TESTS OF EOCKY MOUNTAIlSr WOODS FOR TELEPHONE POLES. 25 



lodgepole pine sliipment averaged about 22 per cent moisture, 

 though the outer shell of the poles was somewhat drier (see. fig. 3). 

 This would tend to make the difference between the strength of the 

 poles and the actual strength of the material in it much greater than 

 was the case. The curve given on page 10 of Forest Service Circular 

 108, The Strength of Wood as Influenced by Moisture, shows that, 

 for eastern spruce, strength in bending will be reduced by about 30 

 per cent when the moisture content is increased from 8 per cent to 

 16 per cent. Tests on lodgepole pine from Wyoming indicate a 

 reduction, under similar circumstances, of about 25 per cent. The 

 curves shown in figure 8 have, however, been plotted with the values 

 as obtained from the tests. 



The curves, arranged in order of the modulus of rupture of the 

 poles from highest to lowest, show the relation between the modulus 

 of rupture of the small, clear sticks and the fiber stress of the poles 

 at the elastic limit and maximum load. The position and number 

 of checks, knots, and other defects, rather than the quahty of the 

 clear wood, determines the grade of a pole. While the curve for 

 the modulus of rupture of the small pieces is erratic, as would be 

 expected from the rather small number of tests averaged for each 

 pole, it shows a tendency to fall with a fall in strength of the poles, 

 indicating the influence of the quality of the clear wood on the 

 strength of the poles. The most important relation shown by the 

 curves is that the ratios between pole and minor strengths are not 

 the same for the different species, indicating that it is not safe to 

 compare species for use as poles on the basis of the strength of their 

 clear material. For example, western red cedar gave an average 

 modulus of rupture for the small, clear beams of 9,305 pounds per 

 square inch, and the lodgepole pine from Montana averaged 12,775. 

 While the strength of the clear material of the pine is thus 37 per 

 cent higher than that of the cedar, the average strength of the poles 

 was a little less than 12 per cent higher. The ratios of the average 

 modulus of rupture of the poles to that of the clear material for two 

 conditions of moisture is as follows : 



As esti- 

 Kind of poles. at 8 per mated i at 



16 percent. 



Western red cedar 0. 74 0. 98 



Lodgepole pine: 



Green cut 



Fire killed 



Engelmann spruce, fire killed .48 .65 



1 On the basis that an increase in moisture from 8 to 16 per cent causes a 25 per cent reduction in strength. 



