Dimension versus Dimensional – Wood Engineering

I design structures. I am an engineer. In this day and age we design buildings so that they don’t fail; don’t fall down, and, in a sense, just barely don’t fall down. Seldom do we `overbuild’. And as such, it’s important to pay attention to detail. Take, for example, nails. There are the nails commonly found on the shelves in the building-supply store, or contractor nail belt, or nail gun, and then there are a type of nails called common nails with specific dimensions. A 10d common nail, technically, in the building code, is a wire nail that is 0.148 inches in diameter, 3.0 inches long, has a nail head diameter of at least 0.312 inches, and a minimum steel strength. But there are quite a number of 10d nails that are also common … 10d sinker, 10d box, to name just two, and short, and gun, finish, brad and … nails. And there are different finishes on nails. 10D sinker nails, commonly used when `hammered’ into place, have a significantly smaller diameter than the 10d common. If an engineered design requires a 10d common, using a 10d sinker in its place may indeed be insufficient. Part of this is remedied by the engineering and building code community specifying actual nail dimensions, instead of just nail names.

Next comes `dimension’ versus `dimensional’. As an engineer I deal with `dimension’ lumber. Dimension lumber is the term used in the model building codes, and the National Design Specification® for Wood Construction (NDS). You won’t find the word `dimensional’ in the IBC (International Building Code) or IRC (International Residential Code) … at least not in reference to lumber. Conversely, you might not be able to find `dimension’ lumber in the building supply store, even though they may have plenty of dimensional lumber. As an engineer, I used to let it bother me when someone used the term dimensional to describe dimension, but I’ve eased up a little. Even some engineers, and an educator or two, use the wrong term.

And then there are stud versus studs. To a builder, a `stud’ is a vertical framing member in a wall. One might say `any’ vertical framing member. Common sizes of studs are 2 x 4 and 2 x 6. But there is also `stud’ grade of lumber. We could even say stud grade of stud. As an engineer designing a wall to meet certain strength requirements, I often specify a grade of `No. 2’ for the studs. It’s often available, and has reasonable strength and stiffness. On the same aisle with the No. 2 grade lumber (studs), there might very well also be `stud grade’ studs. These stud grade studs have significantly less strength and stiffness. (Yikes.) So, if a set of plans says `No. 2’, use No. 2.

Then there’s `select’. When an engineer needs all the strength available in a size and species of lumber, he/she might specify `select structural’. Among other grades of the same species and size, select structural will be the strongest. It will also likely be at the top price-wise, and may or may not be available. But, be careful, `select’ may mean other things in the building-supply store. Select may refer to appearance, not strength. Or it may be part of a brand name.

Dimension, common, select … those stand out as the most confuse-worthy … the `biggies’. There are others. I used to get all wound up about the mix-ups, but now just smile when the mix-up is someone else’s, and just try to be as clear and deliberate as possible with anything I am designing.

Fortunately, in the IRC, stud grade lumber is allowed for vertical framing members, a.k.a. `studs’ (Section R602.2) (unless specified otherwise).