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Re: Chairs for Placement of Post-Tension Cables in Slab

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I have never heard of post tensioning a 4" slab on grade. The people who do this for a living in our area use 8" slabs.
 
Stan Scholl, P.E.
Laguna Beach, CA
 
On Mon, 30 Aug 2004 10:09:13 -0500 "Bill Polhemus" <bill(--nospam--at)polhemus.cc> writes:

I have noticed that in placing cables in a thin slab (such as residential slab on grade) not a lot of consideration appears to be given to the effect of the sudden change of cable trajectory near the anchorage.

 

For example, for a 4-inch slab, with the cable at the center of the slab, it is necessary to “break” the cable downward from the last chair near the edge, to allow for sufficient edge distance at the anchorage. But obviously, the closer the chair to the end of the cable, the higher the “impingement force” at the chair that would possibly crush the chair or cause it to “zing” out of the bottom of the slab.

 

I need some info on the capacity of the chair against crushing, and can’t seem to find it anywhere. Also, often in a residential slab on grade, the chair is set right down on the slab subgrade, so obviously there has to be a limiting “pressure” so that we can prevent the chair from burying itself in the soil under too great load.

 

Has anyone looked at anything like this, and can point me toward relevant data? I’m trying to come up with some notes and/or details for my drawings, and I’m concerned about this possibility.