Q: We're concerned about the loss of strength for staples in wood where the wood is exposed to some wetting and drying. We're also concerned about splitting of the wood when we nail near an end of a piece. What ideas do you have? Thanks in advance.

A: In answering your question, I'm going to discuss the withdrawal strength of the nail, not the lateral or shear strength. Also, note that a staple is considered as two nails, so this discussion also applies to staples.

If you drive a nail into fairly wet wood, the withdrawal strength is as high as for a nail driven into dry wood. But, if the wet wood dries out, the withdrawal resistance may drop substantially as low as 25 percent of the initial values. On the other hand, if the wood fibers deteriorate or the nail corrodes and becomes somewhat roughened, withdrawal resistance may stay fairly constant or may even increase. It's hard to predict and so any potential increases shouldn't be counted on.

Before you give up entirely on nails (but you may have to use screws in some cases), nails or staples in dry wood that doesn't change moisture much, except for seasonal changes (more humid in the summertime and drier in the wintertime) may also lose withdrawal strength as the wood fibers relax over time. To minimize this effect, staples and nails are designed with non-smooth surfaces, using barbs, groves and rings. Adhesives can also be added to the nail or staples.

Splitting varies with species

Some species of wood are more prone to splitting than others. Usually the denser the wood, the more likely it is to split, although swirly grain offsets this tendency.

Regarding the splitting problem, dry wood (under 9 percent MC for softwoods and under 6 percent MC for hardwoods) is much more brittle and prone to splitting. So always check the MC when you have splitting to make sure that the MC is not the main problem.

A second critical item that affects splitting is the tip of the nail or staples. Nails with long, sharp points accentuate splitting in certain species, which may reduce withdrawal resistance. A blunt or flat point without any taper at the end greatly reduces splitting, but its tearing and destruction of the wood fibers when driven reduces withdrawal resistance. A nail tapered at the end but then terminating in a blunt point (for just a few nails, you can blunt the point yourself) will cause less splitting. The withdrawal strength in dense woods remains high, but in lower density woods, such blunting does lower the strength.

A good discussion of more details about nailing can be found at www.woodweb.com/Resources/wood_eng_handbook/Ch07.pdf.


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