Wood Structural Systems
Wood can be used in many popular
structural forms from repetitive small light members to the larger and heavier
framing systems used in commercial projects, such as arenas or storage
facilities. Because wood has a high strength to weight ratio, dead load is a
smaller component of the total load factor than for heavier materials. Usually
the lightest or least involved construction type appropriate for a given span,
that is capable of carrying the design load, is the most preferable.
The
following table of typical spans is presented to aid the designer in selecting
an appropriate wood structural system.
Structural Design Standards
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 (NDS) 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 NDS-ASD.
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 LRFD.
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
various wood products and connections.
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 Council뭩 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, please visit CWC
publications.
Wood Properties
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.
Like all building materials, wood has
unique design properties. By understanding the nature of these properties,
designers are able to maximize the positive attributes of materials and account
for other effects. Unique properties that affect wood design include
hygroscopicity, duration of load effects, system effects, and size effects. Look
for upcoming issues of the Wood (IN)Site, where these wood properties
will be discussed in detail.
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Insulation and Ventilation of Wood-Frame Roof
Assemblies
Part 3 ?Cathedral Ceilings and Low-sloped
Roofs
Michael Steffen
Earlier articles on the topic of
roof insulation and ventilation discussed code requirements, current research,
and principles related to roof ventilation. Here we review special issues
related to cathedral ceilings, low-sloped roofs, and unvented roofs. 
Ventilation of Cathedral Ceilings
Ventilation
is recommended at cathedral ceilings in heating climates, but should be
considered a design option in mixed and cooling climates. Designers and builders
should be aware of potential problems when cathedral ceilings are
ventilated.
Maintaining open ventilation airways is important in
cathedral ceilings. Cathedral ceiling assemblies are particularly susceptible to
increased air leakage from unbalanced ventilation due to the limited volume of
the framing cavities. In some situations, batt insulation may be pushed in too
far during installation, or batts may continue to loft after installation. If
batts contact the roof sheathing, airflow from the intake vents can be
restricted and exhaust vents may pull air from the building interior. Foam
baffles, installed continuously along each rafter bay, can help ensure airways
are maintained.
Even when airways are properly sized and maintained, a
roof with unbalanced intake and exhaust vents can suffer from increased moisture
levels if exhaust vents draw moist interior air against the cold underside of
the roof sheathing.
Framing cavities in cathedral ceilings may be
interrupted by skylights, chimneys, and other large penetrations that can block
the path of airflow from intake to exhaust vents. Special detailing is necessary
to ensure adequate airflow around these penetrations and to avoid ventilation
dead spots.
Cathedral Ceiling Design Options
For
enhanced energy efficiency and protection against ice dams, cold roof designs
can be used in areas where snow remains on roofs for long periods. A cold roof
is a double-layer roof, with insulation filling the framing cavity, and a
separate ventilation cavity provided above. Sleepers [strapping] are installed
over the rafters to create the ventilation cavity. Continuous vents are provided
at the soffit/eave and ridge. A special ridge vent, called a 밄oston Cap?can be
used to maintain airflow while minimizing snow entry into the roof assembly.
Cathedral ceilings are often used for architectural purposes. Where the
roof framing and decking are exposed to the interior, the insulation is placed
on top of the roof deck. This is sometimes referred to as a warm roof.
As for all roof types, airtightness of the ceiling plane is critical.
Wood decking finish on the ceiling side of a vented cathedral ceiling is not
resistant to air leakage. In heating climates, one solution is to use a sealed
polyethylene vapor retarder to achieve the necessary level of
airtightness.
Part 4 of this article will appear in the March issue of
Wood (IN)Site. A longer version of this article is found in the Spring 2004
issue, Number 27 of Wood Design & Building. For more information visit www.woodmags.com. Michael
Steffen is a registered architect and Quality Director at Walsh Construction
Company in Portland, Oregon.
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Points of Interest
To investigate the shear and moment at
any point along the length of a beam or column click the points of interest
button on the Tool bar.
A
point of interest is generated by specifying a "Location from Left" to perform
the analysis. Then click 밃dd?to add this to the list. Several points of interest
can be specified.
After performing a design, the point of interest results will be
shown in the Diagrams window and in Analysis results output.
 
Designers may require shear or moment forces at specific points of
interest. For example, for many types of connections, designers are required to
check the shear capacity of the member at the connection location.
WoodWorks?/sup> Connections provides the effective shear
capacity of a wood member at a connection location. Using the Point of Interest
function, a designer could determine the corresponding design shear force. | | |
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Did you know? Carbon dioxide and other greenhouse gases
are released from the burning of fossil fuels and are thought to be the leading
cause of human-induced climate change. As individuals, we can contribute by
reducing our energy consumption, and where possible, using renewable energy and
materials.
Wood is a renewable material produced with natural solar
energy, compared to steel, cement, and plastics, which are non-renewable and
require the consumption of fossil fuels to produce. Where it makes sense, like
in construction, substituting or continuing to use wood in place of these other
materials can help to reduce greenhouse gas emissions.
Put simply, trees
grow by taking carbon dioxide out of the atmosphere and converting it into
sugars, which are then used to build the wood. When a tree decays or burns, the
carbon contained in the wood is released back into the environment and the cycle
is complete.
To learn more visit The Sensible Environmentalist or contact Dr.
Patrick Moore who has been a leader of the environmental movement for more
than thirty years. As co-founder and former president of Greenpeace, he holds a
PhD in ecology and a BSc in forest biology. Source: Wood Promotion
Network
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The Cedar Strip and The Cedar Panel Cedar is a light and durable wood that is naturally resistant to
decay and insect infestation, as well as offering superior insulating
properties.
Maibec, a
Canadian based company, is the largest manufacturer of eastern white shingles in
the world and has been supplying designers and builders throughout North America
since 1964. More recently, Maibec has developed two new products guaranteed to
save you installation time and resources, without compromising the look and
integrity of tradition.
The Cedar Strip is an arrangement of 5 or 6
shingles spanning a total length of 32 inches. The design guarantees the
alignment of keyway spacing over successive courses, and makes sidewall
installation both faster and easier.
The Cedar Panel is an exterior
sidewall system composed of kiln-dried white cedar shingles affixed to
3/8?exterior plywood over a moisture-eliminating rainscreen, a layer of
perforated asphalt felt with rust-resistant staples front and back, and
weatherproof polyurethane adhesive glue. The complete system provides
long-lasting heavy-duty stability and durability.
The moisture-eliminating rainscreen, Home Slicker, is a trademark
of Benjamin Obdyke Incorporated and protects wall systems from the damaging
effects of moisture. This unique three-dimensional matrix design provides
uninterrupted space for drying, channels for drainage, and a thermal break for
temperature and pressure equalization, allowing moisture to escape quickly. This
ensures decay resistance, protection of sidewall material, and reduces premature
peeling and blistering of finishes.
The panel ends of the Cedar Panel
have both overlapping and interlocking systems between successive panels to
cover and hide all vertical seams with a layer of shingles, and to further
reduce water penetration. The top and bottom edges are also beveled to assist in
panel alignment and to provide a 뱇ocking?system to wedge the top panel into
place. Even buttline panels can be applied in three and four courses with 5?or
7?exposure per course, or a 7?staggered buttline, creating a panel 96?x 22
2/3?with a net surface coverage of 14.25 sq. ft.
Prefabricated corners
are manufactured of identical grade white cedar with matching exposure, texture,
and buttline.
Due to the detailed construction of The Cedar Panel, stain
warranties consist of 7 years (1 coat) or 20 years (2 coats). Maibec has also
teamed up with Cabot?/sup> to provide factory-stained shingles
guaranteed not to crack, blister, peel, or chip for 5 years (1 coat) or 15 years
(2 coats), and boasts a 30-year guarantee against wood decay. Shingles are also
available green, natural and certified (from a well-managed forest) under the Forest
Stewardship Council (FSC). Cabot also offers Cabot?/sup> Bleaching
OilTM to ensure shingles will age uniformly for the traditional
silver grey finish.
Visit Maibec installation details for Applying Single-Course Shingles,
Shingling Over Old Walls, Applying Double-Course Shingles, Corner Treatments,
and Cutting Angles for Gables. For specific installation guidelines for the
systems described above, please visit The
Cedar Strip and The
Cedar Panel.
A list of distributors of Maibec products in North America can be
obtained from there website. For further details on the information discussed
above, and information on other products, pricing, or specifications, visit Maibec or contact their head
office directly at 1-800-363-1930.
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CWC's First Engineering Competition
The
Canadian Wood Council뭩 First Engineering Competition held February 15th in
Ottawa was a resounding success. The CWC challenged engineering students from
Canadian universities and colleges to build a wood catapult with a maximum
allowable mass of 60 kg (132 lbs), and capable of launching a softball a minimum
distance of ten meters. In total nine schools from across Canada participated in
the one-day event.
The winning teams distinguished themselves in the
fields of both design and performance. First place was attributed to Montreal뭩
?ole de technologies sup?ieures. McGill University earned a second place
standing while St. Laurence College was awarded third place. All of the
participating teams however, were noteworthy as they exuded a sense of
professionalism and a dedication to the field of engineering. The CWC would like
to thank all those involved for their implication, with a special mention of
gratitude to the Carleton University team for having hosted the
event.
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Wood Solutions Fair - Vancouver
 Westin Bayshore Resort & Marina
March 10, 2004
Vancouver,
BC
For more information visit www.woodsolutionsfair.com
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NAHB Green Building Conference
 March 14-17, 2004
Austin, TX
For more information visit www.nahb.org |
48th Annual CSI Show & Convention
McCormick Place
April
21-23, 2004
Chicago, IL
For more information visit www.thecsishow.com
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Wood Solutions Fair - Seattle
Washington State Convention Center
April 22, 2004
Seattle,
WA
For more information visit www.woodsolutionsfair.com
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The AIA Show (American Institute of Architects)
McCormick
Place
Booth 1192
June 10-12, 2004
Chicago, IL
For more
information visit www.aia.org
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