516 BELL SYSTEM TECHNICAL JOURNAL 



which is cited here by way of illustration, was directed toward an 

 analysis of the variation in size and variation in modulus of rupture 

 that might be expected to affect the average ground line moment of 

 resistance of random 3 pole groups. Approximately 400 creosoted 

 southern pine and 500 western red cedar, class 3, thirty foot (see 

 Table 2) poles were used in this particular study. It was found that 

 in more than 95 per cent, of the cases the average moment of resistance 

 of such 3-pole groups was higher than the minimum calculated for 

 the given class and length. The result is considered reasonably rep- 

 resentative of what would be found in a similar study of other sizes. 

 It may be concluded that with the new standard fiber stress values 

 as a basis practically all parts of a line when new should be equal to 

 or better than the strength rating for the specified minimum of the 

 class of poles used; and that when the reduced loads under the con- 

 ditions usually obtaining in the higher grades of construction are 

 considered, the bending moment developed at the ground line should 

 rarely, if ever, approach the actual moment of resistance. 



Since the standard ultimate fiber stresses are based upon tests of 

 representative poles, they are believed to be satisfactory for all ordi- 

 nary purposes. They are directly applicable in the engineering of pole 

 lines without further adjustment or compensation for knots, variation 

 in moisture content, or density of wood. In any case, the question of 

 density classification may be limited for practical purposes to southern 

 pine poles; and studies of current production show that approximately 

 75 per cent of such poles passing through the producers' yards could 

 be classified as dense. The creosoting process seems to reduce the 

 variation found in the modulus of rupture values of untreated poles. 

 The comparatively low coefficient of variation of creosoted southern 

 pine shown in Table 1 indicates that for general purposes an attempt 

 to classify pine poles according to density is an unnecessary refinement. 



With the standard fiber stresses as bases, dimension tables for the 

 four species were developed in accordance with the following prin- 

 ciples: 



(a) The tables should specify dimensions in terms of circumference 

 in inches at the top, and circumference in inches at six feet 

 from the butt for poles of the respective lengths and classes 

 except for three classes with "no butt requirement." 



{b) All poles of the same length and class should have, when new, 

 approximately equal strength, or in more precise terms, equal 

 moments of resistance at the ground line. 



(f) All poles of different lengths within the same class should be of 

 suitable sizes to withstand approximately the same breaking 



