What is the variation of hardness in a 2x4?

Question

I am doing a project that involves firing nails into a 2x4 and measuring the amount of energy required for certain depths. However, one variable that I would like to understand/eliminate is the variation in hardness between one point on the 2x4 and another. Any idea what the normal variance is for a white spruce 2x4 or where to find/how to measure this?

Answer 1

I don't have the data in front of me, but I'd guess based on experience that the variance is about 10 to 1 between the wood grain and the soft spots between the rings. There may be data on wood properties, including compressional strength on and off the grain lines, in mechanical engineering or architectural textbooks.

The way the 2x4 is cut can influence the variance, too. If the 4" side of the 2x4 lines up with the rings, the variance on that side will be less than if the 4" side is more closely aligned with the radii of the tree.

Woods with a lot of sap will also show more variance. To measure the variation, you'll need a hardness measuring instrument. Do a search on Brinell and Rockwell; both of those are measures of hardness. Basically, you jam a pointy object  into the wood and measure how many pounds force it takes to penetrate a certain distance.