Point loads on unreinforced residential slabs

[YoungGunner]

So came across this method by Shentu and Al-Nasra for determining strength of point loads on unreinforced slabs. The strength they present for unreinforced slabs is relatively incredible compared to the light loading in residential structures and I'm blown away. I have some questions regarding its use in general and for residential slabs:
1. Would anyone feel good using this for residential applications where builders a) miss the footing b) don't want a footing c) for a remodel where we aren't sure what is below the slab?
2. Why on earth would we need footings in some applications if unreinforced slabs are this strong? I'm not going to stop, but wonder what thoughts people have.
3. ACI requires taking off 2inches in the design thickness for plain concrete, for some fair points per the commentary. Should that be considered for this method they determined?
4. Has anyone heard of bad experiences from this method I should be warned of?

https://www.structuremag.org/wp-content/uploads/2014/08/C-StructuralDesign-Azzi-Apr081.pdf

 

[Once20036]

Lots of old threads on the topic.
I don't use the method.

 

[phamENG]

Interesting. I hadn't come across that article before.

Here are a few threads that mention the 'Shentu Method':
thread507-390051
thread592-213921
thread507-381590 (This one has a list of references from Once20036)

Looks like Enercalc's point load on slab is based on it? I do like the higher capacities. The thing to look into is serviceability. I'm sure you could load the slab like that and not have the mezzanine fall over...but what kind of settlement are you getting? Tall and skinny racks could be more sensitive to that.

Specifically for residential...I'd be very hesitant to use those loads. Again, I didn't see anything about settlement magnitudes. A steel mezzanine holding bags of bulk material may not care if you get 1/4" vertical movement...but that new plaster ceiling probably will. There's also issues with quality of residential slabs. I was involved in several FEMA flood mitigation projects that involved lifting houses...some of them were slabs or had slabs in them and they were lifted, too, so I got to walk underneath them. A couple were beautiful....you'd think the bottom of the slab had been sitting on shored form work. Others were absolutely terrible....5" variations in thickness (on a 3" or 4" slab!).

 

[XR250]

I usually keep it to 1,000 lbs on a residential slab unless i have more information.

 

[Settingsun]

https://www.eng-tips.com/viewthread.cfm?qid=493811

There are some test results in that discussion. The researchers decided the truth is somewhere between the old methods and Shentu's numbers, but that Shentu is quite unconservative. FoS of 6 was recommended on the capacity given by Shentu's method.

 

[JoshPlumSE]

My first objection is that I believe you are not allowed to use unreinforced foundations for any seismic resistance. Maybe R=3 systems that don't require any ductility or seismic detailing. But, I would not use an unreinforced slab to support columns or shear walls.

I have no problems using this method to justify loading from a short storage rack in my own personal garage. But, I'm not exactly submitting calculations to the city for that!

 

[Ron]

The loads given in the article seem incredibly high. Generally such point loads are controlled by shear (storage rack loads on slabs on grade) and would not be anywhere near the loads given.

 

[Once20036]

Josh & Ron - the article states that in order for those loads to be valid, you need to maintain a minimum clear distance between adjacent loads.
This means that the results from the article aren't appropriate for use with any racking systems. This seems to be ignored by everybody I`ve run into that wants to use those loads.

One update to my previous opinions shared in previous threads -
I`ve learned from one of the ACI committee members that they are aware of the article, have debated it, and have determined that it is not going to become part of the ACI standard.
It's been a couple years, but I believe it was because the Shentu was a mathematical excercise and didn't account for some tensile forces that may be present in the concrete from the drying/shrinking process.
Don't quote me on that.

 

[phamENG]

Once20036 - good to know. Thanks. Seems strange, though, since the Shentu method supposedly allows the concrete to be crack...so how applicable are drying stresses? So they are pushed up slightly to a shrinkage crack? Supposedly it's already accounted for.

Not saying I disagree with you...I'll be looking into it more next week (have an industrial change of use with some SOG considerations)...just trying to tease out some ideas and see where the reasoning is. Westergaard is the tried and true method, but it's pretty conservative. Hard to sell the results when there's already a racking system on another part of the floor with no signs of distress, but my analysis says the new, identical one on another part (same construction) doesn't work.