Need a book? Engineering books recommendations...

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

Re: Nail connection factors - displacement issue

[Subject Prev][Subject Next][Thread Prev][Thread Next]

When the ties are higher up, rafters tends to spread out to create substantial horizontal displacement at suppots. Once I modeled it in RISA during my lunch break with pin & roller supports and showed it to my ex-boss more than 7" displacement. He did not want to hear it, and it was sent off like that. Your plywood will help stiffen the rafters, but still displacement can be a deciding factor although strengthwise it may work. Your doubling of rafters should help to some extent. How is it faring displacementwise? Has anybody looked at displacements?

 

 Alexander Sasha Itsekson <sasha(--nospam--at)engstruc.com> wrote:

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
To: "SEAINT Listserver (E-mail)"
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
******* ****** ****** ****** ******* ****** ****** ********



Do you Yahoo!?
Yahoo! Mail Plus - Powerful. Affordable. Sign up now