CROSSARM LOAD CONVERSION 107 



previously stated, what load per pin will the arm support? There are more 

 than twenty million crossarms in the pole lines of the Bell System and each 

 year about a million arms are added. A complete understanding of every 

 problem associated with this important item of outside plant material is 

 manifestly worth while. This paper is intended to contribute to that end. 

 It presents a solution of the problem of converting concentrated vertical 

 loads to comparable loads distributed at each insulator pin position. 



The location of the critical section in crossarms is a basic factor in a study 

 of the problem. The critical section of a crossarm is the section at which the 

 fiber stress is greatest when the arm is loaded. It is the section where the 

 arm may be expected to break if overloaded. To determine its location, the 

 bending moment at various sections along the arm is divided by the section 

 modulus of the respective sections. The quotient in each instance is the 

 fiber stress for each section investigated. The location showing the greatest 

 fiber stress is the critical section. Since horizontal shear is not the control- 

 ling stress in crossarm failures under loads distributed at each pin hole, 

 bending stresses only were considered in this analysis. 



Because of the diflferences in arm shape and in the spacing of pin holes, 

 the location of the critical section is not the same in all arms. It is es- 

 timated that at least three fourths of the arms in the Bell System are lOA 

 and lOB crossarms.^ Both are 10 feet long and 3.25" x 4.25" in cross section. 

 In the lOA arm (Fig. 3), the space between the pin holes is 12 inches, except 

 between the pole pin holes, where the space is 16 inches. In the lOB (Fig. 

 4) the pin hole spacing is 10 inches with a 32-inch space between the pole 

 pins. Both types are bored for wood pins. Most of the arms now in the 

 plant are ''roofed", that is, the top surface of the arm, except the center foot 

 of length, is rounded on a radius of about 4.25 inches. Under the current 

 design, however, the top surface of Bell System arms is fiat, except for the 

 edges, which are beveled. 



Previous studies of both roofed and beveled arms of various types have 

 shown that the critical section of clear arms under vertical loads is either at 

 the center or at the pole pin hole sections. This study is confined to those 

 sections of clear lOA and lOB crossarms of nominal dimensions, both roofed 

 and beveled. Moreover, it was assumed for the purpose of load analysis, 

 that the crossarms are supported at the center only; since, under loads on 

 each side of the pole, the standard crossarm braces provide no significant 

 support when the loads are suflacient to break the arm. 



Roofed lOA Arm 



Let it be assumed that the breaking load concentrated in each end pin 

 hole of a roofed lOA arm is 800 pounds. As shown in Calculation 1 in the 



*10A and lOB crossarms were formerly known as Type A and Type B crossarms, 

 respectively. 



