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RE: 1980s PT Systems

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Paul,
The first PT systems in parking garages in the US were constructed the 1970's. The early engineers of these systems often times did not account properly for shrinkage and creep. The big problem were the intersections that you describe. The ramps constrained the creep, and the garages had a tendancy to pull themselves apart. After the first year though, most of the creep and shrinkage is done. In todays systems, the grease serves to prevent corrosion. You did not say if there were any erupted strands or evidence of corrosion. PT strand can fail catestrophically, but that does not mean that the structure will collapse. Generally the risk of catestrophic failure is in the slab strands, and generally not in the beams.

Your friend may or may not need to be concerned. I have worked on these enough to know when to call for help like Walker Parking.

A full mapped survey of cracking, corrosion, and chloride ingress would be a good first step minimum.

Regards,
Harold Sprague


From: "Paul Crocker" <pcrocker(--nospam--at)reidmidd.com>
Reply-To: <seaint(--nospam--at)seaint.org>
To: <seaint(--nospam--at)seaint.org>
Subject: 1980s PT Systems
Date: Tue, 20 Jul 2004 08:47:33 -0700

Does anyone have any wisdom to share about 1980s PT systems? In visiting a friend recently, I went to look at an existing PT garage upgrade he is working on. There is some cracking in a column at the roof, right at the ramp where beams frame in a different elevations. The beams just miss each other - the top of one is a few inches below the bottom of the other. He believes, probably correctly, that concrete/PT related shrinkage pulled the beams on the two sides of the columns, and caused shear cracking in the column and to an extent running into the beams. While this scenario seems likely, I noted some grease leaching out of one of the cracks in the beam. There is also some grease leaching out of the places where the PT pockets were grouted. Being used to modern encapsulated systems, this worries me quite a bit. Older systems, however, can be different and perhaps this is not quite as bad as I suspect. Does anyone have any experience with this kind of behavior on older systems? I hate to tell my friend that the problem is worse than he believes, but I suspect it might be.

Paul Crocker, PE, SE

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