108 BELL SYSTEM TECHNICAL JOURNAL 



appendix, the bending moment at the center of the arm from the assumed 

 loads would be 44,800 pound-inches, and the fiber stress at the center would 

 be 4600 psi. That calculation also shows that the bending moment at the 

 pole pin holes would be 38,400 pound-inches, and the fiber stress at the pole 

 pin holes 7515 psi. Since the stress at the pole pin holes is greater than that 

 at the center, the critical section of a roofed lOA arm is at the pole pin holes 

 when the arm is subjected to a breaking load at each end pin hole. 



The information wanted, however, is what load at each of the ten pin 

 positions would have produced the same moment and same fiber stress at 

 the critical section? A tentative answer is found by dividing the 38,400 

 bending moment by the "total-per-pin" lever arm*, 120" (see Calculation 

 1) or 320 pounds. Checking to determine whether the location of the criti- 

 cal section changes under loads of 320 pounds at each pin position, Calcula- 

 tion 1 shows that the fiber stress at the pole pin holes and at the center 

 would be 7515 psi and 5257 psi, respectively. Since the stress at the pole 

 pin holes is greater, it is clear that the critical section is there also under 

 equal loads at each pin position; and the 320-pound load per pin is com- 

 parable to the concentrated load of 800 pounds at each end of the arm. 



If a similar investigation were made of a roofed lOB arm and of a beveled 

 lOA arm, it would be found that the pole pin hole section is the critical 

 section of these arms; and that the load per pin comparable to concentrated 

 loads at the arm ends would, like the roofed lOA arm, be equal to the bending 

 moment at the pole pin hole section due to the concentrated load divided by 

 the total per pin lever arm to that section. Figure lb shows a roofed lOA 

 arm that broke under test at a pole pin hole (critical) section from concen- 

 trated loads at the ends of the arm. 



Beveled lOB Arm 



For the investigation of this arm, let a breaking load of 1000 pounds at 

 each end pin hole be assumed. Incidentally, it should be noted that so far 

 as this analysis is concerned, the magnitude of the assumed concentrated 

 loads is of no importance. However, since both computations and tests 

 show the lOB arm to be stronger than the lOA, it seemed appropriate to 

 assume a larger concentrated load for the lOB arm. 



As shown in Calculation 2 of the appendix, the bending moment at the 

 center due to the 1000-pound load would be 56,000 pound-inches and the 

 fiber stress 5882 psi, while at the pole pin holes the bending moment would 

 be 4(),0()() ])()un(l-inches and the liber stress 6885 psi. Here again, under 

 concentrated loads at each end pin hole, the critical section is at the pole 

 I)in holes. 



■'Hy l()t;il-|)er-i)in lever arm is meant the summalion of llie dislances from each ]iin 

 j)osilio)i to the section concerned -in this instance (o the pole pin hole section. 



