32 BELL SYSTEM TECHNICAL JOURNAL 



As the section modulus {S) values for sections containing 1" and 2" hori- 

 zontal knots located immediately below the top segment are 1.97 and 1.11, 

 respectively, (Calculation 5, Table 2) it is apparent that in pinhole sections 

 horizontal knots immediately below the top segment are the more dam- 

 aging. The results of Calculation 5 were accordingly plotted in Fig. 5 and 

 smooth curves drawn to show the section modulus for each of the three 

 sections containing knots of any size. 



Roofed Brace Bolt Hole Section 



The worst position for knots in the brace bolt hole section was assumed 

 to be substantially the same as in the roofed sections between pinholes; 

 and in Calculation 6, the results of which are shown in Table 3, knots up to 

 \\" in diameter were assumed to be so located, viz. immediately below the 

 top segment. 



To check this assumption with respect to worst position, the following 

 analysis was made of the minimum sections: 



Distance from top of section: 



To top of bolt hole 1.78" 



To bottom of bolt hole 2.16" 



Distance from bottom of top segment: 



To top of bolt hole 1.45" 



To bottom of bolt hole 1.83" 



It is apparent that any knot ranging in diameter from 1.78" to 2.16", 

 when located at the top of the section, would enter the bolt hole. The 

 section modulus of any section containing a knot within that size range 

 would be the section modulus of the remaining portion of the section, or 



— , where b is the width of the section and d the depth below the bolt hole. 

 6 



Thus 



c r • • ^ 3.1875 (1.9375)^ . 001:5 



6 (mmnnum arm) = = 1.9943 



6 



It is also evident that any knot from 1.45" to 1.83" in diameter, when 

 located immediately below the top segment, would likewise enter the bolt 

 hole; and that the section modulus, on this basis of knot location, would be 

 the same for any section containing a knot within the size range mentioned. 

 Continuing the analysis the following tests were made: 



2" Knot: 



The distance between the top segment and the bottom of the bolt hole 

 of a minimum section is 1.83". Therefore, a 2" knot located immediately 



