20 



The fibers around' a knot run nearly at right angles to the axis of the 

 tree, so that in a compression test these fibers are subject to a crushing 

 strength across the grain, in which direction they are very weak. 



Season cracks on top of a beam have little effect upon the strength, 

 except as they ma}*- collect water and start rot. Those, however, on 

 the side of a beam near the neutral plane, which for timber is usually 

 a little below the middle, are very injurious, as they greatly increase 

 the liability to shear along this plane. 



Wooden beams, as ordinarily employed, are more apt to fail in this 

 manner than in any other way. By putting a bolt in each end of large 

 beams, thus firmly holding the top and bottom portions together, this 

 danger could be largely avoided. Ring shakes are also prime causes 

 of shearing along the neutral plane. 



Anatomical .structure. — This has, of course, great influence upon the 

 strength of timber, and to it is largely due the difference in strength 

 between the different species. 



But little is known about this subject as yet, except in a general 

 way, as, for example, regarding pine and oak. 



The present time is a little premature for the formulation of full rules 

 for scientific inspection of timber. We only know that for strength 

 we require dry timber, and for a given species the heavier the stronger. 



For the present it is impossible to evaluate the effect of knots and 

 other defects, but we should guard against them as far as possible. 



The safe loads given in the tables herein will then be perfectly safe, 

 and apply to all sizes. 



METHODS OF DESIGNING. 

 PKESENT PRACTICE. 



Many of the railroad companies now use a safe load of 1,000 pounds 

 per square inch for the modulus of rupture for longleaf-pine stringers. 

 The caps, sills, and posts are usually 1 2 by 12 inches, irrespective of load. 



Fig. 2 represents a common type of construction designed by the 

 above considerations and for the same conditions given in Table 111. 



The formula for bending is* 



M = bending moment in pounds per square inch. 

 R = safe load on extreme fiber in pounds per square inch. 

 h = breadth of beam in inches. 

 A=heigh 4 cl beam in inches. 

 Transformed, this becomes 



6M 



I 



R/<* 



See Appendix 1. 



