Wood Screws Allowable Withdrawal Load

Reference data and engineering information about wood screws allowable withdrawal load for material properties applications.

Overview

Engineering reference data for Wood Screws Allowable Withdrawal Load in material science and properties.

Key Formulas

Stress

$\sigma = \frac{F}{A}$

Force per unit area.

Strain

$\varepsilon = \frac{\Delta L}{L_0}$

Change in length per original length.

Hooke's Law

$\sigma = E \varepsilon$

Stress proportional to strain in elastic region.

Thermal Expansion

$\Delta L = \alpha L_0 \Delta T$

Length change due to temperature.

Variables

Symbol	Description	Unit
$\sigma$	Stress	Pa
$\varepsilon$	Strain	—
$E$	Young's modulus	Pa
$\alpha$	Thermal expansion coefficient	1/°C
$\Delta T$	Temperature change	°C

Formula Derivation

The allowable withdrawal load formula for a wood screw in side grain is: $F = 2850 \times SG^2 \times D$ where:

$F$ is the allowable withdrawal load force (lb per inch of penetration)
$SG$ is the specific gravity of the oven-dry wood
$D$ is the diameter of the screw (in)

Key Insight: The allowable load ( $F$ ) incorporates safety and design factors, typically ranging from 10% to 50% of the material's failure load. This means the actual failure load for the screw-wood connection is significantly higher.

Worked Example Calculation

Using the formula, we can estimate the withdrawal force for a No. 10 screw (D = 0.190 in) in Canadian Spruce (SG = 0.45):

Plug values into the formula: $F = 2850 \times (0.45)^2 \times 0.190$
Calculate the specific gravity term: $(0.45)^2 = 0.2025$
Complete the calculation: $F = 2850 \times 0.2025 \times 0.190 \approx 109.6 \, \text{lb/in}$

Therefore, the allowable withdrawal load is approximately 110 lb per inch of screw penetration.

Design Considerations

Specific Gravity (SG): This measure of wood density is critical. The quadratic relationship ( $SG^2$ ) means withdrawal strength increases rapidly with denser wood.
Screw Diameter (D): Strength increases linearly with screw diameter.
Penetration Depth: The formula gives force per inch. Total allowable withdrawal force equals $F$ multiplied by the screw's side-grain penetration depth.
Allowable vs. Failure Load: Always apply the appropriate safety factor. The 2850 constant already incorporates standard factors for seasoned dry wood in side grain.

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