Saturday, February 23, 2013

nailing a built-up wood column


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;

 
3.  the top and bottom (end) nails must be (start/end) 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; 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,

 
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, 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,

 
where
 

n is the number of plies and t is the thickness of the plies, in this case assumed to all be the same.

 
(I provide formulas since some designers like to solve all this stuff using formulas, or spreadsheets, or the like.)

 
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,

 
 ... 15D ≤ end distance ≤ 18D ...
 

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.

 
Now for spacing of nails in a row:

 
 ... 20D = 4.1 in. ≤ spacing in a row ≤ 6 t min = 6 x 1.5 in. = 9 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.
 

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.

 

6 comments:

Matt said...
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Matt said...
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Igor said...

This is great. Thank you for the help.