High-capacity Timber Connectors Introduced to North American
Market
David Mosses
Equilibrium Consulting Inc.
Ready for an alternative to bolts, screws, nails and rivets? There
are two new additions to the family of connectors available to designers and
builders in North America. SFS and BVD are proven systems in
Europe but have only recently been made available to our market. Each of these
patented connectors has its benefits in certain applications.
SFS
manufactures two types of connectors for heavy timber construction ?a
self-tapping tight-fit dowel for steel-wood-steel connections and a long,
threaded screw for wood-wood applications.
The self-tapping dowel, or
WS-T connector, has a built-in drill bit at the tip of the dowel and can
drill through up to three 1/8" steel plates in addition to the timber member. No
need for pre-drilling results in time savings during
fabrication.
The dowel portion of the connector is smooth and is the same
diameter as the drill bit, resulting in a tight-fit connection. This means no
slipping of the connectors compared to standard bolts where the holes must be
oversized. The WS-T is approximately 1/4" diameter and available in a
variety of lengths up to 9" long. It is typically used in applications where
nails, bolts, or rivets would normally be considered. Large groups of these
small diameter dowels can be used to develop high-strength connections for
hangers, trusses and other applications.
The screw connector from
SFS is called the WT-T. It is used for wood-to-wood connections
and can be applied to a number of connection configurations. The screw is
approximately 1/4" diameter and is available in lengths up 12".
It is used where screws, nails, or lag screws would normally be
used, however, the WT-T does not require pre-drilling. The WT-T is
threaded over its full length except over about 1" in the middle of the screw
?this helps to pull the two members together as the screw is tightened. The
result is a strong, clean connection that is easy to install and useful in many
applications.
BVD Systems has developed a connector that is
inserted into the end of a timber member. The BVD connector ?a steel
fitting that comes in six different sizes ?slides into a 2 1/2" diameter hole
that is pre-drilled into the timber up to approximately 24" long.
Once inserted, the steel fitting is criss-crossed by smooth dowels to engage the
timber and to form a mechanical lock between the steel components. Finally, a
non-shrink grout is poured into the connection to form a solid bond. A single
BVD connection can be designed for an allowable to tensile strength as
high as 300 kN (65 kips). This high-capacity means that the full tensile
capacity of the timber can be reached unlike other conventional systems. In
addition, true moment connections can be developed.
These connectors, now available in North America, provide more
options for designers and are inconspicuous once installed. With new connection
options, designers can now re-visit traditional applications as well as develop
new concepts for timber structures.
For more information, please contact SFS intec Inc. at (905)
847-5400 or by email, BVD Systems at (541) 683-5878 or by email, or Equilibrium
Consulting at (604) 730-1422.
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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. This
dependence on the duration of applied load is well documented from studies of
load duration effects for Canadian dimension lumber and on conceptual models.
The duration of load factor is first among the factors applied in the
design process. The base properties (in the US "base design values", and in
Canada "tabulated specified strengths") for lumber and glulam are for standard
duration of load such as live loads on the floors of most assembly, residential,
institutional, and commercial occupancies. The use of the load duration factor
accounts for the influence of the expected duration of the specified loads, and
maintains a margin of safety for all duration of loading conditions.
For
loads applied for a longer or shorter duration than standard, the load duration
factor is used to increase the design values. For permanent loads, the factor is
used to decrease the design values.
In the US, the Load Duration Factor,
CD, is specified in the National Design Specification for Wood
Construction published by the American Forest & Paper Association and
American Wood Council. Typical load duration factors are as follows:
1. Permanent - Dead Loads
2. Normal (Ten Years) -
Occupancy Live Load
3. Shorter Term
(Ten Minutes) -
Wind/Earthquake Load
(Two Months) - Snow Load
(Seven Days) -
Construction Load
(Impact Load) - Impact Load
In Canada, the
Duration of Load Factor, KD, is specified in CSA Standard O86
published by the Canadian Standards Association. There are three classes of load
duration as follows:
1. Permanent - Continuously Applied
Loads
2. Standard Term - Snow and Occupancy Live
Loads
3. Short Term - Wind and Earthquake, Construction, and
Impact Loads
The characteristic behavior of wood structural members to
carry a greater maximum load for short durations than for long durations makes
wood the preferred building material in high wind and seismic areas. To ensure
overall building integrity, it is important that all design specifications
required in building codes and design manuals are complied with.
For
further information on the load duation factor in the US, please visit NDS.
In
Canada, the Canadian Wood Council뭩 comprehensive Wood Design Manual, which
includes a copy of CSA O86-01 can be obtained at CWC
publications.
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How do I create a new custom beam database and one based on an
existing custom database?
To create a new custom beam database, follow the steps
below.
1. Start WoodWorks Database Editor by clicking on the
shortcut in your Design Office folder.
2. Change Database Type to
Custom and Member Type to Beam.
3. Highlight a Material and click
the Add button.
4. Enter unique Material and File names. Check
Material Type (Use I-joist, if member selection is performed according to moment
and shear capacities.) and click Add First Species.
5. Enter a
Species Name, Weight and click Add First Grade.
6. Enter Grade
Name, fill in all other fields and click Add First Section.
7.
Enter Section size, fill in all other fields and click OK. Nominal Size is used
for the pick list in Sizer.
To create a custom beam database based on
an existing custom database, follow the steps below.
1. Start
WoodWorks Database Editor by clicking on the shortcut in your Design Office
folder.
2. Change Database Type to Custom and Member Type to
Beam.
3. Highlight the Material that is the bases for the new
database, click the Save As button, enter a unique File name and click
OK.
4. Double click on the new Material, change the Material name
to a unique Material name and click OK. Click the Use button.
5.
Edit a Species by double clicking on the Species name. To add a new Species,
highlight a Species name and click the Add button.
6. Edit a Grade
by double clicking on the Grade name. To add a new Grade, highlight a Grade name
and click the Add button.
7. Edit the Section by double clicking
on the Section name. To add a new Section, highlight a Section name and click
the Add button.
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Advertise your Engineering or Architectural Services with the CWC for
Free!
Engineering or Architectural firms that have licensed copies of
the latest version of WoodWorks Design Office Software can advertise their
services on the Canadian Wood Council website for free!
The CWC website
receives close to 2,000,000 hits per month from 40,000 to 50,000 visitors.
Hundreds of others call the CWC helpdesk for technical advice. Many of these
people are contractors or owners looking for design professionals to help them
with their wood construction projects. The CWC can put these people in contact
with you.
We propose to post a geographically sorted list of architects
and engineers on the CWC website. We will design the webpage based on your
feedback. If you are an engineer or architect that designs with WoodWorks
software, and you would like to make your services available through CWC, please
contact sales@woodworks-software.com.
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Wood Solutions Fair - Philadelphia
 Adams Mark Hotel
Booth 5 & 6
May 18,
2004
Philadelphia, PA
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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Wood Solutions Fair - Toronto
 Metro Toronto Convention Centre
September 21, 2004
Toronto,
ON
For more information visit www.woodsolutionsfair.com
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Did you know? Did you know that forestry can help
reduce the threat of wild-fires? Actively managing overdense forests and those
prone to disease or fire by removing dead wood and thinning the undergrowth, or
intentionally burning areas that are distant from homes, can help to maintain
forests that are more open and more resistant to natural catastophe. Source: Dr.
Partick Moore, Sensible Environmentalist/Wood Promotion Network. For more
details please visit WPN.
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Finishes for Wood Decks
R. Sam Williams - USDA Forest
Service
 In recent years, wood decks have become an important part of
residential construction. Wood decks can add versatile living space to a home
and provide decades of use. However, wood decks are exposed to high levels of
stress from the severe weather conditions that shrink and swell the wood, thus
increasing the risk of decay and insect attack. Because of these risks, lumber
used in decks is usually pressure treated with a preservative, or the lumber
used is a naturally durable wood such as western redcedar. Applying an
additional finish to wood decks-and with proper maintenance-will minimize the
problems of cracking, raised grain, and mildew growth.
A penetrating
finish applied to wood decks provides better overall performance and is easier
to reapply than a film-forming finish (e.g., paint, solid-color stain). In
addition to the continuous shrinking and swelling of the wood caused by changes
in the moisture content, film-forming finishes are subjected to excessive wear,
especially in high-traffic areas. For these reasons, penetrating finishes, not
film-forming finishes, should be used on wood decks.
Penetrating
Finishes
Penetrating finishes are recommended for use on wood
decks. These finishes include water-repellent preservatives, colored
water-repellent preservatives, and semitransparent stains.
Water
Repellents and Water - Repellent Preservatives
Water repellents
are traditionally formulated with organic solvents such as mineral spirits or
turpentine, a sealer such as linseed oil or varnish, and a water repellent such
as paraffin wax. The solvent carries the oil or varnish and wax into the wood.
During the past 10 years, waterborne formulations have become popular. Some of
these formulations carry the sealers into the wood similar to the solventborne
formulation; however, other waterborne formulations may form a thin film. The
only difference between a water repellent (WR) and a water-repellent
preservative (WRP) is the addition of a mildewcide or preservative. WRPs give
much better performance than WRs. Use of WRs without the mildewcide often leads
to blotchy staining of the wood.
WRPs are also formulated with nondrying
oils that act as solvents. These oils penetrate the wood, but do not dry, and
protect the wood from degradation and mildew attack. Because the oils do not
dry, the deck surface may remain oily until the finish absorbs. This usually
takes several days, depending on the application rate and porosity of the wood.
Several commercial wood treaters use a WR combined with a CCA treatment
for 5/4 radial-edged decking. This dual treatment gives the wood additional
resistance to weathering. Although the WR is supposed to thoroughly penetrate
and saturate the wood, it is still advisable to treat the ends cut during
construction with a WRP. For the treated wood currently available, these
treatments should improve the wood characteristics and extend the product뭩
service life, particularly with sustained maintenance using a
WRP.
Colored Water - Repellent Preservatives
Several
new WRP colored finishes are being marketed both in waterborne and solventborne
formulations that are lightly pigmented, but not to the extent of
semi-transparent stains. These finishes penetrate the wood much like a
traditional WRP but tend to form a thin film. They slightly color the wood but
permit most of the wood grain pattern to show. Compared with the uncolored WRPs,
the added pigment increases the service life of the wood about 2
years.
Semi-transparent Stains
Solventborne,
semitransparent stains have pigment concentrations much greater and coarser than
the colored WRPs. The addition of pigment greatly increases the durability of
the finish compared with that of the WRP. The semitransparent stains penetrate
the wood without forming a continuous layer; consequently, this type of stain
will not blister or peel even if excessive moisture enters the wood. The pigment
also protects the wood surface from sunlight, thus increasing service life. The
binder in the solventborne, oil-based semitransparent stain absorbs into the
wood surface similar to the WRP, and there is no film formation.
If the
decking material was given a factory applied WR finish or if recently finished
with a WRP, a semitransparent stain may not absorb properly. In these
situations, the wood should be allowed to weather for 2 to 3 months before
finishing. This is the only situation where it is beneficial to wait this
long before applying a penetrating finish. Note that lumber should not be
left unfinished for 6 to 12 months as recommended by some product literature or
paint and lumber suppliers.
Choosing a Finish
The
service life of a WRP finish is about 1 year on the exposed surfaces of wood
decks; however, a WRP is the easiest finish to reapply. It absorbs readily into
the end grain of lumber and can prevent water absorption into the end grain much
longer than 1 year. Because WRP finishes are not pigmented, problems with uneven
wear and brush marks are eliminated. (Lap marks occur during finishing of
semitransparent stains when the finish being applied to an area laps over an
adjacent area that has already been finished.)
If you are unsure whether
to stain or use a WRP, apply the WRP to the deck first. You can switch to a
semitransparent stain when the deck needs to be refinished. Even if the deck has
been maintained with a WRP for several years, refinishing with a semitransparent
stain will provide satisfactory performance.
The second easiest finish
to apply is a semitransparent, oil-based stain. Film-forming finishes, such as
paints and solid-color stains, are prone to trap moisture and fail by peeling.
Railings around decks can be finished in the same way as the deck.
However, the railing does not require the abrasion resistance of the finish used
on the deck; it can be designed to avoid trapping moisture, therefore giving
more flexibility in choosing the finish. Paints and solid-color stains may
perform quite well on the railings.
Care of Wood
Decks
The bright color of the wood on weathered decks can be
restored by application of commercial products (called deck cleaners,
brighteners, or restorers). These products do not add color to the deck, but
remove mildew and dirt, allowing the natural color of the wood to show. If all
the natural color has been leached from the surface, the wood will appear silver
gray following cleaning. These commercial cleaning products may remove the
weathered wood surface; therefore, care should be exercised to avoid damaging
the surface of the wood. Aggressive scrubbing with a caustic cleaner can remove
wood from the surface, particularly on softer wood such as western redcedar.
Mildew can also be removed using a liquid household bleach. Dilute the bleach
with 3 parts water and add some detergent, which does not contain ammonia to
avoid the formation of toxic gases. The bleach solution should be rinsed from
the deck with water. If the deck is to be finished after cleaning, allow 1 to 2
days drying time.
For further information on specific products, please
contact Cabot, Minwax, Sansin, Thompsons, and Wolman, or visit your local
building supply store.
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New CWC Publication!
"Energy and the Environment in
Residential Construction" is the latest Canadian Wood Council's publication,
and the first of a new series entitled Sustainable Building Series.
The
publication is a first of its kind. It provides a comparative environmental
impact life cycle assessment of embodied energy and operating
energy of steel, concrete, and wood residential construction. The impact
assessment includes primary energy, global warming potential, air pollution,
water pollution, resource use and solid waste.
Combining the effects of
both the embodied energy of the structure and the operating energy of the
building, will help the design community and governments gain knowledge in
better understanding the impact of their choice of building
materials.
Copies of this bulletin can be obtained by calling
1-800-463-5091 or requested by email. A PDF version will be made available on the CWC
website, and a French version is also under
way.
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