Post Tensioned Concrete Slab with Uniform Bottom Bar Mat

[RFreund]

Typically with post tensioned slab there is very little mild bottom bar reinforcement. However, I occasionally see jobs where the engineer will specify a bottom bar mat. Something like #4 or #5 @ 12" or 18" oc each way. Is there a reason for this or maybe this is how it was done at some point? Just curious if this was related to a minimum or standard practice or something?

Thanks!

 

[Ingenuity]

I assume you are referring to 2-way PT slabs?

ACI 318-14 clause 8.6.2.3 requires minimum bonded reinforcement based upon the magnitude of flexural tensile stresses. If f[sub]t[/sub]<2√f'[sub]c[/sub] then code does NOT require MIN steel, but I would suggest it be provided for limiting crack widths and spacings.

It is very common for UNbonded PT slabs to experience cracks of significant width that are NOT well distributed when insufficient mild steel rebar is provided.

I more commonly see #4@36" B EW, but if varies depending on the design etc.

EDIT: A general mat of bottom rebar also aids in the placement and securing of the PT tendons for installation.

 

[Sheer Force Engineer]

I generally see a bottom mat of reinforcement in a PT slab on the following occasions:

- High degree of crack control required
- Heavily loaded slab or transfer slab
- suspended basement slab (restrained on most/all edges by stiff basement walls)
- Engineer throws it in "just because"



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[RFreund]

I assume you are referring to 2-way PT slabs?

Sorry, Yes, should have clarified.

Thanks for the replies. Seems to be in line with what I was expecting. I just wasn't sure if I was missing something. I have seen it in suspended basement slab applications and transfer slabs, so that makes sense. However, I recently saw it on a set of drawings which weren't either of those conditions and it made me wonder.