Building Design and Construction Handbook - Merritt - Ventech!

10.10 SECTION TEN
10.3 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).
10.3.1 Lumber
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.
10.3.2 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 Tim-