Knots materially affect checking and warping, ease in working, and cleavability of timber.  They are defects which weaken timber and depreciate its value for structural purposes where strength is an important consideration.  The weakening effect is much more serious where timber is subjected to bending and tension than where under compression.  The extent to which knots affect the strength of a beam depends upon their position, size, number, direction of fibre, and condition.  A knot on the upper side is compressed, while one on the lower side is subjected to tension.  The knot, especially (as is often the case) if there is a season check in it, offers little resistance to this tensile stress.  Small, knots, however, may be so located in a beam along the neutral plane as actually to increase the strength by tending to prevent longitudinal shearing.  Knots in a board or plank are least injurious when they extend through it at right angles to its broadest surface.  Knots which occur near the ends of a beam do not weaken it.  Sound knots which occur in the central portion one-fourth the height of the beam from either edge are not serious defects.

Extensive experiments by the U.S.  Forest Service[33] indicate the following effects of knots on structural timbers: 

[Footnote 33:  Bul. 108, pp. 52 et seq.]

(1) Knots do not materially influence the stiffness of structural timber.

(2) Only defects of the most serious character affect the elastic limit of beams.  Stiffness and elastic strength are more dependent upon the quality of the wood fibre than upon defects in the beam.

(3) The effect of knots is to reduce the difference between the fibre stress at elastic limit and the modulus of rupture of beams.  The breaking strength is very susceptible to defects.

(4) Sound knots do not weaken wood when subject to compression parallel to the grain.[34]

[Footnote 34:  Bul. 115, U.S.  Forest Service:  Mechanical properties of western hemlock, p. 20.]

FROST SPLITS

A common defect in standing timber results from radial splits which extend inward from the periphery of the tree, and almost, if not always, near the base.  It is most common in trees which split readily, and those with large rays and thin bark.  The primary cause of the splitting is frost, and various theories have been advanced to explain the action.

R. Hartig[35] believes that freezing forces out a part of the imbibition water of the cell walls, thereby causing the wood to shrink, and if the interior layers have not yet been cooled, tangential strains arise which finally produce radial clefts.

[Footnote 35:  Hartig, R.:  The diseases of trees (trans. by Somerville and Ward), London and New York, 1894, pp. 282-294.]

Another theory holds that the water is not driven out of the cell walls, but that difference in temperature conditions of inner and outer layers is itself sufficient to set up the strains, resulting in splitting.  An air temperature of 14 deg.F. or less is considered necessary to produce frost splits.