24 



BELL SYSTEM TECHNICAL JOURNAL 



Since there is little, if any, engineering interest in the strength of structural 

 members of maximum size, no investigations were made of sections of 

 maximum dimensions. 



50 



U 20 



z 



20 30 40 



DISTANCE FROM CENTER OF ARM -INCHES 



50 



60 



Tz: 



-^ 



7^Z 



^ 



^ 



^=71 



^r 



"T 



'^ 



Fig. 1 — Moment diagram for T}pe A southern pine and Douglas fir crossarms per 



Specification AT-7075: 

 Graph 1 — Resisting moments of arms of nominal dimensions, straight grained and free 



from knots. (Fiber stress 5000 psi) 

 Graph 2 — Resisting moments of arms of minimum dimensions, having maximum slant 

 grain (1" in 8"), and containing knots of the maximum sizes permitted (viz., 

 sizes shown at bottom of arm sketch). (Fiber stress 3250 psi) 

 Graph 3 — Bending moments from a load of 50 pounds at each pin position. 



Section modulus calculations were made of each shape of minimum and 

 nominal size, both with and without knots. Tests have shown that, be- 



