Re: Concrete Columns supporting PT Slab needs drop caps retrofit

Jeff,

 

        I agree with your choice of using a proven and recognized procedure rather than my suggestion which could be academically shown to work but would be untried and unproven in practice.

 

        I was faced with two similar problems on a recent historical building restoration.

 

        The first was to cut a 23 foot wide opening in a load bearing exterior brick wall (for the purpose of garbage bin enclosure) that was 22 inches thick, 80 feet tall, and full of windows.  There were three concerns: one was shrinkage of the concrete beam, which would result in substantial crushing of the brick beyond the ends of the beam; a second was ensuring full contact with the brick along the top of the beam; the third, of course, was how to do it.  We built a substantial garbage containment structure of concrete inside the building to support the weakened wall; we cut half way into the wall from the outside to pour a beam 11 inches wide for the entire outer half of the beam; after that concrete had a few days to cure (and I could begin sleeping nights) we remover the inside half or the wall and poured the final 11 inch wide half of the beam.

 

        The second problem was much more closely related to your problem.  When the building was built in 1912 they built the basement 8 feet wider than the building.  As a result, 8 feet of the basement was under the City sidewalk.  The construction was steel beams spanning 8 feet and a one-way concrete slab 130 feet long with multiple spans of 15 feet; a standard sidewalk was poured on top of this slab.  Over the decades seepage through the construction joint between the wall and the slab had allowed corrosion of some of the steel beams to the point where portions of the bottom flanges of some of the beams could be knocked off with a carpenter's hammer.  Also, over the decades, street traffic loads had increased substantially.  My solution was to provide wide drop strips to reduce the slab spans and to effectively replace the steel beams with reinforced concrete beams.  Also, we sealed the top of the slab and down the wall a few inches below the offending construction joint.  We placed the concrete through 4 inch diameter holes through the sidewalk and slab.

 

        The common element was the concrete mix design.  The features of this mix design were 3/8 inch maximum aggregate, non-shrink admixtures, super plasticizer, and high strength ( in excess of 5,000 p.s.i. for high early strength).  As I recall, the slump was somewhere around 9 inches.  It was flowable!  I think that the most important property was that under its own hydrostatic pressure with modest vibration the concrete was able to attain full contact with the slab and brick wall that it was intended to support.  The contractor liked working with this mix so much that he requested permission to use it for underpinning footings and other similar applications.

 

        As far as your other problems are concerned, I don't pretend to be a prestressed concrete expert.  I do know that Hook's Law applies and that you can not have forces transferring from the tendons to the concrete to prestresse the concrete without both the tendons and the concrete shortening.  This may be why they used this detail, to permit the shrinkage.  All of this stress transfer should have long since taken place, therefore you may well be able to rework the connection detail to make it more rigid without adverse affect.  However, I am not an expert in this matter; you may wish to seek better advice elsewhere.

 

        I hope these comments are of some help to you.

 

Regards,

 

H. Daryl Richardson

----- Original Message -----

From: Jeff Hedman

To: seaint@seaint.org

Sent: Thursday, September 01, 2011 10:38 AM

Subject: RE: Concrete Columns supporting PT Slab needs drop caps retrofit

Daryl,

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.

 

Jeff