Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Nail connection factors

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Sasha,

The 1997 National Design Specification® (NDS®) 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, C_d, 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 lag screw, wood screw, and nail tabulated values are 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.

There is a Technical Report on the AWC website called TR12:  General Dowel
Equations for Calculating Lateral Connection Values
http://www.awc.org/Publications/TR/index.html which provides a mechanics
based approach for evaluating dowel-type connections. It's the basis for the
NDS yield mode equations. You could use this in lieu of the 2001 NDS to
calculate the values.

HTH

Buddy Showalter, P.E.
AF&PA/AWC

*************************
From: "Alexander Sasha Itsekson" <sasha(--nospam--at)engstruc.com>
To: <seaint(--nospam--at)seaint.org>
Subject: Nail connection factors
Thanks for everyone who answered to my post.
To answer some of the concerns about the rafter design, I have in fact
already analyzed the "truss" and I am specifying sistering and additional
2x8 to the existing rafter to take care of bending.
More on question number 2. Why would I be allowed to using clinching
instead of 12d embedment of the nail in the main member. It would
practically serve the same thing. It would prevent the nail pull out during
the shear loading and it would in fact force other yielding (failure) modes
that are already accounted for in NDS' 97 tables. There is nothing that I
could see in NDS that says that I CAN NOT use clinching for that. Is that
what we call engineering judgment?
I do agree though that as far as doubling the shear, it does makes sense for
the double shear connection and NOT for the single shear.
Thanks again,
Sasha Itsekson, SE
Oakland, CA
PS. Buddy, I don't h own a copy of 2001 NDS yet. Could you eloborate on
C_sub_D factor changes in it?
---------------------------------------------------------------------------
From: AWC Info <AWCINFO(--nospam--at)afandpa.org>
To: "SEAINT Listserver (E-mail)" <seaint(--nospam--at)seaint.org>
Subject: Nail connection factors
answers are keyed to your questions:
1) Yes. The 7-day load duration factor is for construction loads and applies
to fasteners as well. See NDS Table 2.3.2.
2) The clinching increase only applies to double-shear nailed connections.
There's more background on this in the 1997 NDS Commentary section 12.3.3.
3) No. As stated above, the clinching provision only applies to double-shear
nailed connections, so using 1997 NDS you'd have to take the penetration
reduction if it's less than 12D. However, in the 2001 NDS, the connection
provisions have been revised and the penetration depth factor is no longer
required for calculated values since all six yield modes are used to
determine design values for dowel-type fasteners (i could provide more on
this in a separate thread if anyone is interested).
4) I think someone responded that you would be adding additional moment to
the rafter with this type of connection. There are adjustment factors in
AF&PA's Wood Frame Construction Manual (WFCM) for One- and Two-Family
Dwellings, 2001 Edition Tables 3.26A-H (which are also incorporated in the
2000 IRC Tables R802.5.1 (1-8)) to account for that if you haven't already
done so.
HTH
Buddy Showalter, P.E.
AF&PA/AWC
***********8
From: Alexander Sasha Itsekson=20
To: seaint(--nospam--at)seaint.org=20
Sent: Thursday, December 19, 2002 2:29 PM
Subject: Nail connection factors
Happy Holidays to everybody,
The architect in his wisdom decided to replace ceiling tie joists with =
the collar ties at the midheight of the pitched roof. The pitch is 4:12 =
and the roof rafters and collar ties are 2x8's.
Anybody who tried to design the rafter to tie connection knows that it =
is difficult to put enough fasteners in the joint to resist the tie =
tension. In this case I have around 2500lbs of tension.
I have a few questions:
1. The table 3.5 of the NDS'97 Structural Connections Supplement =
lists Construction loads and doesn't list roof live load as a typical 7 =
day duration load to be used with the C_sub_D=3D1.25. Can I still use a =
25% increase in allowable shear values for my fasteners for the =
connection at roof framing design?
2. I remember from way back when that if one specifies clinched =
nails, then one can use a 100% increase in allowable shear value. I =
looked through NDS and the only reference that I could find is a 100% =
increase when used in a double shear application. Don't we get an =
increase capacity due to the fact that the nail withdrawal is not an =
issue in lateral nail loading or is it just equivalent to a 12D =
embedment.
3. Using 16d nails connecting two 2x members the provided embedment is =
1.5 inch < 12*0.162=3D1.944. Can I use Cd=3D1.0 if I specify that the =
nails are to be clinched.
4. Any other thing that I should consider in designing this joint =
such as predrilling of nail holes, close attention to edge distances =
etc?
Thanks in advance,
Sasha Itsekson, SE

******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
*
*   This email was sent to you via Structural Engineers
*   Association of Southern California (SEAOSC) server. To
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you
*   send to the list is public domain and may be re-posted
*   without your permission. Make sure you visit our web
*   site at: http://www.seaint.org
******* ****** ****** ****** ******* ****** ****** ********