8 BULLETIN" 77, U. S. DEPARTMENT OP AGRICULTURE. 



is apparent from the various positions occupied by the different 

 shipments of lodgepole pine and western yellow pine that species is 

 not in itself a reliable guide to strength in the selection of material 

 of this form and size. With clear material, consistent differences 

 would probably be found between some of the species, but in the 

 props especially the degree of straightness of the piece and the pres- 

 ence of knots seem of greater effect than the species. 



The results of the tests on the three sizes of 16-foot round beams 

 of lodgepole pine are shown graphically in figures 3 and 4. With 

 the exception of the work to maximum load, all the strength functions 

 decrease in value with increase in the diameter of the beam. The 

 amount of this variation differs in the different kinds of material, 

 being least in the green, greatest in the air-dried, and intermediate 

 in the dead timber. The reason for the decrease in the unit strength 

 of the larger beams is not apparent. It is not due to visible defects, 

 such as cause a decrease in the unit stress in large stringers when 

 compared with small clear pieces cut from them, since the defects in 

 the 8 and 12 inch beams did not differ in kind or in appearance. 

 The fact that stiffness decreases in the same manner as strength 

 corroborates this statement, since such defects as knots and checks 

 do not generally affect stiffness up to the elastic limit. The differ- 

 ence may, however, be due, at least in part, to rate of growth, which 

 increases in all cases with increase in diameter. The smaller beams 

 were apparently cut from the suppressed growth of a fairly even- 

 aged stand. The unit weight of the beams does not vary consist- 

 ently with the strength. The dry weight per cubic foot of the air- 

 dried beams is lowest in the 12-inch size, but for the dead beams 

 the weight is highest in this size. 



The relative values of the dead, green, and air-dried timbers are 

 also indicated in figures 3 and 4. In strength the dead material 

 falls between that of the air-dried and green, being close to the air- 

 dried at the elastic limit and close to the green at the maximum load. 

 The air-dried material is the stiffest, the green and dead having 

 practically equal values for this factor. The closeness of the values 

 for green and dead water-soaked material and a comparison of their 

 curves with those of the dry timbers show very clearly the effect of 

 moisture in reducing the stiffness and strength at the elastic limit. 

 In work to maximum load, which is an indication of toughness, the 

 order of the curves is reversed, the green and water-soaked beams 

 having values about twice as great as the dry material. The posi- 

 tion of the dead water-soaked beams, especially, brings out the fact 

 ^at the greater brittleness of the dead or air-dried material is due to 

 iss and not to a deterioration of the wood. Drying the beams 



