RELATIVE BEXDI.XG STREXGTII OF CROSS A RMS 



29 



With respect to the knot positions considered, it is apparent from an exam- 

 ination of the three curves (Fig. 3) that knots up to approximately 1^" in 

 diameter are most damaging when located immediately below the roofed 

 portion of the arm; and that the worst position for knots over 1|" in diam- 

 eter is at the bottom of the arm. However, since under usual loading 



I 1.5 2 



KNOT DIAMETER- INCHES 



Fig. 3 — Sections between pinholes. Section modulus of crossarm sections containing knots 



of the sizes shown on the base line and located in the positions indicated. The data 



apply to sections of minimum size (3]^" x 4^")- 



conditions knots at the bottom of an arm section are in compression, and 

 thus would have less influence on strength than they would have on the 

 tension side,- it was felt that the strength value shown by Curve 2 may be 

 ignored; and that the values shown by a smooth curve, combining the values 



2 On Page 69 of U. S. Dept. of Agriculture Tech. Bui. 479, "Strength and Related 

 Properties of Woods Grown in the United States" by L. J. Markwardt and T. R. C. Wilson, 

 is the following statement: "Knots have appro.ximately one-half as much elYect on com 

 pressive as on tensile strength." 



