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RE: Concrete flat slab strengthening

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Will you be able to avoid bringing the roof slab into compliance with
today's code.  ACI 13.8.5 requires continuous bottom bars in column strips
now, it probably didn't when it was built.  What about the effects of
unbalanced moment between columns and slab?  Do the drawings provide enough
detail that you can check the existing reinforcing against the new loads?
What about punching shear at the columns with the increased loading that you
are talking about?

Roger Davis
SDS Architects, Inc
205 N. Dewey Street
Eau Claire, WI 54703

-----Original Message-----
From:	Alexander Sasha Itsekson [mailto:sitsekson(--nospam--at)]
Sent:	Monday, January 10, 2000 1:41 PM
To:	List server
Subject:	Concrete flat slab strengthening


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

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.


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