ASD/LRFD Manual - American Wood Council

14 M4: SAWN LUMBER
M4.4 Special Design Considerations
General
With proper detailing and protection, structural lumber
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 on dimensions
as well.
Dimensional Changes
Table M4.4-1 is extracted from more precise scientific
and research reports on these topics. The coefficients are
conservative (yielding more shrinkage and expansion
than one might expect for most species). This level of
information should be adequate for common structural applications.
Equations are provided in this section for those
designers who require more precise calculations.
Design of wood members and assemblies for fire
resistance is discussed in Chapter M16.
Table M4.4-1
Approximate Moisture and Thermal Dimensional Changes
Description
Dimensional change due to moisture content change 1
Dimensional change due to temperature change 2
Radial or Tangential Direction
1% change in dimension per 4% change in MC
20 × 10 -6 in./in. per degree F
1. Corresponding longitudinal direction shrinkage/expansion is about 1% to 5% of that in radial and tangential directions.
2. Corresponding longitudinal direction coefficient is about 1/10 as large as radial and tangential.
Equations for Computing Moisture
and Thermal Shrinkage/Expansion
Due to Moisture Changes
For more precise computation of dimensional changes
due to changes in moisture, the change in radial (R), tangential
(T), and volumetric (V) dimensions due to changes
in moisture content can be calculated as:
X X ∆ MC e
(M4.4-1)
= ( )
o
ME
where:
X 0 = initial dimension or volume
Due to Temperature Changes
For more precise calculation of dimensional changes
due to changes in temperature, the shrinkage/expansion
of solid wood including lumber and timbers can be calculated
as:
X X ∆ T e
(M4.4-3)
= ( )
o
TE
where:
X 0 = reference dimension at T 0
X = computed dimension at T
T 0 = reference temperature (°F)
X = new dimension or volume
∆MC = moisture content change (%)
e ME = coefficient of moisture expansion:
linear (in./in./%MC) or
volumetric (in. 3 /in. 3 /%MC)
and:
T = temperature at which the new
dimension is calculated (°F)
e TE = coefficient of thermal expansion
(in./in./°F)
and:
∆MC = M − M o
(M4.4-2)
where:
M o = initial moisture content % (M o ≤ FSP)
M = new moisture content % (M ≤ FSP)
FSP = fiber saturation point
∆T = T − T o
(M4.4-4)
where:
−60°F ≤ T o ≤ 130°F
The coefficient of thermal expansion of ovendry wood
parallel to grain ranges from about 1.7 × 10 -6 to 2.5 × 10 -6
per °F.
Values for e ME and FSP are shown in Table M4.4-2.
American Wood Council