36 



BELL SYSTEM TECHNICAL JOURNAL 



Rectangular Sections without Bolt Holes 



Here too the most damaging position for knots was assumed to be at the 

 top of the section. In Calculation 8 the section moduU of sections contain- 

 ing knots from I" to 3" in diameter were determined for both minimum and 

 nominal sections. The results are shown in Table 4. As section modulus 

 values for sections containing knots of other sizes than those shown may be 



found so simply by the formula for rectangular sections, 6* = -r- , no curves 



6' 



of the results of this calculation were drawn. 



MoiiENT Diagrams 



From the results of this study as shown in Table 4 and in Figs. 4, 5, 6 and 

 7, section modulus values for clear arms and for arms containing knots of 

 various sizes may be read and multiplied by appropriate fiber stresses to 

 determine the resisting moments throughout the length of such arms. For 

 example, the section moduli of clear arms of nominal dimensions, and of 

 arms of minimum dimensions with the maximum knots permitted under the 

 current Bell System crossarm specification (AT-7075) are as follows: 



Section of Arm 



Pole bolt hole 



Brace bolt holes 



Pole pinholes 



Other pinholes in middle section^. . 



End pinholes 



Other pinholes in end sections^. . . . 

 Unroofed part of middle section. . . 



Roofed part of middle section 



Solid part of brace bolt hole zones'* 

 Between pinholes in end sections. . 

 Extreme ends 



These section modulus values were used in preparing the moment dia- 

 grams shown in Fig. 1. The clear arm of nominal dimensions was also 

 assumed to be straight grained. The fiber stress factor used for it was 5000 

 psi, which is the ultimate fiber stress value that has been employed in the 

 Bell System for many years for sawn southern pine and Douglas fir. The 



^ For the purposes of specifying knot limitations, crossarms under Specification AT-7075 

 are divided into a middle section (between brace bolt holes) and end sections (beyond brace 

 bolt holes). 



^ Where a brace bolt hole zone is less than four (4) inches from a pinhole zone, these 

 zones and the portion of the arm between them are considered as a single zone. 



