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RE: Demolition -- Post-Tensioned Parking Deck

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Fountain,

In 1973 the sheaths used for unbonded PT were pretty poor.  The greased
craft paper and thin PVC sheaths were quite common.  They would cause the
tendons to bind up and suddenly release during construction.  This happened
on at least 3 jobs that I am aware of and caused the jacks to come off the
anchorage reportedly about 2' or 3', and then slam back against the
anchorage.  This surprise gave rise to the OSHA regulation that says you can
not stand behind a jack while tensioning.  It also would make you want to
carry a spare pair of underwear.

During demolition, strands have been known to launch, and they can erupt out
of the slab.  Make sure that no one, and no shoring is directly above,
below, in front of, or behind the line of strand that you are working on.

I would definitely want to verify the loss of pre-tension in the cables by
checking the dead end, and / or exposing the strand somewhere else in the
member to see if the strand is free.  If it is bound up, you will need to
relieve the stress at multiple points with the applied heat method.

With 1970's vintage PT, I would opt for the heat de-tensioning method.  Try
it at the mid point first.  If it works, fine, if not go to the third
points.  The trick is to minimize the length (potential energy) of
potentially bound up strand.  minimizing the potential energy will minimize
the kinetic energy (launch and litigation).

Also with the garage being a 1970's vintage, I would de-tension the strands
that are at the perimeter first.  There was a problem of splitting slabs at
the corners in earlier designs.  It almost killed an engineer with HNTB
several years ago.

You might talk to Brandon McMullin at Rohrer Contracting in Kansas City, MO.
He has repaired and taken down PT parking garages.  He has been much closer
to erupting strands than I have.

Let us know how it goes.

Good luck,
Harold Sprague



> -----Original Message-----
> From:	Fountain Conner [SMTP:fconner(--nospam--at)pcola.gulf.net]
> Sent:	Wednesday, December 29, 1999 2:14 PM
> To:	seaint(--nospam--at)seaint.org
> Subject:	Re: Demolition -- Post-Tensioned Parking Deck
> 
> Thank you, Harold,
> 
> My initial reaction was to take a more agressive approach.
> 
> I'm told the tendons are unbonded, and that the date on the drawings is
> 9/4/73.  I was told earlier that the structure was about 10 years old.  
> 
> The launching of tendons is something I've heard of, but never seen.  I've
> seen a number of unbonded tendons broken during construction, but none of
> them "launched".  About 18 inches is as far as I've seen a tendon "launch"
> from the outside face of the slab.  
> 
> Casey at geiger engineers tells me of a case in Canada where there was an
> "exciting" launch -- some 300 feet.  
> 
> Maybe a girdle around the deck edge would be in order.  But, in the
> interest of economics, we don't want to do anything unnecessary.  
> 
> I appreciate all on this list.  As a loner it helps to bounce things like
> this off some other engineers.
> 
> Fountain
> 
> ----------
> > From: Sprague, Harold O. <SpragueHO(--nospam--at)bv.com>
> > To: seaint(--nospam--at)seaint.org
> > Subject: RE: Demolition -- Post-Tensioned Parking Deck
> > Date: Wednesday, December 29, 1999 11:45 AM
> > 
> > Fountain,
> > 
> > There is a big difference between demolishing unbonded and bonded PT.  A
> > parking deck is probably unbonded which makes it "interesting".
> Unbonded
> PT
> > may get bound up in areas especially if the sheathing may have broken
> down
> > or if it used a greased craft paper sheath (common in the 70's).
> > 
> > A one way system (beams and slab) should have (depending on the code at
> the
> > time it was built) a fair amount of mild reinforcing.  The dimensions
> > suggest that all of the PT could be anchored at the perimeter.  If so
> you
> > can de-tension the strands from the perimeter as well.  
> > 
> > Verify that the strands are de-tensioned.  Unbonded strands under
> tension
> > should never be cut.  You can launch one a long way by just sawing
> through a
> > tensioned strand.  If the ends are not accessible, you can de-tension
> the
> > strand by exposing about a 5' long section and heating the strand to
> relieve
> > the tension.  This takes an experienced guy on the torch.  He must use
> the
> > torch to heat a length of strand, not to cut the strand.
> > 
> > I would suggest:
> > 1.	Shore the structure
> > 2.	Add diagonal cables to control the direction of collapse (you
> > mentioned nearby buildings)
> > 3.	De-tension all of the strands
> > 4.	Knock it down
> > 
> > Regards,
> > Harold Sprague
> 
>