Building Design and Construction

Structural Grading of Wood

Strength properties of wood are closely related to moisture content and specific gravity. Therefore, data on strength properties should be accompanied by corresponding data on these physical properties.
The strength of wood is actually affected by many other factors, including loading rate, load duration, temperature, grain direction, and position of growth rings.
Strength is also influenced by inherent growth characteristics, including knots, slope of grain, shakes, and checks. Analysis and integration of available data have yielded a comprehensive set of principles for grading structural lumber (Art. 10.3.1).
The same characteristics that reduce the strength of solid timber also affect the strength of glued-laminated (glulam) members (Art. 10.3.2). There are, however, additional factors peculiar to glulam flexural members that should be considered.
For example, knots located near the neutral axis, which is a region of low bending stress, have less effect on strength than knots closer to the outer surfaces, where bending stresses are higher. Thus, strength of a flexural member with low-grade laminations can be improved by substitution of higher grade laminations at the top and bottom of the member.
Dispersement of knots in laminated members has a beneficial effect on strength.
With sufficient knowledge of the occurrence of knots within a grade, mathematical  estimates of the effect may be established for members containing various numbers of laminations.


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