Fastener Pullout Strength Calculator

Withdrawal resistance is the axial force needed to pull a fastener out of the member its threads are embedded in, and in wood it is governed almost entirely by density. The National Design Specification for Wood Construction gives the reference withdrawal value for wood screws as 2850 times specific gravity squared times the shank diameter, in pounds per inch of thread penetration, while lag screws use 1800 times specific gravity to the power 1.5 times diameter to the power 0.75 and nails use 1380 times specific gravity to the power 2.5 times diameter. Specific gravity enters squared, which is why species choice dominates the result: red oak at 0.67 develops roughly 2.5 times the withdrawal of spruce-pine-fir at 0.42 for the same screw. Diameter and penetration enter linearly, so doubling thread engagement doubles capacity. Common structural values are 0.55 for southern pine, 0.50 for Douglas fir-larch, and 0.42 for SPF, and the tabulated NDS figures should be used rather than measured densities. Several limits apply. Only thread embedded in the main member counts, and the NDS calls for minimum penetration on the order of six diameters for wood screws. Withdrawal from end grain is substantially weaker than from side grain, and the code does not permit relying on nail withdrawal from end grain at all. The formula already yields a reference design value reduced from ultimate test capacity, so applying an additional safety factor of 3 to 5, as this calculator does, is a conservative practice rather than a code requirement and can double-count the built-in reduction. Adjustment factors for load duration, wet service, and temperature still apply. Connections resisting wind or seismic uplift are generally better made with rated hardware loaded in shear than with fasteners loaded in withdrawal.

Systematic testing of fastener withdrawal in wood began at the US Forest Products Laboratory in Madison, Wisconsin, which opened in 1910 as the research arm of the Forest Service. Its early programs measured nail and screw holding power across dozens of species, and the results published in successive editions of the Wood Handbook established the central empirical finding that withdrawal resistance scales with a power of specific gravity, the square in the case of screws. Before that work, holding power was a matter of carpentry tradition and manufacturer claim rather than measurement. Those relationships were turned into design rules in 1944, when the National Lumber Manufacturers Association published the first National Design Specification for Stress-Grade Lumber and Its Fastenings, the ancestor of the NDS maintained today by the American Wood Council. The specification has been revised on a multi-year cycle ever since, and its connection provisions were substantially reorganized in the 1991 edition around a yield-based model for laterally loaded fasteners, although the empirical withdrawal equations were retained largely unchanged. Standard test methods developed in parallel, with ASTM D1761 codifying how withdrawal, lateral, and head pull-through tests are to be run, which is what allows proprietary structural screws to be evaluated and published in ICC-ES reports with their own tested values.