timbers unaffected by shakes 
or checks, the allowable stress may be taken as one-fourth the value 
listed for green timber. 
TENSION PERPENDICULAR TO GRAIN. 
The tension perpendicular to grain tests are made on specimens 
2 inches square and 2h inches long, the tension area being 1 by 2 
inches. The tension force is applied perpendicular to the grain. 
The values are of use in estimating the resistance of timber to the 
splitting actions of bolts and other fastenings. A factor of 5 should 
be applied to the values in Table 1 to get the allowable stress for 
design; i. e., one-fifth the values given in the tables. 
EXPLANATION OF TABLE 3. 
(See table on p. 45.) 
The figures in Table 3 are presented as an aid to the interpreta- 
tion of data given in Tables 1 and 2 and are explained as follows: 
COLUMN 2. 
The figures given in column 2 are to be applied to the data in 
Table 2. They are, of course, only approximate, as the exact vari- 
ation of any property with change in moisture content is different 
for each species. 1 They will assist in rendering more nearly com- 
parable data which are noncomparable because of differences of 
moisture. 
Example: It is desired to compare the modulus of rupture of air- 
dry locust with that of air-dry bitternut hickory. The hickory 
has a modulus of rupture of 18,850 at 9.2 per cent moisture and the 
locust a modulus of rupture of 20,700 at 10 per cent moisture (see 
Table 2). According to Table 3, a 1 per cent change of moisture 
causes a 4 per cent change of modulus of rupture. Changing the 
hickory from 9.2 to 10 per cent moisture will decrease the strength 
by (10 - 9.2) X 4 per cent = 3.2 per cent; 3.2 per cent of 18,850 = 
600. Then the moduli or rupture of black locust and bitternut hick- 
ory, when placed on a comparable basis, each being at 10 per cent 
moisture, are 20,700 and 18,250 pounds per square inch, respectively. 
The accuracy of this moisture reduction is greatest across small 
intervals. As the interval or difference of moisture increases the 
accuracy becomes less. 
COLUMN 3. 
Study of the data presented in this bulletin has shown that each 
of the shrinkage and strength properties of a given species can be 
estimated with fair accuracy from the average specific gravity, 
since each varies according to some power of the specific gravity. 
1 See Forest Service Bulletin 70 and Circular 108. 
