292 TIM BEE 



logs, and it is useless specifying, as is sometimes done, that 

 the ordinary building timber is " to be free from knots "; 

 loose knots are certainly to be avoided, but one seldom 

 finds these in sound pine timber. 



Very large knots, especially if at the edges of timber 

 which has to stand heavy strains, such as beams or 

 columns, are a cause of considerable risk ; it may safely be 

 said that in the majority of cases where timber is tested as 

 beams or columns it fails at knots. 



Fig. 49 will show the reason for objection to a knot on 

 the underside of a beam carrying a load, where it is 

 much more serious than if on the top side. In bending, 



FIG. 49. -Showing risk of knots on underside of loaded beams. 



the fibres on the underside of the beam are subjected to 

 tension, the fibres of the wood around the knot like those 

 in a piece of cross-grained timber offer but little resistance 

 to tension, and the presence of a knot in this position has 

 the same effect as putting a saw-cut on the underside of the 

 beam. 



A knot is the place where a twig or branch has projected 

 from the parent stem ; some timbers are to be got in con- 

 siderable length with few knots, such as Oregon, California!! 

 redwood, Tasmanian blue gum, kauri, and others, the 

 reason being that these trees do not bear branches within a 

 considerable distance of the ground, whereas European 

 trees, pines and firs used in building, especially the firs 



