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RE: Post Tensioned Slab Attachment

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You should be ok with 3/4" anchors anywhere.  Deeper anchors could be used
within 1/4 the span or so around the columns. I would xray and chip off the
cover for any coring.  Avoid holes near columns like the plague (unless you
want to do a lot of analysis).  For large holes that require de-tensioning,
try contacting Paul Faynor at Stress Express (Laguna Hills, CA), 714 837

Eric Lehmkuhl,  S.E.
KPFF Consulting Engineers, San Diego
(619) 521 8500 phone
(619) 521 8591 fax

-----Original Message-----
From: Drew A. Norman, SE [mailto:DNorman(--nospam--at)]
Sent: Monday, October 23, 2000 8:25 AM
To: seaint(--nospam--at)
Subject: Post Tensioned Slab Attachment

Colleagues and friends,

Returning to active participation in the list after a hiatus, I have a topic
(actually a couple of them) to put before the group.  The specific issue as
it came to my office was a question from an architect client in re how to
attach threaded rods for suspended fan-coil units to the underside of
existing concrete slabs in an office building where his office is creating
plans for a tenant build-out.  He had thought the project was entirely
non-structural and had not involved me until his mechanical consultant got a
plan check correction (kudos to that building official).  He was helpfully
unable to tell me much about the existing building except that it might be
PT.  He of course needed an answer immediately -- the job is design-build
and fast track, demo and prep work has already begun.

A trip to the building department and review of microfiche revealed that the
structure (constructed in the early 1980's to a design by NAM Engineering --
comments by anyone who might be familiar with this firm, as I am, are
invited either on or off list) was as my client suspected of post-tensioned
concrete construction.  Floor slabs are 8-1/2" plates with 1/2 strands at
11" o/c in one direction and grouped (the plans simply call for 20 or 30
strands along a grid line) in the other.

I've cautioned my client (and he has since cautioned the contractor) that
the PT cables make it potentially dangerous to drill or shoot anchors into
the soffit.  This is of course not helpful in terms of the immediate issue
(resolving the plancheck correction) and in fact broadens his problem --
even though the City hasn't asked, he now has to worry not just about his
mechanical engineer's details for support of the fan coil units but also
about connections for his own ceiling support/bracing wires, full-height
metal stud partition tracks, partial-height partition braces, and etc.  Then
there's ducts, sprikler lines and plumbing.  He may also need to cut/core
some openings for pipe/conduit chases and is worried about how to do that.

My client reports that the contractor's reply to his warning about the PT
was that he was using short (3/4") shots for most of these connections (so
that he wouldn't be at risk of hitting the strands) and thought the risk of
drilling/coring/shooting to PT slabs was generally over-stated anyway
because one had to hit a strand in exactly the wrong spot to result in a
serious problem.

I have advised pacometer or other NDT to locate the strands and
design/layout of anchorage devices to avoid them but am interested in what
the list might have to say about any of the questions this little project
raises.  For instance:

What precautions should be taken in making attachments to PT slabs?  Are
certain types of anchors better than others?

Is the contractor correct in his opinion that the risk is generally

Assuming my client has to go ahead and create a new chase in the existing
slab with dimensions large enough to require one or more PT strands to be
cut, has anyone out there successfully re-anchored such cables?  How is such
work done safely?


Drew A. Norman, S.E.
Drew A. Norman and Associates