The Dos and Do-Knots of Wood
Dead branches drop off healthy, living trees all the time, and wood knots appear in the trunk where branches died. Knots are imperfections that cause living wood grain to grow around them.
It makes perfect sense when you think about it. If wood knots are produced by the death of a branch, how could they contribute to the strength of the remaining wood that lives? The human equivalent would be a gall stone, an imperfection which painfully weakens the performance of a gallbladder. Remove the gallstone, and the bladder recovers. Dead branches leave wood knots behind. Such imperfection is just part of life as we know it.
Wood knots make beautiful wood displays. Ironically, it is the very curves and bends in wood grain that make knots so attractive to view. The wood grain waves its way around the knot. If you have a closer look, notice how the grain pattern slopes up to 90 degrees to get around wood knots. Remarkable. This is why many consumers enjoy having decorative wood with knots.
While wood knots carry visual appeal, they adversely affect wood strength.
Wood strength in knots can be assessed in two ways: tensile strength and compressive strength. These measurements allow us to predict how much load (weight) wood can safely bear before collapsing. Tensile stress elongates or expands an object. Compressive stress shortens or compresses the material.
Since knots are defects which weaken lumber, this weakening effect is more serious when the lumber is subjected to forces perpendicular to the grain and/or tension than when under load along the grain and/or compression. Please note that the extent to which wood knots affect the strength of wood depends on their position, size, number, and condition.
Wood knots weaken wood strength. In fact, knots materially affect cracking (known is the US as “checks”; known in the UK as “shakes”), warping, and the ease of working the lumber. They are defects which lower the lumber’s value for structural purposes where strength is critical. Manufacturers account for the presence of wood knots in lumber grading and knots are classified according to their form, size, soundness, and the firmness with which they are held in place. Frankly, the knot lowers the wood grade.
Aside from wood strength, wood knots also skew wood moisture content (MC) readings. Although a wood plank may have reached its prime MC, known as equilibrium moisture content (EMC), MC consistency can be just as erratic around a wood knot as is the wood grain pattern. Checked knots occur in the initial stages of drying. These defects can be controlled by using higher relative humidities and by drying to a higher final MC but it is impossible to prevent them. Encased knots loosen during drying. The dried knot is smaller than the knothole and sometimes falls out during handling or machining as nothing can be done to prevent the loosening of dead knots during drying. Fewer dead knots will fall out if the final MC of the lumber is kept as high as possible before machining.
Great builders understand the circumstances in which wood knots have an impact on wood strength. Consumers benefit from knowing and remembering that wood knots are a perfectly natural indication of wood’s growth process. You just don’t want to put too much weight on them.
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