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Re: Concrete Columns supporting PT Slab needs drop caps retrofit

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Jeff,
 
        Sounds like you need a drop cap about 16" to 18" in diameter with a vertical dimension of maybe 8".  This is just a guess, I haven't done any calculations.
 
        If so, here is a procedure that may work.
1.)  Roughen the top few inches of the column the appropriate distance down from the slab.
2.)  Pour a drop cap of the appropriate dimensions using non shrink grout of non shrink concrete with fine aggregate only.
3.)  Place a steel hoop around the cap and tension it like you would a hose clamp.
 
        This should provide a reliable shear friction between the new cap and the column and the cap diameter should provide your punching shear capacity.
 
Regards,
 
H. Daryl Richardson
   
----- Original Message -----
Sent: Tuesday, August 30, 2011 3:56 PM
Subject: Concrete Columns supporting PT Slab needs drop caps retrofit

I was hired to perform a structural evaluation of the existing construction on a 6 building condo development.  the development is a subgrade garage with concrete columns and PT slab above supporting up to two stories of wood framed construction.  Originally it was only the distress report, but as my investigation revealed considerable construction defects, the owner then asked me to perform structural calculations on the existing structures to verify they were engineered correctly.  Everything was OK, until I got to investigating the punching shear capacity of the PT slab at the columns.  I hadn't noticed it before, but the concrete columns supporting the PT slab were only 12" square....with NO drop caps.  As a result the punching shear force is about 30% ? 40% higher than the punching shear capacity.  Now my question, I need to attach a drop cap to the concrete column, but am unsure of the best way.  With the connection force being around 70k on some columns, I don't think I can attach the retrofitted drop cap to the concrete column without a million bolts.  I was thinking of fabricating bent plates that would sandwich around the concrete columns and then welding them together, thus providing a direct path to the foundation.  I would like to hear if anyone else has any better/more cost effective ideas.  Pouring concrete around the columns to make them bigger is not an option since there are two assigned parking spaces per unit.  Making column bigger would eliminate at least one parking spot per column (8 total per building).  Does anyone else have any better ideas as far as increasing the punching capacity of the PT slab or retrofitting a drop cap to the top of the columns?