RELATIVE BENDING STRENGTH OF CROSS A RMS 41 



2. It is shown that the critical section of a crossarm is located at the pole 

 pinholes. The practical value of this observation is that it emphasizes the 

 need for keeping the pole pinhole sections and the portion of the arm be- 

 tween them reasonably free from strength reducing defects. 



3. Only by breaking tests can the actual bending strength of crossarms 

 be determined. The relative bending strengths, however, of two or more 

 arms of different types or quality may be estimated with sufficient accuracy 

 by means of the moment diagram, regardless of the fiber stress used in its 

 construction. 



4. If the fiber stress factor employed is dependable, the moment diagram 

 may be used to estimate the minimum factor of safety that would obtain 

 for an arm of any type or any assumed quality. In this connection, it is 

 believed that the strength of Bell System crossarms is well above the mini- 

 mum required to support the loads ordinarily carried. 



5. The section modulus curves of Figs. 4, 5, 6 and 7 will simplify the con- 

 struction of moment diagrams for arms of the same sizes shown in the figures 

 but dififering with respect to type and quality. 



The uses listed lead to the general conclusion that the crossarm moment 

 diagram is a convenient and reasonably reliable engineering tool. 



APPENDIX 



Computation I. Moment of Inertia of Top Segment of Minimum (J^" x 



4^") Section between Pinholes: 



The moment of inertia {IT) of a segment {T) with respect to an axis 

 through its center of gravity and parallel to its base may be found by the 

 formula 



IT = Ibb - Ax'' 



where I bb is the moment of inertia of the segment about the axis BB,A 

 the area of the segment and .v the distance between the two axes. The 

 values I BB, A and x are given by: 



1 sfi 1 2 sin^ a cos a 1 , . 



Ibb = \Ar 1 H -. (1) 



[_ a — sm a cos a J 



A = Ir^ (2a - sin 2a) (2). 



x = i'^^ (3) 



