Visual strength grading ~ ``process by which a piece of timber can
be sorted, by means of visual inspection, into a grade to which
characteristic values of strength, stiffness and density may be
allocated''. Definition from BS EN 14081-1.Characteristics:Knots ~ branch growth from or through the main section of timber
weakening the overall structural strength. Measured by comparing
the sum of the projected cross sectional knot area with the cross
sectional area of the piece of timber. This is known as the knot
area ratio (KAR). Knots close to the edge of section have greater
structural significance therefore this area is represented as a
margin condition at the top and bottom quarter of a section. A
margin condition exists when more than half the top or bottom
quarter of a section is occupied by knots.
MKAR = Margin knot area ratio.
TKAR = Total knot area ratio.T
W
knots projected
Projected knots
relative to
sectional area
of timber
w
4
w
4
margin areas
each x Tw
4
w
2
Materials Testing---Softwood Timber Grading---1