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RE: Post Tensioned Slab Attachment

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"I disagree strongly with the contractor's thoughts that drilling through is
not an issue.  I don't know what your post-tensioning force is but I doubt
it's trivial.  Damaging the strand, or cutting the strand can send a missile
out the side of your building at the anchorage point, and who knows how many
buildings it can go through before it stops."

I've worked on a fairly large number of PT projects over the past few years,
and have gotten a number of phone calls about broken cables.  These usually
result from someone drilling or coring places they shouldn't be drilling or
coring.  Generally, nothing bad has happened, which is probably the
experience of the contractor who thinks it's no big deal.  I whole heartedly
agree with you that it is potentially dangerous, it just doesn't always turn
out that way.  From a structural point of view, if they hit one cable, you
can often analyze the design without it and get it to work.  If they drill
in the midst of a band, things can get bad quickly, as they can hit lots of
tendons with little drilling.  Even when they are being careful, I steer
them away from banded areas.  

As far as drilling for embeds or use of shot pins is concerned, if you have
a copy of the drawings for the building, you can determine where the tendons
are high or low, which gives you greater liberties about the depth you can
go before you will hit anything.  You could consider creating a zone map of
areas where you get greater or lesser safe depths... assuming the PT was
installed as shown.  Otherwise, not allowing anything deaper than minimum
cover minus a reasonable distance for a factor of safety is the norm.  I
have heard of contractors watching the dust come up from the drill and
watching for purple plastic shavings to see if they are hitting PT sheathing
and thus can't go deeper.  I'm not endorsing that method and don't approve
of it, just one of those stories that I've heard that might amuse people.  I
have also hear of contractors using that method as a fail safe check when
they are sure they are clear of PT by using x-rays or something of that
sort.  That approach sounds a little wiser.  I have heard that it is
difficult to distinguish between rebar and PT on an X-ray, though, so the
"safe" areas to cut a large hole might be very limited as you could end up
needing to cut to miss both of them to be sure you miss PT.  

Paul Crocker