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RE: Demolition -- Post-Tensioned Parking Deck

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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

Harold Sprague

> -----Original Message-----
> From:	Fountain Conner [SMTP:fconner(--nospam--at)]
> Sent:	Wednesday, December 29, 1999 9:08 AM
> To:	seaint(--nospam--at); StrucTX
> Subject:	Demolition -- Post-Tensioned Parking Deck
> I'm proposing on an interesting project with a demolition contractor, and
> will appreciate any technical guidance, warnings, advice you can offer.  
> An existing post-tensioned parking deck with 4 elevated levels is to be
> demolished to make way for a new building.  I haven't yet seen either the
> building or the drawings, but am told it's about 200 x 250 feet with 5"
> slab, beams in one direction, and basic column grid 28 x 58 feet.  The
> municipality is requiring "demolition design", because it's very close to
> the surrounding buildings.
> I'll be looking at the building tomorrow (Thursday) with the contractor,
> and perhaps the owner.
> As a side note, I don't understand the logic of eliminating a perfectly
> satisfactory parking structure, when  parking is *already* at a premium. 
> But that's not my decision...
> Thanx, all,
> Fountain E. Conner, P.E.
> Gulf Breeze, Fl. 32561