Min. Standard Thickness of Two Way Slab

[bookowski]

In my area (Northeast US) I do not typically see two way elevated slabs less than 8". Do others see/use less than this and if so what is a lower bound? I would think that 6" is pretty thin once tolerances and unknowns are accounted for, i.e. the 'd' being off by 1/2" in the field is a much larger percentage at 6" slab compared to 8". 7"?

 

[JedClampett]

For a 6 inch slab to work the column spacing would have to be about 18 ft (l/36; 60 ksi bar; drop panels), but it seems as a practical matter (closely spaced columns; drop panels practically on top of each other; and the reasons you state above), that someone would have to be trying to make that work. I've never used less that 8 inches.
But I bet you'll get a response from womeone who does it all the time.

 

[bookowski]

I'm assuming no drop panels as they are not typical here.

 

[TDIengineer]

Take a look at section 9.5.3.3of ACI 318-11.

there are equations in that section for minimum thickness h between supports.

 

[bookowski]

Yes but 9.5.3.4 allows different values as long as deflections are computed. Even using the table, assuming no drops and no beams w/Fy=60, then a 7" slab is good for 17.5 clear span (so say ~19' c-c). Running the fea probably pushes that a little further. Was just curious if anyone is using less than 8" since it seems to work by calculation for many situations.

 

[KootK]

I've used 7" and 7.5" often for residential floor plates. That's the minimum that I feel comfortable putting conduit etc in and it's tight at that.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[slickdeals]

I have used 7" and 7.5" slabs on 28-30 ft spans, with careful studies of long-term deflections

 

[asixth]

I've gone 6" for parking slabs and 7" for retail.

 

[Ingenuity]

The thinnest slab I have been involved in (on repair, not design) was a 4-1/2" PT slab, with a 3/4" set down/depression at exterior balconies, so at the edge anchorages it was 3-3/4" thick. Slab spans were short, and they were one-way spanning, not 2-way.

It was most ridiculous slab design I have ever seen - design was probably dated early 1970's. We were tasked with repairs to corroded PT anchorages, and destressing was undertaken by chipping concrete behind the live-end anchorages. Bastard of a project.

I have a distaste for VERY thin slabs, be it RC or PT.

 

[rowingengineer]

I have gone to 150mm slab supported on walls.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[Tomfh]

I have done a few alterations and additions on buildings from the 30s,40s and 50s

These often have 4 1/2", sometimes even 4". Supported on beams/walls though, not columns.

I've also come across really thin one way ribbed slabs from that era. 2.5" spanning one way between the ribs with a central layer of mesh, 2.5" wide ribs with a single bar bottom, and no shear reinforcement.



We specify 150mm slabs all the time.

 

[hokie66]

bookowski,
My approach to selection of flat plate thickness is to consider punching shear first. This of course involves spans and column sizes, but if a flat plate works in punching shear, it will usually work in bending strength and serviceability.

 

[jayrod12]

We've done a bunch of 6" slabs but those were all short spans in both directions. For anything of real size we start at 7 or 8.

 

[bookowski]

Thanks for the input - exactly what I was looking for.