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Re: Concrete flat slab strengthening

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-----Original Message-----
From: Sprague, Harold O. <SpragueHO(--nospam--at)bv.com>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Date: 10 Ocak 2000 Pazartesi 23:34
Subject: RE: Concrete flat slab strengthening


>Alexander,
>
>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
>columns
> 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
>
>Post-Tension:
> 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.
>
>Regards,
>Harold Sprague
>
>> -----Original Message-----
>> From: Alexander Sasha Itsekson [SMTP:sitsekson(--nospam--at)ida-se.com]
>> 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
>> INGRAHAM DEJESSE ASSOCIATES INC.
>> Berkeley, CA
>> (510) 527-7223 ext. 209
>>
>>
>
>