Need a book? Engineering books recommendations...

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

RE: Lag Screws

[Subject Prev][Subject Next][Thread Prev][Thread Next]
I would not install a 1/4" diameter lag screw into the edge of a 2x bottom
chord of a manufactured truss for a number of reasons:

1. The load you are applying has not been or may not have been considered in
the original design of the truss. The additional weight might overload the
truss so I would be cautious and probably go back to the truss manufacturer
if you know who it is.

2. The withdrawal value depends on the species and specific gravity of wood
member and the length of embedment. You need to check a wood design manual
such as the AITC Timber Construction Manual. You are looking for the Lead
Hole Diameters for Wood Screws (Tables 6.29 thru 6.31) of the 3rd Edition
AITC manual (I'm sure that there is a newer version than my 1985 edition).
This is why Simpson would not be able to tell you the withdrawal values.

AITC states under the paragraph "Withdrawal":
"If possible, structural designs should be such that wood screws are not
loaded in withdrawal. When such loading is unavoidable, the tensile strength
of the wood screw at its net (root) section should not be exceeded. Loading
in withdrawal from end grain is not permitted."  The section references
Table 6.30 for withdrawal values based on gauge of screw and the specific
gravity of the wood used in the truss.

2. You take a great risk of splitting the bottom chord when installing a
1/4" diameter lag screw into 1-1/2" thick lumber (bottom chord of the truss)
even with a pilot hole. If you split the truss chord you can create failure.
I would suggest you seek another form of connection. 

If the truss can handle the additional load, there are hangers that will
saddle the 2x chord and the steel can be connected to that. I would do a
little research on this first as well as discussing your intentions with the
truss manufacturer. If you can't locate the truss company - don't use the
truss to hang the steel from.


Dennis S. Wish, PE


California Professional Engineer

Structural Engineering Consultant

dennis.wish(--nospam--at)verizon.net

http://www.structuralist.net

 

"Apathy is Lethal!" Speak out and Vote - but make sure you get the facts
right without the spin from either side; Verify their claims at
FactCheck.Org; http://www.factcheck.org

 


-----Original Message-----
From: Michael L. Hemstad [mailto:hemstad.ml(--nospam--at)tkda.com] 
Sent: Friday, October 22, 2004 1:06 PM
To: seaint(--nospam--at)seaint.org
Subject: Lag Screws

List,
I have a project where I need to hang a small steel beam from the bottom
of some prefab wood (2x) trusses to carry the track for a moveable wall.
The beam will cross about 7 trusses.  It's stiff enough to spread out
the load to the trusses.

The simplest idea I have come up with is to use lag screws through holes
in the top flange of the beam, into the bottom chord.

Simpson, for example, makes a 1/4 inch diameter lag screw, with catalog
values for strength in shear.  However, they corporately refuse to give
me a withdrawal value for it.

The NDS has values--rather large values--tabulated for generic lag
screws.  I could just use those.  However, in my earlier days (say, 20
years ago) I occasionally fastened pieces of wood together with lag
screws.  We shied away from the smaller ones because they often broke
during installation.  We drilled pilot holes, we waxed threads, we
sacrificed black chickens before we screwed them in.  5/16 screws broke
maybe one time out of four; the 1/4 inch ones broke more often than not.

Relating this fascinating story to the Simpson engineer, she said that
her understanding was that current lag screws were better than those of
twenty years ago, and theirs, being of hardened steel, were better than
the ones we bought at the hardware store.  She simply was not allowed to
give me a withdrawal value.

It would be easy to just use a bigger screw, but the truss manufacturer
is going to be excited enough as it is with the 1/8 inch pilot hole I'm
calling for in his bottom chord.  And I could bring straps up the side
of the truss chord, but it's at about midspan and there is probably a
metal truss plate there that doesn't want holes drilled in it, and
vertical or diagonal members that preclude strapping over the top of the
chord.

So, finally, the question:
Can anyone comment on the quality and viability of 1/4 inch lag screws?
Or does anyone have a better way to solve this simple problem?

Thanks,

Mike Hemstad, P.E.
TKDA

******* ****** ******* ******** ******* ******* ******* ***
*   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 
******* ****** ****** ****** ******* ****** ****** ******** 

---
Incoming mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.778 / Virus Database: 525 - Release Date: 10/15/2004
 

---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.778 / Virus Database: 525 - Release Date: 10/15/2004
 


******* ****** ******* ******** ******* ******* ******* ***
*   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 
******* ****** ****** ****** ******* ****** ****** ********