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

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

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Jordan Truesdell, PE writes:

"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.

Jordan"


Jordan,
I certainly wouldn't argue with most of what you say.  I understand the
strictures against lap splices, better than I understand the exceptions.
I have a fear of round tanks with all the ring bar laps lined up,
waiting to split the wall and peel apart as soon as I'm off the job
site.  I do happen to agree strongly with the prohibition against
lapping tie bars as we use them for PEMB, which is in a little trench
under the slab, with no confining steel.  Really not a good place for a
lap.

Yes, we do spec A706 bar for the ties to go along with our welded
detail.  If getting that bar is a concern, it could be done with A36 rod
or angle shapes too.  Rebar or round rods, being less spread out in
section, take longer to corrode in case something goes wrong with the
concrete encasement.  In other words, if you lose 1/4 inch all around a
#10 bar, you've still got something left; if you lose 1/4 inch all
around a 1/2 inch angle, you're toast.  (This concern really only
applies to exceptional situations, like plating plants and places that
generate large electrical fields.)

I hear what you're saying about the ties being susceptible to retrofit
cutting.  About all I can say is that the owner could cut in a shallow
trench and maybe not destroy anything.  I will say that most of the
contractors around here aren't interested in the liability of a building
falling down due to their work; and they actually do recognize that
while #4 at a foot can usually be safely cut, two #8 bars in a trench
may actually be doing something and should be respected.  But it's no
guarantee.

Thankfully, we usually work for the owner; so, our drawings may actually
get filed in a file drawer rather than the round kind.  But, that's no
guarantee either.

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



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