THE EFFECT OF SPIRAL GRAIN ON THE STRENGTH OF 



WOOD 



By Thomas R. C. Wilson, 



Engineer in Forest Products, Forest Products Laboratory 



The occurrence of spiral or cork-screw grained trees has long 

 been a subject of interest to foresters. Many theories have been ad- 

 vanced to account for the phenomenon and experiments are now under 

 way to determine if it is inheritable. 



Severe spiral grain has been recognized by many as disqualifying 

 timber for uses in which strength and resistance to shock are im- 

 portant. However, many timber producers and users, while recogniz- 

 ing as defects knots, pitch pockets, rot, shakes, severe checks, and cross 

 gram resulting from mismanufacture, have entirely neglected spiral 

 grain as a source of danger or weakness except perhaps in extreme 

 cases. 



It has long been observed in the testing laboratory that spiral grain 

 is as weakening as other forms of cross grain and it has been consid- 

 ered more dangerous because of the probability of its passing unno- 

 ticed or not being recognized as a source of weakness. It had not 

 been possible previous to the war to carry out tests to give a quan- 

 titative measure of the effect of spiral or other forms of cross grain, 

 but when the problem of specifying material for use in airplane con- 

 struction arose, it soon became apparent that more exact knowledge of 

 the effect of deviations of the grain of wood from parallelism with the 

 edges or axis of the piece was needed. In order to secure, such in- 

 formation a series of tests was made on Sitka spruce, Douglas fir, 

 and white ash, at the Forest Products Laboratory. These tests go a 

 long way in clarifying cross grain and spiral grain as defects and, be- 

 cause of the depreciating influence of these defects upon the strength 

 properties of the wood and the frequency with which spiral grain oc- 

 curs in many stands, indicate that their control is a subject worthy 

 of study by the silviculturists. 



Before proceeding to further discussion of these tests, it is desir- 

 able to have exact definitions of some of the factors to be treated. 



