Wood is a
naturally occurring renewable material affected by
species, natural growth characteristics and moisture
content all of which contribute to variability of
its structural properties. Because of its cell
structure, wood has different strength properties in
different grain directions and is therefore
categorized as an anisotropic material.
CSA O86 provides resistance equations and
specified strength values for various wood products
and connections. Links to these values can be found
below under design values.
Like all building materials, wood has unique
design properties. By understanding the nature of
these properties, designers are able to maximize the
positive attibutes of materials and account for
other effects. Unique properties that affect wood
design include:
- Hygroscopicity - The tendancy for wood to
absorb moisture. This can affect both
dimensional stability and strength. A calculator
to account for this can be found
here.
- Duration of Load effects - Wood has a
greater capacity to carry short term loads
compared to sustained loads enhancing its
performance in seismic and high wind zones.
- System effects - Wood systems have the
ability to distribute and mutually support loads
increasing the efficiency of wood framing
systems.
- Size effects - Research has shown that
smaller wood members are stronger per unit area
than larger members.
Detailed information on wood design and wood
properties can be found in
Introduction to
Wood Design
Structural Design Standards
CSA Standard O86-01
Engineered Design in Wood is the referenced wood
design standard in Canada. This consensus-based
standard is referenced by Part 4 of the National and
Provincial Building Codes and is written in the
limit states design (LSD) format. It provides
resistance equations and specified strength values
for lumber, glulam, panels, composite components
such as stressed skin panels, shearwalls and
diaphragms, fastennings, poles and piles and
proprietary structural wood products such as
I-joists.
Major revisions in the 2001 edition of CSA O86
included: new design procedures for shearwalls and
diaphragms,inclusion of design values for
construction OSB, changes to connection design and
modifications to sawn lumber and glulam design
procedures. The Canadian Wood Council's
comprehensive Wood Design Manual, has been updated
to reflect the changes to the design standard
including member and fastenings design examples,
tables and reference material. The manual also
includes a copy of CSA O86-01. For further details
see the
Wood Design Manual.
Wood designers in the US can use either an
Allowable Stress Design (ASD) format or a Load and
Resistance Factor Design (LRFD) approach. The
referenced ASD design standard is the ANSI/AF&PA
National Design Specification for Wood Construction.
The ASD Manual, published by the American Forest and
Paper Association, brings together all required
elements for design of wood structures in one
comprehensive package. It includes the NDS and
Supplement, material design information and design
examples. For further information, please visit,
http://www.awc.org/Standards/nds.html .
The AF&PA/ASCE 16-95 Standard for Load and
Resistance Factor Design (LRFD) for Engineered Wood
Construction serves as the code recognized alternate
basis for wood structures designed using the LRFD
methodology. The LRFD Manual brings together all
required elements for LRFD design of wood structures
in one package that includes the design standard
along with 5 supplements and 4 guidelines. For
further information, please visit,
http://www.awc.org/Standards/lrfd.html.
The American Wood Council publishes a series of
accompanying supplements, commentaries and updates
found
here.
