Punching shear on S.O.G. 3

[EngDM]

Is there an easy or well defined method of determining punching shear resistance for a point/line load on a slab on grade? I've ran these in SAFE before, and some of the load goes straight to the soil depending on its bearing/subgrade modulus. I'm wondering if theres a hand calc method that's accurate enough to get an idea of what's going on without having to set up a model. Of course I could just check punching assuming nothing below, but that would be overly conservative.

 

[skeletron]

The Shentu Method might be worthwhile to check out.

 

[hawkaz]

Be careful with the Shentu Method. It allows for very large loads on a slab on grade

City of Los Angeles recognized this method many years ago but has since retracted allowing it. I have not been able to find any documentation stating why they reversed themselves. Anyone know anything more?

 

[skeletron]

Yes. Agree on that @hawkaz . It is an ultimate load method and I was surprised how big the load calculated out to be when compared to a standard punching shear check. Beware that this is the method that Enercalc uses in their module.

 

[XR250]

How are any of these methods accurate without knowing the actual subgrade modulus and bearing capacity?

 

[EngDM]

How are any of these methods accurate without knowing the actual subgrade modulus and bearing capacity?

I have both of these for the requirements of the OP. Just want to know a way to do it without modeling in SAFE.

 

[GaStruct]

@XR250 the methods acknowledge it is critical but without involving a geotech you would just have to assume a conserve value if you wanted to use it. The structure mag article gives a nice chart to use as a basis. I estimate bearing capacity for footings often so not much different.

Note that the structure mag article is for point loads only and no other loads should be within the radius calculated.

 

[TLHS]

I don't generally explicitly check punching shear. I use one of the methods for point loads in ACI 360-R or the US military slab on ground manuals. It's not really a pure punching shear problem because of what XR250 is saying.

I'll also sometimes just assume load goes straight into the subgrade below the applied load using a prismatic distribution if it's a small load, reasonably good subgrade and a small applied bearing pressure for the sake of simplicity and I just want to generate a paper trail. (i.e if I can fire it down at 45 degrees (or any other reasonable distribution logic) through the slab and any good compacted gravel and get a nice low bearing pressure I might just stop there)

This is assuming we're talking about a conventional, unreinforced or minimally reinforced slab on grade. If you've got something where you've got a bunch of bending strength and stiffness you're trying to take advantage of then you have to look at it differently. Conventional slabs on grade are really just a nice surfacing material that help distribute load pretty locally into the subgrade and their 'failure' is just with significant servicability cracking rather than anything critical. You have to consider whether this is a level of service that is reasonable for the type of load you're applying. If you need more reliability then you shouldn't be putting it on a slab on grade.

 

[haynewp]

I've used the GRDSLAB spreadsheet a decent amount in the past for point loads. Look at the tab called "Slab on Grade". I don't certify the spreadsheet's accuracy.

Floor/Diaphragm Systems

www.steeltools.org

 

[jerseyshore]

I've used the GRDSLAB spreadsheet a decent amount in the past for point loads. Look at the tab called "Slab on Grade". I don't certify the spreadsheet's accuracy.

Floor/Diaphragm Systems

www.steeltools.org

We've used GRDSLAB for a long time now. Used to use the charts in the PCA book for racking, but for a single post, the first tab (SOG) is very handy. Results were always pretty close to whatever old calcs were used. Enercalc has a point load calculator as well.

 

[RFreund]

So the Shentu method is based on testing, I believe, by the Storage Equipment Manufacturers Association and The Second Research and Design Institute of Chinese Mechanical Industrial Department. I've tried to reach out for this testing and from what I've gathered, it doesn't exist, or no one can find it (or maybe they don't care to try). I would love to be proven wrong.
In any case, I did find the attached paper which compares methods and recommends a FOS of 6 with the Shentu method.
I'm conflicted, on one hand the Shentu method seems a little too aggressive. However, some other methods seem too conservative for what is potentially some small slab cracks (exaggerating little here). Sure, not good in some cases, but in others that could be worth the risk.

 

[lexpatrie]

Insert mean comment about EnerCalc.

Conventional slabs on grade are really just a nice surfacing material that help distribute load pretty locally into the subgrade and their 'failure' is just with significant servicability cracking rather than anything critical.

I don't really agree, it depends on what they are using it for. A hotel? Sure, fine. There's well established literature (and the PCA manual, as I recall) when you are dealing with forklift loads and the durability of the slab on grade is critical to operations, like, say, a precast plant.

You can check punching shear and apply a bearing load upwards directly under the load, appropriately sized (I think this is how spread footings are checked, AFK curently), but I'm not convinced this is how a slab with bars at 18" square grid will behave but I'm not aware of any testing, either. (My sense here is this punching shear check would be deeply conservative).

I did notice that structure magazine article some years ago. I can maybe make an FAQ entry on it someday. If there are other resources besides the Tomanovich spreadsheet (grdslab) [note these date from the old 8,3 filename convnetion in MS-DOS].