Need a book? Engineering books recommendations...

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

RE: Post Tensioned Slab Attachment

[Subject Prev][Subject Next][Thread Prev][Thread Next]
I don't believe that the risk is overstated at all.  Let's say that the core
that you need is 12" located at/near the face of column.  Typical banded
tendons are groups of three or four tendons at 6" to 12"oc with at least one
bundle passing over the center of the top of column E.W.  For a 12" core
there is the possibility that the contractor could cut at least one bundle
and maybe two if they are unlucky.  This would be a serious problem.
(reduction of dead load balancing, reduction of joint ductility etc.)
Please note that any damage to the tendon can cause failure.  The damage can
be as slight as cutting one of the seven strands in the tendon and may not
result in a broken tendon for some time (ie after contractor is off job)

I would tend to agree that a 3/4" shot pin is unlikely to damage a tendon
but you might puncture the protective sheathing and provide an opening for
moisture to attack the tendon (not recommended).  If you are planning to
hang anything off the ceiling, other than very light loads, 3/4" embed is
not really enough.  Drilling in an epoxy or mechanical anchor will require
the contractor to locate any reinforcing and PT prior to commencing any
drilling operations.  The anchor should be located at least 6" horiz. away
from any tendon. I believe Hilti has ICBO approval for use of KB-II's and
their HY-150 epoxy system for overhead installation in seismic zones but you
must have continuous special inspection and uncracked concrete etc...

If a chase or core must be located at face of column (very strongly not
recommended), the slab/column joint should be checked for adequate shear
capacity especially for unbalanced moment conditions (parking
structure/heavy storage loads at one area only, seismic activity). Punching
shear failure of flat plates during seismic activity (especially if the slab
loading is already unbalanced) is not uncommon.  ACI has been publishing a
lot of research papers on this very subject recently.

Proceed with the utmost care and caution before cutting/drilling/blasting
into a PT slab.  I would recommend that all PT and reinf. adjacent to each
work area be located prior to any work commencing.  I would also provde a
map of no-go areas where either the PT is near its low profile point or
significant tension can be expected in the concrete reducing the holding
ability of any new anchor(s).  

In the end it comes down to what the contractor does.  If you have provided
thorough guidance and they ignore it then the contractor has a problem.

Good Luck

Nick
-----Original Message-----
From: Drew A. Norman, SE [mailto:DNorman(--nospam--at)dnormanse.com]
Sent: Monday, October 23, 2000 8:25 AM
To: seaint(--nospam--at)seaint.org
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
overstated?

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?

TIA

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