CROSSARM LOAD CONVERSION 



113 



APPENDIX 



Calculation 1. Bending Moments and Fiber Stresses in a Roofed 10 A 

 Crossarm — (See Figure 3) 

 Notation: 



W = 800 pounds concentrated load 



P = Load per pin 

 Mc = Bending moment at arm center 

 Mp = Bending moment at pole pin hole 



fc = Fiber stress at center 



fp = Fiber stress at pole pin hole 



Sc = Section modulus of center section** 



Sp — Section modulus of pole pin hole section^ 



^'POLE PIN 

 HOLE 



12" > 



w 



-^- 



'T 



12" 



56"- 



Fig. 3 — Loading diagram for a roofed lOA crossarm. 



Concentrated Load: 



Mc = W X 56 = 800 X 56 = 44,800 pound-inches 

 Mp = W X 48 = 800 X 48 = 38,400 pound-inches 

 fc = Mc ~ Sc = 44,800 -^ 9.74 = 4600 psi 

 fp = Mp -^ Sp = 38,400 -^ 5.11 = 7515 psi 

 Load per Pin: 



Mc = 56P -f 44P + 32P + 20P + 8P = 160P 



Mp = ASP + 36P -f 24P + UP - 120P 



(Note: The total-per-pin lever arms are 160" to center and 120" to the 



pole pin hole). 

 Since under IF load/ is maximum at pole pin hole, the P load that would 



result in same/ is P - 38,400 -^ 120 = 320 pounds. Thus 

 fp = nop -^ Sp ^ (120 X 320) ^ 5.11 = 7515 psi 

 fc = 160P H- Sc = (160 X 320) h- 9.74 = 5257 psi 

 Conclusion: 



Under both IF loads and P loads, the critical section is the pole pin hole 

 section. 



'Sc = 9.74 and Sp = 5.11 for clear roofed 3.25" x 4.25" crossarms. (See Pages 27 and 

 28 of Bell Sys. Tecli. Jour., Jan. 1945). 



