Nailing Together a Built Up Lumber Column
It is common in wood frame and post and beam construction to use multiple pieces of Dimension Lumber (`2 by’) material, nailed laminated together, to make up structural posts and columns. Properly fastened together, typically by nails, the resulting piece acts nearly the same as a solid wood section of similar dimensions. This article deals with the `properly nailed’ part. The National Design Specification for Wood Construction (NDS), Section 15.3.3,provides the prescription for nailing the pieces together. This prescription (set of rules) is summarized below.
1. Adjacent nails are to be driven from opposite sides.
2. All nails must penetrate all the pieces (plies, or laminations) and at least three-fourths into the outermost lamination.
3. The top and bottom (end) nails must be between 15 and 18 nail diameters from the top and bottom.
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.
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 us come up with the nailing pattern for a 3-ply 2 x 6 `column’ that is 8 ft tall.
I will `attack’ the above 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 7)? A piece of 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 (face-to-face). All three pieces have a thickness of 1-1/2 in.; so the minimum thickness is 1-1/2 or 1.5 in.
Item 7 may be cast in terms of a formula as follows:
(Is) d > 3 tmin (?),
where d is, of course, the wide dimension of the pieces.
In our case, then, is 5-1/2 in. > 3 x 1-1/2 in. = 4-1/2 in.?
Yes; we need more than one row of nails; try two rows of nails.
Second, let us come up with the required nail length (Item 2). 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 into the last lam, or,
L ≥ 2 x 1.5 in. + ¾ x 1.5 in. = 4.125 = 4-1/8 in., where, of course, L is the nail length.
So, we need nails that are at least 4-1/8 in. long. The nails could be 4-1/2 in. long, and just begin to come out the opposite face. They can be even longer, but then must clinched (protruding tips pounded over).
This leads to selecting an actual nail.
Table L4 of the NDS provides length and diameter information for nails used in construction. From the Table we find that the `30d’ common wire nail has a length of 4-1/2 in. and diameter (D) of 0.207 in. (Perfect!)
Once nail diameter information is known we can deal with the other items.
With regard to end distance (Item 3), the first and last nails must be between 15 and 18 diameters from the end (top and bottom of the column).
Doing the math, 15 diameters is 15 x 0.207 in. = 3.1 in. and 18 diameters is 3.7 in.
A reasonable end distance specification, then, would be 3-1/2 in.
With regard to the spacing of nails in a row (Item 4), 20 diameters is 4.1 in. and 6 times the smallest lamination thickness is 9 in.
A reasonable spacing of nails in a row is 9 in.
With regard to the spacing of the rows (Item 5), 10 diameters is 2.1 in. and 20 diameters is 4.1 in.
Let us try a spacing of 3 in., and see if that also accommodates the edge distance requirement (Item 6). The nails cannot be closer to the edge than 5 diameters, or 1 in., or farther from the edge than 20 diameters or 4.1 in. If we use 2 rows spaced 3 in. apart in the 5-1/2 in. wide face, we are left with 1-1/4 in. distance. (Perfect!)
Summarizing: 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.
References
National Design Specification for Wood Construction, American Wood Council, Washington, D.C.
