Lag Screw Withdrawal Force Formulae and Calculator

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Lag Screw Withdrawal Force Formulae and Calculator

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Lag Screw Reference Withdrawal Force Formula and Calculator per. AWC-NDS

The lag screw reference withdrawal design value in wood, W, in lbs/in. of thread penetration, for a single lag screw inserted in the side grain of a wood member, with the lag screw axis perpendicular to the wood fibers within the range of specific gravities, G, and lag screw diameters, D. Reference withdrawal design values, W, should be multiplied by all applicable adjustment factors, See AWC-NDS-2018 to obtain details of additional adjusted withdrawal design values, W' · C_D.

Preview: Lag Screw Reference Withdrawal Force Calculator

Eq. 1
W = 1,800 · G^3/2 · D^3/4

Where lag screws within wood are loaded in withdrawal from end grain, reference withdrawal design values, W, shall be multiplied by the end grain factor, C_eg eg = 0.75.

Eq. 2
W = [ 1,800 · G^3/2 · D^3/4 ] · C_eg

lag screws are loaded in withdrawal based on length of full penetration

Eq. 3
W = [ 1,800 · G^3/2 · D^3/4 ] · L

lag screws are loaded in withdrawal based on length of full penetration in end grain

Eq. 4
W = [ 1,800 · G^3/2 · D^3/4 ] · C_eg · L

Where

lag screw reference withdrawal

W = reference and adjusted withdrawal design value for fastener, lbs per inch of penetration
G = specific gravity of wood species, lbs/in²
D = nominal diameter, in
C_eg = end grain factor = 0.75

Source:

National Design Specification for Wood Construction (AWC-NDS)
American Wood Council