WOOD - Materials for Civil and Construction Engineers - page 406

Each of these agencies writes standards and specifications for particular
species or combinations of species that are produced within their operating
region. For example, the SPIB provides the grading rules for all species of
Southern Pine, the WWPA governs grading of Ponderosa Pine, and the
NLGA provides grading rules for all grades produced in Canada.

■ 10.5.1 Hardwood Grades

The National Hardwood Lumber Association bases the grading of hardwood
on the amount of usable lumber in each piece of standard length lumber. The
inspection is performed on the poorest side of the material and the grade is
based on the number and size of clear “cuttings” that can be produced from
a given piece of lumber. Cuttings must have one face clear of strength-reducing
imperfections and the other side must be sound. Based on “cutting” quanti-
ty, the wood is given a classification of Firsts, Seconds, Selects , and Common
(No. 1, No. 2, No. 3A, or 3B), with Firsts being the best. Frequently, Firsts and
Seconds are grouped into one grade of Firsts and Seconds, FAS.

■

Softwood Grades
Softwood for structural applications is either graded by visual inspection or
is machine stress graded. The purpose of grading is to ensure that all lumber
graded to a specific grade designation has at least the minimum mechanical
or load-carrying capability with respect to critical design parameters. Under
machine stress grading, each piece of wood is subjected to a bending stress,
then based on the mechanical response, the wood is graded, as shown in
Table 10.3. The grade designation identifies the minimum extreme fiber
bending stress, tensile stress parallel to grain, compressive stress parallel to
grain, and modulus of elasticity of the wood.
For visual classification, the basic mechanical properties of the wood are
determined by testing small clear wood specimens. These results are then ad-
justed for allowable defects and characteristics for each class of wood. Unlike
stress graded lumber, visual stress grade properties are defined for each
species of softwood. Table 10.4 is an example of design values for grades of
Eastern White Pine.
Visual grade designations include Yard, Structural , or Factory and Shop ,
with subgrades of Select, Select B, Select C, and No. 1, No. 2, and No. 3 com-
mons, appearance, and studs. Other commonly used ratings are Construction,
Standard , or Utility and combinations such as No. 2&BTR (Number 2 and
Better) and STD&BTR. Not all grades are used for all species of wood.
Yard lumber frequently refers to some specialty stress grades of lumber
such as those used for light structural framing. Structural lumber typical-
ly comprises pieces 50 mm to 125 mm (2 in. to 5 in.) thick and is graded
according to its intended use. Grading categories include light framing,
joists and planks, beams and stringers, and posts and timbers. Factory and
shop lumber includes siding, flooring, casing, shingles, shakes, and finish
lumber.

10.5.2