Wood Screws Allowable Withdrawal Load

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

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):

1. Plug values into the formula: $F = 2850 \times (0.45)^2 \times 0.190$
2. Calculate the specific gravity term: $(0.45)^2 = 0.2025$
3. 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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References

• Original Source