ASD/LRFD Manual - American Wood Council

M2: DESIGN VALUES FOR STRUCTURAL MEMBERS
M2.1 General Information
Structural wood products are provided to serve a wide
range of end uses. Some products are marketed through
commodity channels where the products meet specific
standards and the selection of the appropriate product is
the responsibility of the user.
Other products are custom manufactured to meet the
specific needs of a given project. Products in this category
are metal plate connected wood trusses and custom structural
glued laminated timbers. Design of the individual
members is based on criteria specified by the architect or
engineer of record on the project. Manufacture of these
products is performed in accordance with the product’s
manufacturing standards. Engineering of these products
normally only extends to the design of the products
themselves. Construction-related issues, such as load path
analysis and erection bracing, remain the responsibility of
the professional of record for the project.
M2.2 Reference Design Values
Reference design value designates the allowable
stress design value based on normal load duration. To
avoid confusion, the descriptor “reference” is used and
serves as a reminder that design value adjustment factors
are applicable for design values in accordance with referenced
conditions specified in the NDS – such as normal
load duration.
Reference design values for sawn lumber and structural
glued laminated timber are contained in the NDS
Supplement: Design Values for Wood Construction. Reference
design values for round timber poles and piles,
dowel-type fasteners, split ring and shear plate connectors,
and timber rivets are contained in the NDS. Reference design
values for all other products are typically contained
in the manufacturer’s code evaluation report.
M2.3 Adjustment of Design Values
Adjusted design value designates reference design
values which have been multiplied by adjustment factors.
Basic requirements for design use terminology applicable
to both ASD and LRFD. In equation format, this takes the
standard form f b ≤ F b ′ which is applicable to either ASD or
LRFD. Reference design values (F b , F t , F v , F c , F c , E, E min )
are multiplied by adjustment factors to determine adjusted
design values (F b ′, F t ′, F v ′, F c ′, F c ′, E′, E min ′).
Reference conditions have been defined such that a
majority of wood products used in interior or in protected
environments will require no adjustment for moisture,
temperature, or treatment effects.
Moisture content (MC) reference conditions are 19%
or less for sawn lumber products. The equivalent limit for
glued products (structural glued laminated timber, structural
composite lumber, prefabricated wood I-joists, and
wood structural panels) is defined as 16% MC or less.
Temperature reference conditions include sustained
temperatures up to 100ºF. Note that it has been traditionally
assumed that these reference conditions also include
common building applications in desert locations where
daytime temperatures will often exceed 100ºF. Examples
of applications that may exceed the reference temperature
range include food processing or other industrial buildings.
Tabulated design values and capacities are for untreated
members. Tabulated design values and capacities
also apply to wood products pressure treated by an approved
process and preservative except as specified for
load duration factors.
An unincised reference condition is assumed. For
members that are incised to increase penetration of preservative
chemicals, use the incising adjustment factors
given in the product chapter.
The effects of fire retardant chemical treatment on
strength shall be accounted for in the design. Reference
design values, including connection design values, for
lumber and structural glued laminated timber pressuretreated
with fire retardant chemicals shall be obtained
from the company providing the treatment and redrying
service. The impact load duration factor shall not apply
to structural members pressure-treated with fire retardant
chemicals.
American Wood Council