26 



CALIFORNIA TANBARK OAK. 



TABLE 11. Strength of small dear pieces of tanbark oak, green, air-dry, and kiln-dry, 

 size 2 by 2 inches in section Continued. 



GREEN. 



AIR-DRY (10 TO 20 PER CENT MOISTURE). 



KILN-DRY (5 TO 10 PER CENT MOISTURE). 



1 The values in this column are based on a shrinkage of 18 per cent volume. The fiber saturation point 

 is taken as 30 per cent moisture. 



Clear, straight-grained specimens free from defects are needed in 

 determining the strength of the wood itself. The results of tests 

 made on this class material can also be used for comparison with 

 similar tests on other kinds of wood. Pieces 2 by 2 inches in section 

 have been found weh 1 suited to tests of this kind. For bending, 1 

 they are cut about 30 inches long, and for compression parallel to 

 the grain and compression perpendicular to the grain, from 6 to 10 

 inches long. The blocks for shear parallel to the grain are cut with 

 a projecting lip that is sheared off under test. In making a bending 

 test the beam is supported at the ends and loaded at the middle. 

 The supports for the beam are on the weighing platform of the test- 

 ing machine, so that the load on the beam can be determined at any 

 time during the test. This load is applied by a crosshead which 

 can be forced down on the test specimen by means of heavy screws 

 turned by a train of gears. The deflection or bending of the beam is 

 measured by an apparatus (PL VIII, fig. 1 ) consisting of a light steel 

 frame on which is mounted a movable pointer. In making a test, 

 the frame is rested on two nails driven into the beam near the ends 

 and the pointer attached to the center of the beam in such a way 



1 For a detailed description of methods used in the tests, see Forest Service Circular 38 (revised), Instruc- 

 tions to Engineers of Timber Tests. 



