19 

 Making- W=W" wo have 



or 



c = o oj for uniformly loaded beams or 



R/r 

 c = ., .. , for concentrated loads at middle. 



Problem: What is the length of grip necessary at the end of a white- 

 pine stringer 14 feet long and 16 inches high? 



The table gives for these dimensions 7.3 inches as the grip required 



for a longleaf-pine beam uniformly loaded. 



, K'. . . . . J 4.0 1 



Ihe value of -tt- in this case is equal toj^= *- k 



Also we must add 30 per cent on account of the difference in the 



character of the load. Hence we have 7.3 X +-5 X 1.3 = 7.0 inches. 



l. Ji 



INSPECTION. 



Scientific inspection of timber requires a knowledge of the elements 

 affecting the strength thereof. 



The following are the principal elements: Moisture condition; weight 

 per cubic foot; size of piece: position in tree; defects, such as knots, 

 cross-graining, ring shakes, and season checks; anatomical structure; 

 character of the secretions, such as resin, etc.; method of treatment 

 previous to use. 



Moisture condition. — This is the chief element affecting the strength 

 of timber. The strength of a thoroughly seasoned piece is from 50 to 

 100 per cent more than that of a green piece. To say that a certain 

 piece of timber has a strength of a certain amount is of no service 

 whatever in determining its relative strength unless the moisture con- 

 dition is specified. 



Weight. — In material of the same species, having practically the same 

 anatomical structure and character of secretions, the strength varies 

 directly as the weight for the same condition of moisture. 



Size of piece. — This has some effect, but it is much less than has been 

 generally supposed. The chief difficulty is in the seasoning of large 

 pieces. If this is done carefully no allowance need be made in the safe 

 loads given on account of size, except for more frequent defects. 



Position in tree. — The strength varies with the position in tree. In 

 old trees of pine, from 150 to 200 years of age, the strongest portion of 

 the butt log will be at about one-half the radius from the center. As 

 we go higher in the tree, the central part, though weaker than in the 

 butt log, becomes the strongest portion of the cross section. 



Defects. — Large knots should not be allowed to come at the middle of 

 a beam, either on top or bottom, as they are a source of weakness in 

 compression as well as in tension, though not quite to the same extent. 



