Wood Screw Withdrawal Design Value (NDS 12.2.2)
Why a screw that holds in fir strips in spruce.
Example
You enter
- Specific gravity G (0.50 DF-L, 0.42 SPF) 0.5
- Screw diameter D (in; #10 ~ 0.190) 0.19
- THREAD penetration into holding member (in) 1
- Load-duration factor CD 1
You get
- Reference withdrawal W 135.4 lb/in
- Withdrawal capacity 135 lb
Details, formula, and sources
NDS W = 2,850 G^2 D lb/in scales with the SQUARE of specific gravity. A #10 screw (D 0.190) in DF-L (G 0.50) gives 135 lb/in; the same screw in softer SPF (G 0.42) drops 30% to 95.5 lb/in. Times the penetration for the withdrawal capacity. Axial withdrawal only, not the lateral connection or the head pull-through. A design aid, not the engineer of record's stamped design.
W = 2,850 G^2 D (lb/in); Z_w = W x p x CD.
The NDS 2018 12.2.2 reference wood-screw withdrawal design value, by name; the 2,850 empirical constant is named.
The NDS is free to view at awc.org (ANSI/AWC NDS); the 12.2.2 wood-screw-withdrawal equation is in the published standard.
Estimate. AHJ and licensed professional govern.
Field names used by the API: g, d_in, p_in, cd, w_lbin, z_w
- Withdrawal value W = 2,850 G^2 D per inch of penetrationNDS 2018 12.2.2
- G^2 sensitivity the square-of-specific-gravity law makes species the dominant variableNDS 2018 12.2.2
- Axial only the withdrawal value, not the lateral connection or the head pull-throughscope of this tile