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RE: Concrete flat slab strengthening

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Conventional Reinforced Concrete:
	You mentioned the most obvious caveat on the concrete side
(shoring). Also consider:
		1.	Jack the slab to relieve the DL 
		2.	Scarify / prepare / test the surface to get adequate
bond from new to old concrete.  Dowels can be used, but they are more
expensive.  There is a good history for raking surfaces to get new concrete
to bond to existing.  With a 4" thickness you don't need to account for a
different modulus of elasticity.
		3.	Use a low shrinkage concrete.  I wouldn't use
shrinkage compensating, but you can reduce shrinkage with a good mix and
good curing.
		This is how Filigree wide slab is done.  The also have a
mechanical anchor, but it does not see the horizontal shear load.  The bond
is adequate.

Structural Steel:
	For the "steel beams from above" solution. Also consider jacking the
slab to preload the steel beams.  

Composite steel:
	Another possibility is a system of composite steel beams:
		1.	Provide steel beams below the slab connected to the
		2.	Preload the steel beams 
		3.	Drill holes through the slab
		4.	Shoot shear connectors to the top flange of the
steel beam
		5.	Grout the shear connector holes full
		6.	Remove preload after grout is cured

	Another potential solution is external PT in which the PT strand is
harped similar to prestressed concrete.  Use either cast-in-place concrete
anchor blocks / spacer blocks, structural steel, or a combination.  This can
be a very cost effective solution.

You can get load cells to tell you how much load you are putting on the
structural elements during jacking for the shoring / jacking solutions.
Obviously the rams will be calibrated to indicate the amount of PT force.

You should always factor in long term deflections. 

Harold Sprague

> -----Original Message-----
> From:	Alexander Sasha Itsekson [SMTP:sitsekson(--nospam--at)]
> Sent:	Monday, January 10, 2000 1:41 PM
> To:	List server
> Subject:	Concrete flat slab strengthening
> Hello,
> I am working on the renovations for the existing 4 story concrete building
> with flat slabs (10" thick at the floors and 8" thick at the existing
> roof)
> and columns on the 23' by 25' grid.  The building is approximately
> 155'x166'
> in plan with a few oddly sized bays.  The building is a historical
> structure.  It is used to be SEARS store and is being converted to lofts
> (surprise!).
> They are adding an additional story on top of this building.  Obviously
> the
> new roof is curved and none of the new bearing walls are centered on top
> of
> the columns.  The existing roof slab needs to be leveled and is
> overstressed
> with all the additional dead and live loads.
> There a two ways of retrofitting of the existing roof slab that were
> discussed with the architect:
> 1. Steel framing raised several inches above ex. slab and supported on top
> of existing columns (columns are fine for the additional loads as the
> floor
> slabs were designed for 125 psf live loads) with concrete filled metal
> deck.
> This was my preferred solution.
> 2. A minimum 4" reinforced concrete filling to bring the slab thickness to
> 11".  Of course the existing slab will need to be shored prior to pouring
> of
> new concrete and new topping shall be dowelled into ex. concrete.
> The architect's choice is to use the topping slab, of course.
> Questions:
> 1.  What are the potential problems with the second choice in your
> experience?  I have always used steel framing to reinforce existing slabs
> but this was ruled out by the architect.
> 2. Are there any alternatives?
> 3. Shall we be concerned with long term deflections?
> I appreciate your opinions.
> Thanks in advance,
> Alexander Sasha Itsekson
> Berkeley, CA
> (510) 527-7223 ext. 209