RELATIVE BENDING STRENGTH OF CROSS A RMS 



25 



cause of the distortion of the grain that occurs around them, knots are fully 

 as injurious to the strength of structural timbers as knot holes. ^ Therefore, 

 in dealing with sections containing knots, it was assumed for 'the purposes 

 of this study that the knot extended across the section in the same manner 

 as a hole having a diameter equal to the diameter of the knot. It was 

 also assumed that the knot was located in, or reasonably close to, the most 

 damaging position in the arm section. 



In the calculations of the section modulus of all roofed arm sections, it 

 was necessary first to compute the moments of inertia of the whole or parts 

 of the top segments of such sections (viz. nominal and minimum sections 



Fig. 2 — Brace bolt hole section containing a f inch knot located immediately below 

 the top segment (knot and bolt hole shaded). 



between pinholes, and nominal and minimum pinhole sections). Accord- 

 ingly, four such computations were made and the results used in calculating 

 the section moduU of all the roofed sections investigated. The details of 

 the four computations are shown in the Appendix. To insure uniformity 

 in the results, the degree of precision used in these computations was con- 

 siderably greater than is ordinarily employed in dealing with timber prod- 

 ucts. All of the work, however, was done on a computing machine, and it 

 was just about as easy to carry the operations to eight decimal places (which 

 was the capacity of the machine used) as to a lesser number. As a matter 

 of interest in this connection, it was found by actual trial in Computation I 

 that absurd results would occur if fewer than five decimal places were used. 



For convenience, all of the section modulus calculations were made in 

 tabular form. In such form the procedure employed would not be readily 



1 Pg. 6 Dept. Circular 295, U. S. Dept. of Agriculture, "Basic Grading Rules and Work- 

 ing Stresses for Structural Timbers," by J. A. Newlin and R. P. A. Johnson. 



