Wood Screw Withdrawal Design Value (NDS 12.2.2)
Why a screw that holds in fir strips in spruce: 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.
Formula and source
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.
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