Design Values for Lumber and Timber

Design values for an extensive range of sawn lumber and timber are tabulated in the supplement to the National Design Specification for Wood Construction, (NDS), American Forest & Paper Association (AF&PA).


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.

Glued-Laminated Timber

Design values for glulam timber, developed by the American Institute of Timber Construction (AITC) and Engineered Wood Systems (EWS) in accordance with principles originally established by the U.S. Forest Products Laboratory, are included in the supplement to the NDS. The principles are the basis for the Standard Method for Establishing Stresses for Structural Glued-Laminated Timber (Glulam), ASTM D3737 which specifies procedures for obtaining design values that account for the effects of knots, slope of grain, density, size of member, curvature, number of laminations, and other factors unique to glulam. The satisfactory performance??of structures made with glulam members conforming to AITC and EWS specifications and manufactured in accordance with Structured Glued-Laminated Timber, ANSI A190.1, demonstrates the validity of the methods used to establish glulam design values.

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