RFI - Interior Slab for uplift resistance 1

[slickdeals]

Would any of you have access to this paper "Uplift resistance of interior footings of low-rise buildings"

Or any other research that shows the contribution of a slab-on-grade to uplift resistance?

Imagine an interior column of a single story large warehouse type building. The columns have individual spread footings, soil above them and a slab-on-grade. How do you calculate the effective area of the slab for uplift resistance?

Do you calculate how much the slab can "cantilever" from the column in each direction based on the reinforcing provided? The compare this moment capacity to the moment produced by the self weight of the slab?

 

[BAretired]

I suppose it would depend on the thickness and reinforcement of the slab, but for a 5" slab with mesh reinforcement, I would not consider much more than the dimensions of the footing, maybe an extra foot each side.

If uplift is critical, I would prefer to drop the footing to engage the weight of soil or alternatively to use piles with pullout resistance. There is always the possibility that the slab could be removed at some time in the future.

BA

 

[steellion]

I agree with BARetired. I think if you're really in a bind and just need a little more capacity for a project already designed/built, you can justify assuming a soil cone coming from the edges of the footing at the soil angle of friction and that defines your quantity of soil and area of slab-on-grade you can use. But that's as far as I would go.

 

[MiketheEngineer]

Ditto

 

[ron9876]

I am a bit more aggressive. Soil cone plus 2' past edge of soil cone. If not then it seems to me that you would use slab over cone plus shear capacity of slab.

 

[jberg]

If you have access to "Foundation Design" by Das or Boyles, the authors discuss the additional effect of the soil shear cone. Very good information.

JWB

 

[msquared48]

There is also the sucking effect of the pullout... In order to lift the footing up, air has to get under the footing. Kinda like starting a wedge up an incline - more force needed to initiate the movement than to keep it moving.

I just add extra concrete and consider the soil where appropriate. Some of these structures go without slabs for a time as their owners run out of money, or other delays. I would just consider the slab and the sucking force as part of the factor of safety.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[SteelPE]

I only consider the soil and slab directly on top of the footing. I had a discussion with another engineer the other day who does the same thing, however, if he is desperate (being challenged by another engineer) he will consider an increased portion of the slab.

Be safe and just the extra concrete and just let the contractor b***h about it. In the end, it will probably end up being a small portion of concrete.

 

[rowingengineer]

you can purchase the paper from

http://www.nrcresearchpress.com/toc/cjce/18/3

ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[rowingengineer]

I take a bit more of the slab into account when I design footings, I use the first rupture apporch when the footing is intergral and when septrate I use a similar apporach to the above article.

ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[slickdeals]

RE,
Does that paper have a formulation based on testing? Do you have that paper and if so, would you mind sharing the design information pertaining to it?

 

[JRGse]

I use the weight of the slab for a distance beyond the edge of the footing. This distance is based upon the moment capacity that a slab has as a “cantilevered member” beyond the edge of the footing. At times, I have increase the slab reinforcing to bar in order to increase the uplift resistance.

 

[a2mfk]

Sounds like its an engineering judgment decision based on these responses, and the several different methods and opinions I have gathered from SEs over the years. Plenty of SEs put hairpins in their slabs to help in this situation, especially with PEMB foundations. The problem with a 4"-5" SOG that is reinforced only with WWR for shrinkage is that you don't know until after it is placed where it will crack (for sure). But this is where hairpins and additional bars can come in.

And not to start another thread since this issue has been beat to death, but trying to get this equation to balance: W < 0.6D, it makes me feel a bit more comfortable using every ounce of dead load I can get my hands on (being liberal)...

 

[Ron]

slickdeals...I run into this a lot in using a slab for uplift/OT resistance on canopy structures. If the concrete is tied together (reinforced or wire mesh) the slab can be used for uplift/OT resistance. I use half the spacing between canopy columns, for instance, as the amount of concrete dead load I can use for resistance.

This only considers ultimate failure, not strain compatibility. In my scenario, the concrete would fail, but keep the structure from failing.

 

[rowingengineer]

Slickdeals,
They do have a formula for designing the uplift force of a footing without joints or shrinkage cracking. The formula is based on testing but is really a best fit of the data, with a large variation.
Ps=0.6 x Ks^0.2 x H^1.4 x b^0.56 x fr
Ps, is the uplift force on the slab at the first crack load (N); K, is the modulus of soil (N/mm3); h is the slab thickness (mm); B is the width of square footing (mm); and f, (=0.6 x fc’^0.5 (recommended this is factored down due to this be a non-structural element)) is the modulus of rupture (MPa). The ks values has very little bearing on the equation thus they suggest the use of 100 000 kN/m3 thus ks= 0.1. hopefully I copied this all correctly.

This is based on footing below the slab not integrated with the slab. When integrated with the slab you can take advantage of this as well as the strength will increase by the footing depth at the column location.


ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”