CONSTRUCTION - MATERIALS TESTING - SOFTWOOD TIMBER GRADING 1

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.