ASD/LRFD Manual - American Wood Council

ASD/LRFD MANUAL FOR ENGINEERED Wood Construction
57
M8.4 Special Design Considerations
General
With proper detailing and protection, SCL can perform
well in a variety of environments. One key to proper detailing
is planning for the natural shrinkage and swelling
of wood members as they are subjected to various drying
and wetting cycles. While moisture changes have the largest
impact on lumber dimensions, some designs must also
check the effects of temperature. While SCL is typically
produced using dry veneer, some moisture accumulation
may occur during storage. If the product varies significantly
from specified dimensions, the user is cautioned
from using such product as it will “shrink” as it dries.
In addition to designing to accommodate dimensional
changes and detailing for durability, another significant
issue in the planning of wood structures is that of fire
performance, which is covered in Chapter M16.
Dimensional Changes
Durability
Designing for durability is a key part of the architectural
and engineering design of the building. Wood
exposed to high levels of moisture can decay over time.
While there are exceptions – such as naturally durable species,
preservative-treated wood, and those locations that
can completely air-dry between moisture cycles – prudent
design calls for a continuing awareness of the possibility of
moisture accumulation. Awareness of the potential for decay
is the key – many design conditions can be detailed to
minimize the accumulation of moisture; for other problem
conditions, preservative-treated wood or naturally durable
species should be specified.
This section cannot cover the topic of designing for
durability in detail. There are many excellent texts that devote
entire chapters to the topic, and designers are advised
to use this information to assist in designing “difficult”
design areas, such as:
The dimensional stability and response to temperature
effects of engineered lumber is similar to that of solid sawn
lumber of the same species.
Some densification of the wood fiber can occur in
various manufacturing processes. SCL that is densified
will result in a product that has more wood fiber in a given
volume and can therefore hold more water than a solid
sawn equivalent. When soaked these products expand and
dimensional changes can occur.
Adhesive applied during certain processes tends to
form a barrier to moisture penetration. Therefore, the
material will typically take longer to reach equilibrium
than its solid sawn counterpart.
For given temperatures and applications, different
levels of relative humidity are present. This will cause the
material to move toward an equilibrium moisture content
(EMC). Eventually all wood products will reach their EMC
for a given environment. SCL will typically equilibrate at
a lower EMC (typically 3% to 4% lower) than solid sawn
lumber and will take longer to reach an ambient EMC.
Normal swings in humidity during the service life of
the structure should not produce noticeable dimensional
changes in SCL members.
More information on designing for moisture and temperature
change is included in M4.4.
• structures in high moisture or humid conditions
• where wood comes in contact with concrete
or masonry
• where wood members are supported in steel
hangers or connectors in which condensation
could collect
• anywhere that wood is directly or indirectly
exposed to the elements
• where wood, if it should ever become wet,
could not naturally dry out.
This list is not intended to be all-inclusive – it is
merely an attempt to alert designers to special conditions
that may cause problems when durability is not considered
in the design.
More information on detailing for durability is included
in M4.4.
8
M8: STRUCTURAL COMPOSITE LUMBER
American Forest & paper association