Floor Slabs on Grade

[medeek]

In a previous topic I had questions on how to determine the amount of lineal load a slab on grade could safely handle.

I have recently come across another reference that addresses this subject: Designing Floor Slabs On Grade by Boyd C. Ringo and Robert Anderson (second edition)

Just wondering what others experience is using this reference for slab design.

A confused student is a good student.

 

[MrHershey]

Great book. Use it quite a bit. Best out there in my opinion.

 

[KootK]

It's a classic reference for good reason. Thumbs up.

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.

 

[medeek]

I think someone had previously directed me to take a look at the Army and Air Force Technical Manual (ARMY TM 5-809-12) Table 3-2 for wall loads on slabs. Comparing the values found in this table with the values found in Table 11 in Designing Floor Slabs On Grade there is a huge disparity.

Reading the fine print I realized that the Army manual is using a safety factor of 5.625 whereas the other reference is using the author suggested value of 2.5 instead.

Yikes! No wonder I was having such a hard time getting my bearing walls to work on the slab, my highest wall loads were about 900 plf which I could not get to work with a 6" slab.



A confused student is a good student.

 

[WiSEiwish]

Concrete Floors on Ground by Tarr and Farny is a good reference. This is the book put out by the Portland Cement Association. It includes thickness design for different loadings as well as a host of other topics such as moisture, toppings, and subbases.

 

[Jerehmy]

I have the PCA one and the Ringo/Anderson one. I like the Ringo/Anderson one better, but I don't think you would go wrong picking either.

This is the one area where I really wish I had a design program. There were situations where it would have helped immensely I just can't recall off the top of my head.

 

[WiSEiwish]

CSC has a design module for slabs on grade, though I don't think it has a line load option.

 

[DaveAtkins]

The factor of safety for slab-on-grade design is quite high, I think to prevent "unsightly" cracking.

Under a 900 plf line load, a 6" slab will not sink into the ground, but it might crack. But I don't think it would crack, because, again, the FS is so high.

DaveAtkins

 

[WARose]

Most references (and spreadsheets out there too) really don't take into account all the various loading conditions (near edge, corner, etc). Therefore, most of the time I model it on something like STAAD (i.e. plates with an elastic base). That typically gives me what I need to know.

 

[XR250]

I rarely see cracks in slabs in practice that are from line or point loads. The reality is that slabs perform alot better than they should as long as the subgrade is decent.
I see people installing 7000lb capacity, 2 post car lifts on 4" slabs without incident (not that I would condone this). Not only is there a point load, but a pretty decent OT moment as well.
I would not worry about a 900 plf load on a 6" slab or a 4" slab for that matter.

 

[spats]

There's a spreadsheet program available at steeltools.org called GRDSLAB.xls that will design for a wall/line load on a slab.

 

[medeek]

Nice spreadsheet however he is using the same safety factor as the TM 5-809-12 (5.625) and a 6" slab (k=200, f'c=3000) is only good for 775 plf.

At least now I can see where this number is coming from: FS = MR/Fb (Modulus of Rupture/Allow Bending Stress).

So who is correct? The Army Corp. of Engineers or Boyd Ringo? How does he arrive at a safety factor of 2.5 versus 5.625. What is everybody out there using in practice when they have to put a wall on top of a pre-existing slab. It seems like there is a lot of latitude in this field when it comes to analyzing certain situations.

A confused student is a good student.

 

[CCox]

The Army Corp of engineers method is in the Boyd Ringo book. Look on Scribd and you can download it. The table I have shows a SF = 2.5. For a 6" slab, k=200 and f'c=3000, I get a wall load of 1744 plf.

CJC

 

[CCox]

Actually, maybe in the Ringo book, it is not the Corp of Engineers method? They say it comes from the Army/Air Force Technical Manual. Maybe that is something different.

 

[medeek]

Read paragraph 2 and 3 of the Boyd Ringo book, page 54. Ringo explains that the Army manual is setting the allowable bending stress at 1.6 * sqrt(f'c) which gives a SF of 5.625. In the next paragraph he then explains that table 11 and 12 of his text is based on a SF of 2.5, which he recommends. In his tables he also includes some values calculated at 5.625 SF for comparison sake.



A confused student is a good student.

 

[GerryBertier]

CCox-
The difference is the safety factor.

If your not worried about floor cracking, you can design the wall foundation as thickened (along with plain concrete bending/shear) and based on allowable bearing values assuming a rigid support, say 1500 psf rather than a flexible support such as the subgrade modulus.

The problem you will have is different settlement between completely different elements cast monolithically.

While that might seem like a big deal, interior footings are designed that way fairly often and without issue, especially if you don't have sensitive floor finishes, but say carpet.

Isolating wall foundations completely from slabs is an expensive task.

 

[medeek]

I guess the selection of the safety factor is an arbitrary number and is up to the judgement and discretion of the designer/engineer. My question now is what is a reasonable safety factor in this situation and why? With a safety factor of 2.5 will the concrete crack but not affect its function? Perhaps the higher safety factor by the Corp. is to try and eliminate all unsightly cracking.

A confused student is a good student.

 

[CCox]

I have never never looked directly at the army manual so I don't know why they only specify a high SF. I tend to think the SF would have to do with limiting cracks where you don't want them (think caustic environment like a vat) or there is a higher uncertainty of loading conditions. Remember, it is only 1 method. FEM will generally yield a thinner slab.

 

[Ron]

Keep in mind that Army (military) construction is often done under adverse conditions with locally available resources that might or might not have been tested, thus a higher FS. For commercial construction, we can control conditions thus allowing a lower FS.

 

[medeek]

I can buy that.

A confused student is a good student.