AWC 2012 NDS
National Design Specification - Design Values for Wood Construction
|Publication Date:||1 January 2012|
This Supplement is a compendium of reference design values for structural sawn lumber, structural glued laminated timber, and round timber piles and poles. These reference design values have been obtained from the organizations responsible for establishing design values for these products. Reference design values in this Supplement are provided as a courtesy for use with the design provisions of the National Design Specification® (NDS®) for Wood Construction, 2012 Edition.
Reference design values for lumber in this Supplement are obtained from grading rules published by seven agencies: National Lumber Grades Authority (a Canadian agency), 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. Grading rules promulgated by these agencies, including reference design values therein, have been approved by the Board of Review of the American Lumber Standard Committee and certified for conformance with U.S. Department of Commerce Voluntary Product Standard PS 20-10 (American Softwood Lumber Standard).
Reference design values for most species and grades of visually graded dimension lumber are based on provisions of ASTM Standard D 1990-07Σ1 (Establishing Allowable Properties for Visually Graded Dimension Lumber from In-Grade Tests of Full-Size Specimens). Reference design values for visually graded timbers, decking, and some species and grades of dimension lumber are based on provisions of ASTM Standard D 245-06 (2011)Σ1 (Establishing Structural Grades and Related Allowable Properties for Visually Graded Lumber). Methods in ASTM Standard D 245 involve adjusting strength properties of small clear specimens of wood, as given in ASTM Standard D 2555- 06 (Establishing Clear Wood Strength Values), for effects of knots, slope of grain, splits, checks, size, duration of load, moisture content, and other influencing factors, to obtain reference design values applicable to normal conditions of service. Lumber structures designed on the basis of working stresses derived from ASTM Standard D 245 procedures and standard design criteria have a long history of satisfactory performance.
Reference design values for machine stress rated (MSR) lumber and machine evaluated lumber (MEL) are based on nondestructive testing of individual pieces. Certain visual grade requirements also apply to such lumber. The stress rating system used for MSR and MEL lumber is regularly checked by the responsible grading agency for conformance to established certification and quality control procedures.
For additional information on development and applicability of lumber reference design values, grading rules published by individual agencies and referenced ASTM Standards should be consulted.
Structural Glued Laminated Timber
Reference design values in this Supplement for structural glued laminated timber are developed and published by the American Institute of Timber Construction (AITC) and APA-The Engineered Wood Association (APA) in accordance with principles originally established by the U.S. Forest Products Laboratory in the early 1950s. These principles involve adjusting strength properties of clear straight grained lumber to account for knots, slope of grain, density, size of member, number of laminations, and other factors unique to laminated timber.
Specific methods used to establish reference design values have been periodically revised and improved to reflect results of tests of large structural glued laminated timber members conducted by the U.S. Forest Products Laboratory and other accredited testing agencies. The performance history of structures made with structural glued laminated timber conforming to AITC or APA specifications and manufactured in accordance with American National Standard ANSI/AITC A190.1-2007 (Structural Glued Laminated Timber) has demonstrated the validity of methods used to establish structural glued laminated timber reference design values.
Round Timber Piles and Poles
Reference design values in this Supplement for round timber piles and poles are developed by the Timber Piling Council of the Southern Pressure Treaters' Association in accordance with principles originally established by the U.S. Forest Products Laboratory in the early 1950s and contained in ASTM D 2899-03 (Standard Practice for Establishing Allowable Stresses for Round Timber Piles) and ASTM D 3200-74(2005) (Standard Practice for Establishing Allowable Stresses for Round Timber Construction Poles), respectively. These principles involve adjusting strength properties of clear straight grained poles to account for knots, slope of grain, density, size of member, and other factors unique to timber poles.
Specific methods used to establish reference design values are contained in D 2899 and are used by D 3200. These methods have been revised to reflect results of fullsize tests of timber piles.
Conditions of Use
Reference design values presented in this Supplement are for normal load duration under dry conditions of service. Because the strength of wood varies with conditions under which it is used, these reference design values should only be applied in conjunction with appropriate design and service recommendations from the NDS. Additionally, reference design values in this Supplement apply only to material identified by the grade mark of, or certificate of inspection issued by, a grading or inspection bureau or agency recognized as being competent.