The following table summarizes typical moisture content ranges for various wood species relative to their maximum compressive strength (at 2% moisture).

Wood Species	Condition	Moisture Content (%)	Relative Strength vs. 2% MC
Red Spruce	Air Dried	14%	~70-75%
Red Spruce	Kiln Dried	6%	~95-98%
Longleaf Pine	Green Wood	20%	~50-55%
Douglas Fir	Green Wood	24%	~40-45%

Note: Relative strength values are approximate and interpolated from the general trend described.

Strength Calculation

The compressive strength parallel to grain of wood decreases as its moisture content increases above the reference point of 2% moisture.

General Formula

The adjusted strength range based on a given moisture content is calculated as:

\text{Adjusted Strength} = \text{Base Strength Range} \times \text{Reduction Factor}

Example: Red Spruce at 16% Moisture

From the provided example for Red Spruce at 16% moisture content (reduction factor = 0.5):

Base Strength Range:

Lower Bound: $18 \text{ MPa} \ (2610 \text{ psi})$
Upper Bound: $39 \text{ MPa} \ (5655 \text{ psi})$

Reduced Strength Calculation:

\text{Lower Bound}_{\text{adjusted}} = 18 \text{ MPa} \times 0.5 = 9 \text{ MPa} \ (1305 \text{ psi})

\text{Upper Bound}_{\text{adjusted}} = 39 \text{ MPa} \times 0.5 = 19.5 \text{ MPa} \ (2828 \text{ psi})

Therefore, the compressive strength range for Red Spruce at 16% moisture is 9 - 19.5 MPa (1305 - 2828 psi).

Definitions

Moisture Content (MC): The weight of water in wood expressed as a percentage of the oven-dry weight of the wood.
Reduction Factor: A multiplier (between 0 and 1) used to adjust the reference compressive strength for wood at a specific moisture content above 2%. The factor is determined from strength vs. moisture content charts or equations for a given wood species.

References

Original Source

Source: engineeringtoolbox.com