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Re: Demolition -- Post-Tensioned Parking Deck

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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.


> From: Sprague, Harold O. <SpragueHO(--nospam--at)>
> To: seaint(--nospam--at)
> 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
> may get bound up in areas especially if the sheathing may have broken
> 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
> 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
> the tension.  This takes an experienced guy on the torch.  He must use
> 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