Slab on grade design guidance 3

[StructureMan44]

What is a good resource for design a slab on grade subjected to a column load? I have a column with an 18kip vertical load and soil with 150 pci subgrade modulus and I'm wondering if I can use a 12" slab. Hopefully there are tables that will eliminate the need for FEA.
Of the two resources I have ACI-360 appears best suited for a grid of posts and TM 5-809-1 appears best suited for walls at the center and edge of slab.

 

[KootK]

Link

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.

 

[StructureMan44]

Thank You KootK, this is very useful. Is this what you rely on?

 

[KootK]

You're very welcome. Yeah, it is what I rely on. I can use it to make quick decisions in simple situations. With exposing my previous indiscretions, it's also considerably more technically sound than what I'd been doing before...

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.

 

[hawkaz]

Does L.A. still allow this method? I thought they retracted their approval of it (I could be mistaken)
I could not find reference to this method in more recent City of L.A. Information Bulletins.

 

[Once20036]

My concern with the article is that I haven't been able to determine what they consider to be a failure - ie, what happens at the specified loads.
Is cracking of the slab considered?
Is settlement considered?
Is the load provided something more akin to a collapse load?

To get to the bottom of these questions, I spent a week digging into the bibliography and a) couldn't answer that question, and b) found that at least one of the items didn't exist (item 9).

Please also not that the paper is not appropriate to use for racking loads and this violates the spacing requirements

I`ll use the paper with a significant additional FS, or utilize more traditional methods.

 

[dik]

Once20036:

On first glance, the loadings appear to be too high and I might consider using 1/2 the tabulated values...

Dik

 

[Okiryu]

I do geotechnical engineering but sometimes use this USACE manual for slab-on-grade recommendations:

http://www.apd.army.mil/epubs/DR_pubs/DR_a/pdf/web/tm5_809_12.pdf

Perhaps it can serve as a reference for you.

 

[txeng91]

Here's a nice spreadsheet:

http://www.steeltools.org/communiti...umentKey=7f9760d0-9fd8-468a-b7d3-46554c7cbe2c

It's old but still a good reference. I believe the main failure mechanism is typically rupturing of the concrete on the tension face.

 

[txeng91]

Also worth noting the spreadsheet will probably give you significantly lower values then the article kootk posted since it is based on Westergaards theory instead of an elastoplastic model. Looking at those tables kind of makes me feel bad about making so many contractors cut through slabs to install footings for columns, which I have typically done for loads more than like 10 kips for 4"-6" slabs...[pre][/pre]

 

[Once20036]

Regardless of the article, I personally would not put a 10k load on a 4" slab.
The traditional methods have been used successfully for years. Someone publishes an article (again, with no mention of how they determine "failure") and suddenly we triple those loads?

For comparison, for a 4" slab:
My typical slab notes = 2.5k allowable
British TR-34 = 7.8k allowable
Ringo's slab on grade book = 5.5k allowable
This new article = 21k allowable.

My first course of action is always the traditional methods that are well established and tested.
If additional capacity is necessary, I consider cracking, settlement, etc and discuss those risks with the project manager / client.

 

[dik]

Can you elaborate on the methods for British TR-34 and Ringo's SOG book?

Thanks, Dik

 

[Once20036]

TR-34 is the British standard for slabs on grade.

https://www.thenbs.com/PublicationIndex/documents/details?Pub=CS&DocID=299679

I`ve found comprehensive design procedures to be woefully lacking in ACI and appreciate the document. It provides guidance on how to deal with edge conditions, corner conditions, closely spaced posts, etc.

Ringo wrote a book in the 90s. I haven't seen any new or ground breaking research or information in the book, but its a great summary of the different design methodologies that are common in the states. He reviews the different methodologies (Army, PCA, WRI, ACI, etc) and provides a recommendation as to which methodology is most appropriate for a given load configuration (line load, point load, distributed load, etc).

https://books.google.com/books/abou...on_Grade.html?id=84VGAAAAYAAJ&source=kp_cover

It was 2013 that I was getting deep into the weeds on this design aspect. If I remember correctly, the british methodology is looking at bending stresses (top and bottom) created in the slab and comparing that to the rupture strength of the concrete. They recommend permitting cracking in the bottom of the slab and limited your flexural stresses in the top of the concrete.

I think Ringo is just a summary of the traditional concrete design methodologies.

 

[dik]

Thanks, Dik

 

[MER3]

I am also looking for a good reference for slabs on grade. Where I am, we evaluate existing slabs as well and occasionally design new ones. We usually get questions relating to large concentrated loads. Would ACI 360-10 be a good reference for this?