CROSSARM LOAD CONVERSION 109 



Calculating the load for each of the 10 pin positions in the same manner 

 as for the roofed lOA arm, we have, tentatively, a load per pin of 400 pounds. 

 However, in checking to determine whether the location of the critical sec- 

 tion changes under loads of 400 pounds at each pin position, we obtain re- 

 sults quite different from those in Calculation 1 ; for Calculation 2 indicates 

 a liber stress of 6885 psi at the pole pin holes, but a higher stress (7563 psi) 



: at the center, which shows that the location of the critical section does 

 change. Moreover, this change would occur whether the loads were 400 

 pounds per pin or 4 pounds per pin. But let us now consider the 400- 



I pound load. 



If a concentrated load of 1000 pounds results in a fiber stress at the pole 

 pin hole section of 6885 psi and causes failure, that stress is the maximum 

 ultimate fiber stress for the arm. It is, therefore, not reasonable to suppose 

 that the same arm would have endured a higher stress (7563 psi) at the 

 center if it had been loaded at each pin position. If 6885 psi is the maxi- 

 mum stress for the arm, the maximum moment it would endure at its center 

 would be 65,500 pound-inches (viz. 6885 multiplied by 9.52, the section 

 modulus of the center section). The maximum load per pin would be 364 

 pounds (viz. 65,500 divided by 180, the total-per-pin lever arm to the center) ; 

 and this load of 364 pounds, not 400 pounds, distributed at the 10 pin posi- 

 tions is comparable to the 1000-pound concentrated load. Thus, while the 

 critical section of a beveled lOB arm is at the pole pin holes when the load is 

 concentrated at the arm ends, it shifts to the center when the load is dis- 

 tributed at each pin position; and, moreover, the load is less than the load 

 per pin tentatively computed. 



A graphic illustration of this shift of the critical section is shown in Fig. 2. 

 Graph 1 in this figure is the graph of the resisting moments of a clear, 

 straight-grained beveled lOB arm, 3.25" x 4.25" in cross-section, and having 

 an assumed ultimate fiber strength in bending of 6000 psi. Each point 

 in the graph is equal to the section modulus of the section under considera- 

 tion multiplied by 6000 psi. Graph 2, which is the graph of a concentrated 

 load at the end pin position, was drawn from the zero moment under the 

 end pin to the point of greatest moment possible without intersecting re- 

 sisting moment Graph 1. Since the point of coincidence between Graphs 

 1 and 2 is the pole pin hole section, that section is the critical section for a 

 concentrated load at the end pin. The magnitude of this concentrated 

 load is equal to the resisting moment at the pole pin hole, 34,860 pound- 

 inches (viz. 5.81 inches^ x 6000 psi) divided by the 40" lever arm, or 871.5 

 pounds. The load per pin, tentatively figured, would be 34,860 pound- 

 inches divided by 100 inches or 348.6 pounds. Graph 3 is the graph of a 

 load (P) of 348.6 pounds at each pin hole. Under such loading, however, 

 the bending moment at the center of the crossarm would be 62,748 pound- 



