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RE: Concrete Nails

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I guess I'm wondering why is there a hearing? I see no mention of what
the problem was unless I missed something.

If the double 2x6 is sitting/bearing on the foundation, it is
essentially a post. It will not transfer any load to the stem unless it
moves. The fastening of the 2x6 to the concrete would limit the unbraced
length for buckling. If the concrete nails have some withdrawal
capacity, they could also contribute to unbraced length shortening in
the other direction.

If the post is only 5 feet long unbraced length, the double 2x6 is good
for about 15 kips (DF #2), so its way strong for your situation.

The potential problems I could see with this are not using pressure
treated wood or naturally decay resistant wood in contact with concrete,
capacity of the footing since you do not have the stem to spread the
load (unless you are trying to take the load through the concrete nails
which I previously said would not work if the bot. of the post is
bearing on concrete). The thickness of the footing may not be enough to
take the punching shear and check soil bearing. If you are in Canada,
you have low footings due to frost line and the soil bearing capacity is
probably close to 3 ksf at that depth, so you should be okay there too.

If you want to force the load through the concrete nails, you would need
to cut the bottom of the post so it hangs off the stem of the footing.
Then, as long as the dowel equations work for the specified load, I
don't see a problem.

HTH,
-gerard
Santa Clara, CA

-----Original Message-----
From: Daryl Richardson [mailto:h.d.richardson(--nospam--at)shaw.ca] 
Sent: Saturday, May 03, 2003 8:32 PM
To: seaint(--nospam--at)seaint.org
Subject: Concrete Nails

Fellow engineers,

        A colleague of mine, a registered P.Eng., is facing a discipline
committee hearing regarding the suitability of one of his structural
designs.  He has asked me to sit in as an expert witness on his behalf
and I have agreed to do so.  One of the key issues will be the
suitability of the use of concrete nails in securing a double 2x6 "post"
to a concrete foundation wall.  To help me prepare for this role I would
appreciate any comments you are prepared to send.

        The situation details are the following.

        The hearing date is May 13, but some documents must be filed by
May 6 (not yours, of course).

        The project is standard residential wood frame construction.
The house is a single family "split level" located in south/central
Alberta.  Seismic is virtually zero; wind is fairly high; but neither of
these are applicable to the case.  Usually a structural engineer is not
involved with a single family residence in Alberta unless there is some
special circumstance such as an unusual beam arrangement, a retaining
wall, or some problem such as the one I am describing develops.

        The floor system is 2x10 floor joists supported on laminated
wood beams (possibly MicroLam or ParaLam both of which are made op of
approximately 1/8" laminations with the glue plane vertical and
totalling 3.5" in width) which are, in turn, supported on columns.
Normally, there is a pocket cast in the foundation wall to support the
end beam beam reaction; but on this project the pocket was omitted,
hence the problem.

        The foundation wall is 8" thick; the strength and age of the
concrete are unknown.  Normally, concrete strength would be about 2,500
p.s.i. and it could be normal or sulphate resistant.  One may speculate
that the concrete age might be from 5 to 15 days.  Below the wall is a
footing 6" thick and 16" wide.  There is no requirement for reinforcing
steel in residential concrete basements although some builders place ten
millimetre or #4 bars horizontally at 15" centres.

        Since the beam pockets were omitted my colleague got involved.
His solution was to place a 2x6 "on the flat", against the wall and nail
it to the wall using ten 5" long by 3/16" diameter concrete nails.  He
then took a second 2x6 and nailed this directly to the first using "an
appropriate number of ordinary wire nails".  This provides a 3" seat for
the laminated beam.  The double 2x6 are approximately 5' long or less
and are also in contact with the wall's strip footing.  The slab on
grade basement floor is placed directly on top of the wall footing
encasing the double 2x6 post.  Unless I have been misinformed there has
been no indication of a failure taking place.

        Please comment on a) the suitability of using concrete nails in
this way, b) the capacity of such nails, or c) anything else related to
this situation you want to.  All replies will be appreciated.

        Thank you.

Regards,

H. Daryl Richardson



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