38 BELL SYSTEM TECHMCAL JOURNAL 



follows, therefore, that the average strength of any lots of southern pine or 

 Douglas fir arms produced under Specification AT-7075 may be expected 

 to lie well above the Graph 2 limit. 



Graph 2 and a bending moment graph for vertical loads at each pin 

 position are of considerable value to the material design engineer, since the 

 degree of parallelism between the two will show whether a consistent 

 strength relationship exists throughout the length of the crossarm. As a 

 matter of interest in this connection, moment diagrams were used as a guide 

 in setting the knot limitations shown in Specification AT-7075. 



Resisting and bending moment graphs may also be used to determine the 

 location of the critical section of a crossarm by noting the point of coinci- 

 dence between a maximum bending moment graph and the resisting moment 

 graph for a clear arm. It can be shown by such graphs that this point in all 

 types of Bell System crossarms, designed for vertical loads, is located at the 

 pole pinholes. If the comparison were made between a maximum bending 

 moment graph and the resisting moment graph of an arm containing all of 

 the maximum defects permitted, the location of the point of coincidence 

 between the graphs might or might not fall at the pole pinholes, depending 

 on the magnitude and location of the defects allowed. It should be noted, 

 however, that for such arms the critical section locations so determined apply 

 only when the arms are actually of the assumed minimum quality; and, 

 since the probability of such being the case is so extremely remote, it is 

 concluded that the maximum stress or critical section locations in arms of 

 that quality are of academic interest only, and that for all practical purposes 

 the critical section of any 3j" x 4|" x 10' crossarm is located at the pole 

 pinhole. 



This conclusion does not mean that every arm broken in service or under 

 test will break at the pole pinhole; for, obviously, if some other section is rela- 

 tively weaker because of some hidden defect which reduces its section 

 modulus or its fiber strength, it will break at such section regardless of any 

 mathematical determination of the break location. But the conclusion 

 does mean that, generally speaking, when a crossarm breaks the break will 

 occur at, or be closely related to, the pole pinholes. To check the accuracy 

 of this conclusion, an examination was made of all available crossarm 

 strength test data in which the break locations were recorded. The exam- 

 ination revealed that, out of 258 arms tested, the breaks in 219, or 85 per 

 cent, were either at, or directly related to, the pole pinholes. Six per cent 

 of the breaks were located between the two pole pinholes, and 9 per cent at 

 points outside the pole pinholes. 



As an illustration of another use to which such a moment diagram may be 

 put, the following specific example is cited. Before the present standard 

 Bell System specification for crossarms was drafted, it was decided to 



