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

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Michael L. Hemstad wrote:

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.
That's interesting, as I've used the tie into wwf/spliced bar concept for my PEMB foundations. Oddly enough, the commentary on 12.15.5 seems to be of absolutely no help in determining. One could easily argue that the in-slab rebar is completely in tension and fully utilized. Their suggested examples of "arch tie" or "tenstion element in a truss" are applicable. However, their example of what is not - circular bands for hoop stress in a water tank - seems like the perfect case for this requirement, as they too are completely in tension and fully utilized, as well as being critical to carrying the load. One might argue, as they have with the tension hoops in the tank, that the large number and close spacing of wires (in wwr) or rebar (in rebar-grid slabs), would be in the same category. Also, single purpose rebar ties - whether below the slab or in the slab - would certainly be subject to this rule.

As to your explanation of your standard, I have two questions. First, do you require A706 rebar for your ties, or do you let them weld A615 in the field? I honestly don't know the requirements for A615 welding, except that there are special considerations, so I don't allow it. Second, any work requireing a pit or deep trench which occurs after your drawings go out will destroy the tension tie, whether it's welded, mechically spliced, or lab spliced. Once that tension tie is cut, it doesn't matter how it was spliced to begin with. I suspect that once the permit it pulled, my PEMB foundation drawings are quickly filed in the closest refuse recptacle, as I know that I'm a "necessary evil" or "cost of doing business" to the fabricators, which makes special notes regarding the tie area mostly useless.


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