ASD/LRFD Manual - American Wood Council

M1: general requirements for structural design
M1.1 Products Covered in This Manual
This Manual was developed with the intention of covering
all structural applications of wood-based products
and their connections that meet the requirements of the
referenced standards. The Manual is a dual format document
incorporating design provisions for both allowable
stress design (ASD) and load and resistance factor design
(LRFD). Design information is available for the following
list of products. Each product chapter contains information
for use with this Manual and the National Design Specification®
(NDS®) for Wood Construction. Chapters are
organized to parallel the chapter format of the NDS.
• Sawn Lumber Chapter 4
• Structural Glued Laminated Timber Chapter 5
• Round Timber Poles and Piles Chapter 6
• Prefabricated Wood I-Joists Chapter 7
• Structural Composite Lumber Chapter 8
• Wood Structural Panels Chapter 9
• Mechanical Connections Chapter 10
• Dowel-Type Fasteners Chapter 11
• Split Ring and Shear Plate Connectors Chapter 12
• Timber Rivets Chapter 13
• Shear Walls and Diaphragms Chapter 14
An additional Supplement, entitled Special Design
Provisions for Wind and Seismic (SDPWS), has been
developed to cover materials, design, and construction of
wood members, fasteners, and assemblies to resist wind
and seismic forces.
M1.2 General Requirements
This Manual is organized as a multi-part package for
maximum flexibility for the design engineer. Included in
this package are:
• NDS and Commentary; and, NDS Supplement:
Design Values for Wood Construction,
• Special Design Provisions for Wind and Seismic
(SDPWS) and Commentary,
• Structural Wood Design Solved Example Problems.
M1.2.1 Bracing
Design considerations related to both temporary and
permanent bracing differ among product types. Specific
discussion of bracing is included in the product chapter.
M1.3 Design Procedures
The NDS is a dual format specification incorporating
design provisions for ASD and LRFD. Behavioral equations,
such as those for member and connection design, are
the same for both ASD and LRFD. Adjustment factor tables
include applicable factors for determining an adjusted ASD
design value or an adjusted LRFD design value. NDS
Appendix N – (Mandatory) Load and Resistance Factor
Design (LRFD) outlines requirements that are unique to
LRFD and adjustment factors for LRFD.
The basic design equations for ASD or LRFD require
that the specified product reference design value meet or
exceed the actual (applied) stress or other effect imposed
by the specified loads. In ASD, the reference design values
are set very low, and the nominal load magnitudes are set
at once-in-a-lifetime service load levels. This combination
produces designs that maintain high safety levels yet
remain economically feasible.
From a user’s standpoint, the design process is similar
using LRFD. The most obvious difference between
LRFD and ASD is that both the adjusted design values
and load effect values in ASD will be numerically much
lower than in LRFD. The adjusted design values are lower
because they are reduced by significant internal safety
adjustments. The load effects are lower because they are
nominal (service) load magnitudes. The load combination
equations for use with ASD and LRFD are given in the
model building codes.
American Wood Council