It is common in wood frame and post and beam construction to use multiple pieces of Dimension Lumber (`2 by’) material, nailed laminated to one another, to make up posts and columns. Properly fastened together, typically by nails, but sometimes by bolts, the resulting piece acts (nearly) the same as a solid wood section of similar dimensions. This article deals with the `properly’ part. The National Design Specification for Wood Construction (NDS), Section 15.3.3, gives us the prescription for nailing the pieces together. I summarize the prescription (rules) as follows:
1. adjacent nails are to be driven from opposite sides of the post or column;
2. all nails must penetrate all the pieces (plies, or laminations) and at least three-fourths into the outermost lamination;
4. nails in a row must be spaced not closer than 20 nail diameters apart and not farther than six times the thickness of the thinnest of the plies;
5. rows of nails must be spaced between 10 and 20 diameters apart;
6. nails must be located between 5 and 20 diameters from the edges of the column; and
7. two more rows must be provided where the wide face dimension of the `lams’ exceeds three times the thickness of the thinnest piece.
To illustrate the above let’s consider a 3-ply 2 x 6 `column’; and let’s come up with a nailing pattern for such a column that is 8 feet tall. I will `attack’ these rules in a different order than given (but one that makes better sense to me).
First, do we need more than one row of nails (item 5)? 2 x 6 Dimension Lumber is actually 1-1/2 inch (in.) x 5-1/2 in. The 5-1/2 in. wide faces will be nail laminated to one another. All three pieces have a thickness of 1-1/2 in., also being the minimum of the three. So, in `formula form’:
... (is) d > 3 tmin ? ...
or, in our case, ... (is) 5-1/2 in. > (3)(1-1/2 in.) = 4-1/2 in.? Yes; we need more than one row of nails. Let’s try two rows.
Second, let’s come up with the required nail length. Three plies of 1-1/2 in. each will give a total thickness of 4-1/2 in. The nails must be at least long enough to penetrate three-fourths of the last lam, or,
So, I need nails that are at least 4-1/8 in. long, ideally about 4-1/2 in. long, and they can be even longer, but I must then clinch them (pound over the protruding tips).
In `formula form’,
Lmin = [(n-1) + 0.75] x t,
n is the number of plies and t is the thickness of the plies, in this case assumed to all be the same.
Choosing a nail size ... the `30d’ common wire nail has a length of 4-1/2 in. and diameter (D) of 0.207 in. (NDS Table L4).
Perfect! (with regard to length).
Now to deal with the end distance:
In formula form,
or, in our case, 15 (.207) = 3.1 in. ≤ end distance ≤ 18 (.207) = 3.7 in.
Let’s pick end distance = 3-1/2 in.
Let’s pick spacing of nails in a row = 9 in.
For row spacing:
... 10 D = 10(.207) = 2.1 in. ≤ row spacing ≤ 20 D = 4.1 in.
Let’s try 3 in., and assume only two rows.
This leaves us with edge distances of, assuming things centered, ...
5-1/2 – 3 = 2-1/2 ... divided by 2 = 1-1/4 available each edge.
The edge distance requirement is:
... 5D = 5 (.207) = 1 in. ≤ edge dist. ≤ 20 D = 4.1 in.
So, yeah, let’s space the rows 3 in. apart and provide at least 1 in. edge distance. Two rows works perfectly.
Two rows of 30d common wire nails; 9 in. (max.) o.c. each row, rows spaced 3 in. apart and not closer than 1.0 in from edges; first and last nails of each row 3-1/2 in. from ends.
National Design Specification for Wood Construction, American Wood Council, Washington, D.C.