114 



BELL SYSTEM TECHNICAL JOURNAL 



Calculation 2. Bending Moments and Fiber Stresses in a Beveled lOB 

 Crossarm — (See Figure 4) 

 Notation: 



W = 1000 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^ 



nuLC 



-4- 



w 



-lr- 



- 10" >K — 10" — >K — 10- — *■ 



-1* 



r- 



^^*jj 



40 



Fig. 4 — Loading diagram for a beveled lOB crossarm. 



Concentrated Load: 



Mc = W X 56 = 1000 X 56 = 56,000 pound-inches 

 Mp = W X 40 = 1000 X 40 = 40,000 pound-inches 

 fc = Mc ^ Sc = 56,000 -^ 9.52 = 5882 psi 

 fp = Mp -¥ Sp = 40,000 -^ 5.81 = 6885 psi 

 Load per Pin: 



Mc = 56P -f 46P -f 36P + 26P -f 16P = 180P 

 Mp = 40P + SOP + 20P + lOP = lOdP 

 P = 40,000 ^ 100 = 400 pounds 



fP = 



lOOP 100 X 400 



sp 



5.81 



= 6885 psi 



, 180 180 X 400 _,- . 

 ^'=31= 9.52 =75^3P^^ 

 Conclusion: 



Critical section shifts under P loads, and arm will not support 400 pounds 

 per pin. 



^Sc = 9.52 and Sp = 5.81 for clear beveled 3.25" x 4.25" crossarms. (See Calculation 

 3 of this appendix.) 



