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All engineered wood
components are manufactured in a relatively "dry" state. The
moisture content of engineered wood products at the time of
manufacture ranges from approximately 4 to 12%. During the
manufacturing process, the wood-based resource must be dried to
these levels to insure that a good glue-bond is developed. A range
of values is given because some adhesive systems used in some
products have different moisture requirements.
It is also important to realize that these are not average
moisture contents as traditionally measured. If a certain adhesive
system requires a maximum 6% moisture content to develop an
adequate glue bond, then every piece must meet that maximum during
fabrication. A traditional average where 50% are above the maximum
and 50% are below just doesn't work. Only those pieces that were
at or below the maximum will ever get to the market place.
Traditional "dry" lumber on the other hand is dried to a much
higher moisture content, typically 19%, although some lumber is
dried to 16%. Because of natural variability, the range of
moisture content of the lumber pieces in a given bundle may vary
widely. A given lumber element may even have moisture gradients
along the length or across the width.
In service, however, such as in a residential structure, after 4
to 8 months of drying, all wood elements will reach an equilibrium
moisture content of from 6-10%, depending on the season and
location of the structure. As the engineered wood products are
very close to this normal equilibrium moisture content as
manufactured, and because they are typically shipped in a
waterproof protective wrapping, they take on little or no
additional moisture and their dimensions vary imperceptibly during
this period. The sawn lumber, however, during this period dries
down through a relatively large range of moisture content. Along
with drying comes an equally significant shrinkage. A 14" deep
sawn lumber element can shrink as much as ¾" in its depth as it
cycles from the as-dried to in-service equilibrium moisture
content. This difference in behavior between solid sawn lumber and
engineered wood can lead to structural failure if the designer is
not careful.
APA EWS I-joists and APA EWS Rim Board products are manufactured
in 9-1/2", 11-7/8", 14", and 16" depths. It is no accident that
these sizes are not compatible with, and are larger than,
traditional lumber net depths for 2x10's, 2x12's, 2x14's, and
2x16's. There are many applications in roofing systems and
especially residential floors, where other elements are used in
conjunction with the I-joists for the express purpose of
transferring load through the floor system without overloading the
floor joists. Some examples of these other elements are blocking
panels over an interior bearing wall and rim or starter joists. In
these cases, the vertical load from the structure above the plane
of the floor is transferred through the floor into the
structure/foundation below by way of direct bearing on the
blocking panels, rim or starter joist.
Because the load is transferred in direct bearing, it is essential
that the blocking panels, rim or starter joist be the same height
as the floor joist. Solid sawn lumber cannot be used in
applications like these because of the very likely potential for
shrinkage. Shrinkage by as little as 1/8 of an inch (3 mm) can be
enough to transfer the vertical loads from the walls above
directly to the floor joists, thus inducing possible bearing or
reaction overload conditions at these locations. The solution to
the problem is to use engineered wood products for these
applications. They are manufactured in the correct depths and have
the same dimensional stability properties.
While the previous discussion concerns vertical loads, the same is
true of lateral loads such as those caused by wind and seismic
events. The small gap between the floor sheathing above and the
sawn lumber rim joist or blocking panel below resulting from
shrinkage of the lumber members can have a small but negative
impact on the performance of the structure during the design
event. Even greater however, will be the impact on the deformation
of the structure caused by the potential slip at this location
under design lateral loads. While not necessarily life
threatening, these greater deformations can result in increased
damage to the non-structural components of the building, such as
drywall, windows and doors, cabinets and interior and exterior
finishes. If the deformations are excessive, they can cause the
structure to be irreparable.
Every application where solid sawn lumber is used in conjunction
with engineered wood must be looked at very carefully with respect
to the different moisture states of materials at the time of
construction. The safest alternative is to not mix engineered wood
with solid sawn lumber in any situation where load sharing might
be an issue.
Ray Clark
Product Support Specialist
Wood Products Support Help Desk
APA - The Engineered Wood Association
Southern Forest Products Association
Structural Insulated Panel Association
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