﻿12 BULLETIN 1128, U. S. DEPARTMENT OF AGRICULTURE. 



short and sharp and may be confused with breaks in brash wood. 

 By turning the member over after the first weight is applied, the 

 compression side, already partially failed, becomes the tension side 

 under the new load; and when the new compression side fails, the 

 tension side, already fractured squarely across, fails with it. To 

 test the resiliency of such members, apply the load on one side only, 

 and do so with moderation. 



SHAKES. 



Shakes are long tangential cracks or separations in the wood fiber. 

 They are the result of an actual rupture of the wood due to wind, 

 felling stresses, or other causes and are exceedingly detrimental to 

 strength. Old shakes which have occurred while the tree was still 

 standing are often stained and readily visible to the naked eye. 

 This is also true where lumber has been exposed to the weather and 

 dirt has filtered into the cracks. But where they are neither dis- 

 colored nor opened up the rupture is not so easily detected. 



PITCH POCKETS. 



Pitch seams or pockets are lens-shaped cavities or openings be- 

 tween the annual rings. They contain resin or pitch either in solid 

 or liquid form; hence the name. These defects result from injury 

 to the living tree, but the cause of injuiy is as yet unknown. Pitch 

 pockets may indicate more serious wounds. They are very common 

 in Douglas fir, but may be found in other resin-producing softwoods, 

 including spruce. 



While pitch pockets reduce the strength of wood, the reduction is 

 not as serious as is generally supposed. General specifications re- 

 garding the presence of these defects have been worked out for wing 

 beams of spruce and Douglas fir (69, p. 21). 



WORM HOLES. 



Worm holes are caused by the larvae of three main types of wood- 

 boring insects. The powdery or granular matter, the excrement or 

 f rass of either the adults or the " worms," or larvae, with which these 

 galleries or burrows are usually filled, need not be confounded with 

 decay, since there is no difficulty in separating the two defects. In 

 decay the transition from the soft, spongy, or friable wood to the 

 normal hard material is gradual, while in the worm holes, usually 

 circular or somewhat flattened when seen in cross section, the line 

 between the firm wood and the trass or finely excreted wood is very 

 sharp. 



(1) Ambrosia beetles or "pinworms." The small adult beetle bores into the 

 green saw log or green lumber and deposits its eggs. The larvae hatching from 

 these eggs extend other burrows at an angle from the parent galleries. Mois- 

 ture is necessary for this type of insect. There may be a blackish discoloration 

 extending around the galleries, particularly those in the sapwood. This is the 

 result of the activity of a wood-staining fungus which does not cause decay in 

 the wood and therefore need not be considered as weakening the material. 



(2) Borers. The adults of borers, as a rule, require bark under which to 

 lay their eggs. The larvae hatching from these eggs burrow under the bark 

 through the sapwood and sometimes into the heart wood; the holes are often 

 large. 



(3) Powder-post beetles. Powder-post beetles cause the most dangerous type 

 of defect, and their presence may be detected by fine, powdery droppings com- 

 ing from the wood. The eggs are laid either in the pores of the wood or under 

 the bark, depending upon the type of insect causing powder post. 



