WOOD - Materials for Civil and Construction Engineers - page 412

in specific heat beyond the simple sum is due to the wood-water bonds ab-
sorbing energy. An increase in temperature increases the energy absorption
of wood and results in an increase in the specific heat.

Thermal Diffusivity Thermal diffusivity is a measure of the rate at which a ma-
terial absorbs heat from its surroundings. The thermal diffusivity for wood is
much smaller than that of other common building materials. Generally, wood
has a thermal diffusivity value averaging 0.006 mm/sec. (0.00025 in./sec.), com-
pared with steel, which has a thermal diffusivity of 0.5 mm/sec. (0.02 in./sec.).
It is because of the low thermal diffusivity that wood does not feel hot or cold
to the touch, compared with other materials. The small thermal conductivity,
moderate density, and moderate specific heat contribute to the low value of
thermal diffusivity in wood.

Coefficient of Thermal Expansion The coefficient of thermal expansion is a
measure of dimensional changes caused by a temperature variance. Thermal
expansion coefficients for completely dry wood are positive in all directions.
For both hard and soft woods, the longitudinal (parallel to the grain) coeffi-
cient values range from 0.009 to 0.0014 mm/m/°C (0.0000017 to 0.0000025
in./in./°F). The expansion coefficients are proportional to density and there-
fore are five to ten times greater across the grain than those parallel to it.
When moist wood is heated, it expands due to thermal expansion and
then shrinks because of the loss of moisture (below the fiber saturation point).
This combined swelling and shrinking often results in a net shrinkage. Most
woods, at normal moisture levels, react in this way.

■ 10.7.3 Electrical Properties

Air-dry wood is a good electrical insulator. As the moisture content of the wood
increases, the resistivity decreases by a factor of three for each 1% change in
moisture content. However, when wood reaches the fiber saturation point, it
takes on the resistivity of water alone.

10.8 Mechanical Properties

Knowing the mechanical properties of wood is a prerequisite to a proper de-
sign of a wood structure. Typical mechanical properties of interest to civil and
construction engineers include modulus of elasticity, strength properties,
creep, and damping capacity.

■

10.8.1 Modulus of Elasticity

The typical stress-strain relation of wood is linear up to a certain limit, fol-
lowed by a small nonlinear curve after which failure occurs, as shown on