WOOD - Materials for Civil and Construction Engineers - page 413

MPa ksi

30

4

25

3

20

15

2

10

1

5

0

0

0 0.1 0.2 0.3 0.4

FIGURE 10.9 Typical stress
strain relationship for wood.

Strain, %

Figure 10.9. The modulus of elasticity of wood is the slope of the linear por-
tion of the representative stress-strain curve. The stress-strain relation of
wood varies within and between species and is affected by variation in mois-
ture content and specific gravity. Also, since wood is anisotropic, different
stress-strain relations exist for different directions. The moduli of elasticity
along the longitudinal, radial, and tangential axes are typically different.

■ 10.8.2 Strength Properties

Strength properties of wood vary to a large extent, depending on the orien-
tation of grain relative to the direction of force. For example, the tensile
strength in the longitudinal direction (parallel to grain) is more than 20
times the tensile strength in the radial direction (perpendicular to grain).
Also, tensile strength in the longitudinal direction is larger than the com-
pressive strength in the same direction. Common strength properties for
wood include modulus of rupture in bending, compressive strength parallel
and perpendicular to the grain, and shear strength parallel to the grain.
Some of the less common strength properties are tensile strength parallel to
the grain, torsion, toughness, fatigue strength, and rolling shear strength.

■

10.8.3 Creep

Under sustained loads wood continues to deform or creep. The design values
of material properties contemplate fully stressing the member to the tabulat-
ed design values for a period of 10 years and/or the application of 90% of the
full maximum load continuously throughout the life of the structure. If the
maximum stress levels are exceeded, the structure can deform prematurely.

■ 10.8.4 Damping Capacity

Damping is the phenomenon in which the amplitude of vibration in a ma-
terial decreases with time. Reduction in amplitude is due to internal friction
within the material and resistance of the support system. Moisture content