18 BULLETIN 556,, IT. 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 '2\ 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. 



