Slab-on-Grade Thickness in High Seismic Areas

[chief45]

Based on the parameters below, would anyone see the need to increase slab-on-grade thickness from 4" to something thicker:

- Seismic Design Category= D
- Live load= 150 PSF (IMPORTANT: there are NO significant point loads acting on the slab)
- Modulus of subgrade reaction= 280 psi/in.
- Geotechnical report recommends conventional floating slab-on-grade

What I'm trying to get at is this: based on the above, why even consider thickening the slab? In my office there is this notion that if the Seismic Design Category is D (or worse) that you automatically bump up the slab thickness. I have scoured the ACI standards and the internet (including this forum) for such justification and I'm coming up with nothing! In this scenario,adding thickness to the slab I feel is adding unnecessary cost and material.

Any input would be appreciated.

Thanks

Danny

 

[mike20793]

Only if you are using it as a diaphragm. On my high seismic projects, I include that the slab on grade is not designed as a structural diaphragm in the structural notes, unless I am using it as a diaphragm.

 

[chief45]

Thank you mike20793. No diaphragm action.

One important piece of information I left out is this: in the geotechnical report they do state that differential settlement could approach 2" however this does not specifically point out that the slab will settle differentially.

Thanks again.

 

[KootK]

In high seismic codes, there are often requirements for tying foundations together. Sometimes that's done with discrete grade beams, sometimes it's done using the SOG. Where it's the SOG, the required load resistance might steer one towards a thicker SOG. Some will use SOG thickenings for this purpose. It's efficient but can mess with the control joint layout.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[mike20793]

Good point, KootK. In my office, we use grade beams/tie beams to tie the foundations together. I wasn't aware you could thicken the SOG to do the same thing. Do you thicken the whole thing or just at specific areas?

 

[KootK]

If it would have otherwise been a 4" SOG, I'll usually just add local thickenings quite like what we do for block wall support. For a higher end / institutional building, where the SOG was going to be 5-6" anyhow, I'll just try to use the slab on grade to save the expense of the thickenings. I don't generally feel comfortable using the slab on grade unless it's reinforced with rebar as opposed to welded wire fabric.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.