Thursday, September 1, 2011

RE: Concrete Columns supporting PT Slab needs drop caps retrofit

You are right in having caution.  Locate any embedments with a megger, and I prefer drilling holes with a hammer drill and not a core drill.  A hammer drill bit will bounce off embedded steel. 

Regards, Harold Sprague

Subject: RE: Concrete Columns supporting PT Slab needs drop caps retrofit
Date: Thu, 1 Sep 2011 10:38:01 -0600


I also agree that the conical caps are the way to go.  I am going to order the Concrete International publications that outline this procedure.  I had thought of retrofitting some stud rails as well but am nervous about drilling into the slab for fear of hitting the PT tendons.  The owner is also nervous about this as well, which is why I began researching retrofit ideas where there would be extremely limited drilling into the PT slab.  I am submitting my report today on my findings with regard to the structural integrity of the project buildings.  Even after removing all live load from the column loads, punching shear is still a problem.  This is a little off subject, but the pt slab to wall connection is also a problem.  The slab is only connected with #5 bars @ 24" o.c., which is borderline for lateral load attachment.  However for portions of the building where the garage walls retain 11'-0" of soil, the connection is not sufficient to transfer the soil loads into the PT slab.  Another issue with the slab to wall connection is the lack of positive connection to shear walls.  The #5 bars have a 6" long, 2-1/2" diameter foam sleeve around the bars in the top of the garage wall.  I have seen this at PT slab corners where the most shrinkage would occur, but generally there has been a much more positive connection to the wall at midwall of shear walls, etc., especially in a moderately high seismic area where I am.  Any input on why they would have put this type of connection for all wall to PT slab connections.  If they wanted it done differently, there were not any other details in the plan specifying to do otherwise.