Design values for lumber are contained
in grading rules established by the National Lumber Grades Authority
(Canadian), Northeastern Lumber Manufacturers Association, Northern
Softwood Lumber Bureau, Redwood Inspection Service, Southern Pine
Inspection Bureau, West Coast Lumber Inspection Bureau, and Western
Wood Products Association.
The rules and the design values in them
have been approved by the Board of Review of the American Lumber
Standards Committee. They also have been certified for conformance
with U.S. Department of Commerce Voluntary Product Standard PS 20-94
(American Softwood Lumber Standard).
In addition, design values for visually
graded lumber may be established in accordance with ASTM D1990,
‘‘Standard Practice for Establishing Allowable Properties for
Visually-Graded Dimensional Lumber from In-Grade Tests of Full- Size
Specimens.’’
Design values for visually graded
timbers, decking, and some species and grades of dimension lumber are
based on provisions of ‘‘Establishing Structural Grades and
Related Allowable Properties for Visually Graded Lumber,’’ ASTM
D245.
ASTM D245 also specifies adjustments to
be made in the strength properties of small clear specimens of wood,
as determined in accordance with ‘‘Establishing Clear Wood
Strength Values,’’ ASTM D2555, to obtain design values applicable
to normal conditions of service.
The adjustments account for the effects
of knots, slope of grain, splits, checks, size, duration of load,
moisture content, and other influencing factors. Lumber structures
designed with working stresses derived from D245 procedures and
standard design criteria have a long history of satisfactory
performance.
Design values for machine stress-rated
(MSR) lumber and machine-evaluated lumber (MEL) are based on
nondestructive tests of individual wood pieces. Certain visual-grade
requirements also apply to such lumber.
The stress rating system used for MSR
lumber and MEL is checked regularly by the responsible grading agency
for conformance with established certification and quality-control
procedures.
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