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PEMB Tension Ties--was Nashville, TN area foundation help

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Daryl Richardson wrote:
"Jim,

        I use bars to lap with the hairpins at the column lines.

Regards,

H. Daryl Richardson"


A number of people have talked about lapping the hairpins into either
WWF, bars in the slab, or bars under the slab.

ACI 318 (either 99 or 02), section 12.15.5, seems to prohibit this.  I
suppose the prohibition could be taken as applying only to bars
specifically used for the tension tie, i.e. not slab bars, although I
don't read it that way.  We usually detail a welded splice using an
angle.  Since there aren't many of them, and the expense is in the
welding, I usually size them by the tie bar area times 60/36 (even
though both the rebar and the angle, in working stress terms, are
allowed to be stressed to 24 ksi).  We also allow a mechanical splice as
an option.  I stagger the laps; ACI 12.15.5 says a minimum of 30 inches,
but I usually use 4 feet.

I haven't done hundreds of PEM Buildings, so take that into
consideration, but I've run into trouble before in trying to use the
slab rebar. Usually someone wants a trench drain or machine foundation
or something like that; often 6 months after the building is done, so I
may not even find out about it.  And in my opinion, using passive
resistance might be iffy.  To really get full passive resistance, you
have to move the dirt quite a bit.  Then it rains, or freezes and thaws,
or somebody trenches in a lawn sprinkler pipe along the building ("well,
gee, I didn't want to tear up the sod") and the building moves again.
So, I calculate the required wall area based on At-Rest pressure or a
little more, and it takes 20 feet of wall to resist the force.  Then I
calculate the reinforcing for a 20 foot length of wall to behave like a
beam, and it's huge, and I'm back to tension ties.

While it's true that these under-slab bars can get cut, too, they aren't
as susceptible to it as the slab itself (in terms of future work).  And,
if carefully marked on the drawings, they are a little more likely to
attract attention to their importance.

The other reason we like the discrete tension ties under the floor slab
is that it frees us from having to dictate the construction sequence to
a contractor.  They often like to put the floor in after the building is
up (it's a good way to stretch the construction season in northern
climates).  If the floor is the tension tie, that doesn't work.

A few thoughts...

Mike Hemstad, P.E.
TKDA
St. Paul, Minnesota

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