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Re: 2001 NDS nail penetration

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Title: Re: 2001 NDS nail penetration

Nels/Bill,

NDS(r) yield equations are not empirical. Take a look at Technical Report 12 on the AWC website: http://www.awc.org/Publications/TR/index.html

The equations provide a mechanics-based approach for connection design. Modes Is and Im are simply bearing equations. Modes II-IV are interaction equations of dowel bearing and dowel bending. Review Part III of TR12 and you'll see the mechanics-basis used to derive the equations.

Wrt the penetration reduction, here's an explanation for the change when we switched to the 2001 NDS:

The 1997 National Design Specification(r) (NDS(r)) for Wood Construction only required the designer to check 3 yield mode equations for screwed and lag screwed connections or 4 yield mode equations for nailed connections in single shear.  The penetration depth factor was assumed to account for the other modes. The 2001 NDS eliminated the penetration depth factor, Cd, for nails, wood screws, and lag screws. The removal of this factor was coupled with the requirement to check all 6 yield limit equations per section 11.3.1. This change allows the effect of reduced penetration on strength to be calculated in a consistent manner with the yield mode equations. Nails in double shear are now calculated using the double shear equations in the 2001 NDS.

 
The 2001 NDS still has provisions for the minimum penetration permitted. For lag screws, this penetration limit is 4D excluding the tip. For nails and screws, this penetration limit is 6D including the tip, except in cases where 12d or smaller nails are used in double shear. When this exception occurs, the side member must be at least 3/8" thick and the nails must extend at least 3D beyond the side member and be clinched.

 
The 2001 NDS tabulated lag screw, wood screw, and nail values were calculated using penetrations of 8D, 10D, and 10D respectively.  For users that rely on tabulated values for design rather than the calculation method, values for connections with reduced penetration can be conservatively calculated using the table footnotes. Note that main member thickness is assumed to be sufficient to provide full penetration of the fastener, except where noted in the table footnotes.

 
Here's a link to a white paper on the AWC website that further describes changes to the 2001 NDS:
 
http://www.awc.org/Publications/papers/NDS2001article.pdf

HTH

Buddy

John "Buddy" Showalter, P.E.
Director, Technical Media
AF&PA/American Wood Council
1111 19th Street, NW, Suite 800
Washington, DC 20036
P: 202-463-2769
F: 202-463-2791
http://www.awc.org

The American Wood Council (AWC) is the wood products division of the American Forest & Paper Association (AF&PA). AWC develops internationally recognized standards for wood design and construction. Its efforts with building codes and standards, engineering and research, and technology transfer ensure proper application for engineered and traditional wood products.

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>The guidance provided herein is not a formal interpretation of any AF&PA standard.  Interpretations of AF&PA standards are only available through a formal process outlined in AF&PA's standards development procedures.

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From: "Nels Roselund, SE" <njineer(--nospam--at)att.net>
To: <seaint(--nospam--at)seaint.org>
Subject: Re: 2001 NDS nail penetration
Bill,
I set out to answer your question about whether the Tabulated values in the
NDS are reduced. I started out to make the calculations my self based on
the Yield Limit Equations of the NDS. First I found that I can't find my
'97 NDS -- I'm using the 2001 edition now. Then I found that the values I
calculated do not match the NDS. That may mean that the values are reduced,
but it may also mean that I've made an error in my work. First I need to
make sure that I've made the calcualtions correctly, so I have to trouble
shoot my mathcad calc. I've run out of time to fiddle with this right now.
I'm back to my old complaint that the NDS equations are not intuitive --
there is no way that I can see to be able to look for errors in the formulas
I've entered by asking, "does this make sense?" The Yield Mode Equations
aren't based on common sense as far as I can tell -- I guess they are
empirical.
Buddy Showalter -- do you have the answer?
Nels
Nels Roselund
Structural Engineer
South San Gabriel, CA
njineer(--nospam--at)att.net