Grading of Lumber

Подпись: Copyright © Marcel Dekker, Inc. All rights reserved.Lumber is graded visually and by the machine stress-rated system (MSR). The majority of lumber is graded visually by experienced inspectors who are familiar with lumber grading rules. Lumber grading rules establish limits on the size and characteristics of knots, the number of shakes or splits, and the slope of the grain. Visual evaluation also takes into account any imperfections caused by manufacturing such as torn grain or chip marks. Figure 1.4a shows a typical grade for visually graded lumber.

In machine stress rating, lumber is evaluated by mechanical stress-rating equipment. Lumber is fed through a machine that subjects each piece of wood to a nondestructive test that measures the modulus of elasticity and bending stress. The machine auto­matically takes into account size and characteristics of knots, slope of the grain, density, and moisture contents. The machine automat­ically stamps lumber as machine rated and indicates the values for

Grading of Lumber Grading of Lumber
Grading of Lumber
Grading of Lumber

‘Nominal

Bending

Stress

(1650psi)

 

Moisture Content at Time of Surfacing (S-DRY)

Ю

 

Modulus of Elasticity (1.5x10s psi)

 

Подпись: Copyright © Marcel Dekker, Inc. All rights reserved.

Figure 1.4 Gradings of lumber: (a) visually graded; (b) machine rated.

 

fiber stress in bending and the corresponding modulus of elastic­ity. Figure 1.4b shows a typical grade for machine-rated (MSR) lumber.

Another method of grading is to grade lumber according to its use and appearance. Lumber is classified as Select, Finish, or Common. Select and Finish are used when fine appearance is re-

 

quired. Common grades are more suitable for general construc­tion.