Evaluating the Relative Bending Strength of Crossarms 



By RICHARD C. EGGLESTON 



/^\'ER a million crossarms are produced annually in the United States. 

 ^^ In the open wire lines of the Bell System alone there are no\^ about 

 20 million arms in use. It is natural, therefore, that public utility engineers 

 should have an interest in the strength of such an important item of outside 

 plant material; and, consequently, an interest in any tool or means of evalu- 

 ating the strength of such material. It is believed that the moment diagram 

 is a convenient and reasonably reliable tool for estimating the loads an 

 arm will support, for measuring the effect of knots of various sizes and of 

 pinhole locations on arm strength, and for answering similar questions 

 relating to the bending strength of crossarms under vertical loads. 



Two moment diagrams are shown in Fig. 1 for Bell System Type A cross- 

 arms; and in the pages that follow are presented the method used in con- 

 structing the diagrams and a discussion of their use. While the calculation 

 results apply particularly to the type and quality of arm referred to, they 

 would also be of value as a time saving reference in future studies that may 

 be proposed relating to the strength of the same or other types of arms 

 involving different knot allowances. 



The resisting moment of a beam is the product of its section modulus by 

 the unit stress on the remotest fiber of the beam. The section modulus of a 

 beam of uniform cross-section is constant and readily determinable. The 

 section modulus, however, of a beam of nonuniform cross-section, such as a 

 crossarm, varies because of the different cross-sectional shapes and dimen- 

 sions involved. 



In this study the following five different shapes were recognized: 



(1) Roofed section between pinholes 



(2) Roofed pinhole section 



(3) Roofed brace bolt hole section 



(4) Rectangular pole bolt hole section 



(5) Rectangular section without bolt holes 



The dimensions of the sections investigated were as follows: 



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