445 



only be obtained by cylinder processes that reduce the strength of the 

 timber. The unit stresses used in the design of creosote structures should, 

 therefore, in these cases, be decreased below standards established for 

 natural wood. 



UNIT STRESSES FOR DESIGN. 



The relation of strength of large sticks, involving defects, to small 

 and perfect pieces, taken from the parent beam, is reported in Circular 

 115, Forest Service. The strength of large and small sizes is not a ques- 

 tion of geometrical magnitude, but of the existence of defects in the large 

 sticks such as knots, shakes, checks and the presence of inferior growth. 



Study has been given to the failure of large beams under longitudinal 

 shear. It is apparent that, in the case of large beams of seasoned timber, 

 the failure is due to longitudinal shear rather than to bending. In green 

 beams, also, this form of failure is frequent. Therefore, shearing stresses 

 should be taken account of in the design. The result of later tests confirm 

 the early results that the strength of large pieces is not increased by sub- 

 sequent seasoning, except in case of select grades. In other words, unit 

 stresses for design should usually be based upon strength of green timber. 



NEW SPECIES AND SUBSTITUTES. 



The eucalypts of California and the South have been tested. They 

 are among the strongest of our woods. The quality of the various species 

 differs greatly, varying in kiln dry state from 25,000 pounds per sq. in. 

 to 13,000 pounds per sq. in. in modulus of rupture. Tests have been com- 

 pleted on tan-bark oak, which formerly was left stripped of its bark in 

 the woods. 



GENERAL STUDIES OF SPECIES. 



Tests of red gum are completed. 8 Tests of various species of hickory 

 collected from various site conditions have been made and the report com- 

 pleted. These latter tests established relations between rate of growth 

 and strength, locality and strength, and species and strength. It appears 

 that the most fundamental factor governing the strength of wood of any 

 species is the specific gravity, or, in the conifers per cent of summer wood. 



8 Bulletin No. 58, Forest Service. "The Red Gum," by Alfred K. Chittenden. 



