Floating Slab vs. Stem Wall Foundation for Grain Storage bins 1

[lindsaylu]

I have been asked my opinion on what seems to be a rather large floating slab. The problem is that I am a steel connection/erection engineer and have little to no experience in the Grain bin foundation field.
The bin is 48'(diam)x74'(wall height) with corrugated steel walls (Sukup design). We're assuming 4000 psf soil capacity. The initial numbers (from consultant) are of a floating slab with a max diameter of about 56'(inside diam of donut is 40'). This doesn't fit the area the owner wants to put the bin, so we're wondering whether a ring wall or stem wall foundation would reduce the footprint of the foundation. Seems to make sense to me, but the EOR has said it typically doesn't.
If someone could give me a little insight into this process I would really appreciate it. Even a yes or no would set the record straight.
Thanks,

Lindsay

 

[JStephen]

Try omitting the hole in the center?
Try a square slab rather than round to fit the property better.
Check with the geotech to see if there are other options- higher bearing at deeper depths, or use piers, or possibly higher bearing with increased allowable settlement.
If the design does not allow partial uplift with wind loading, consider removing that restriction.

 

[hawkaz]

Using very rough numbers- 110 pcf product, 54' of product height (assuming 20' of clear space under the hopper)- You would have 10,750k of product. Based on 4000 psf soil bearing, you would need almost 2700 ft^2 of contact between the concrete foundation and the soil. This equates to a 52 ft X 52 ft footing.

This isn't too far off from the floating slab size proposed.

Since this doesn't fit your site, have you considered drilled piers and a cap slab?

 

[lindsaylu]

Just to clarify - this is not a hopper bin - it is a flat bottom. Thanks very much for your input. Does this mean that a stem wall is not a great idea? I am interested in the idea of piers and a cap slab, as well as an irregularly shaped pad.

Another question. I'm having a hard time finding good info on the pre-engineered bins themselves. Do the contents of the bins typically bear on the slab (reduced to 8" in the middle of the donut), or is the load carried out to the increased depth donut?

 

[hawkaz]

Talk to your geotechnical engineer and see if the soil is suitable for piers. They should be able to provide allowable loads.

You need to get plans from the bin manufacturer to verify exactly how it is loading the slab.

 

[dhengr]

Basically you have a uniform max. load on the slab, inside the bin of (height)(commodity wt.). You have a line load at the bin walls consisting of bin DL, plus roof loads and lateral load distributions from one side to the other, plus a downward frictional component for the commodity hanging up on the wall during unloading. Finally, the bin walls probably have vert. stiffeners in the lower portion of the bin which tend to concentrate the wall loads a bit. The EOR has offered the cleanest and simplest found. design; it appears he’s assumed an 8' doughnut centered under the bin wall dia. It must be reinf’d. for tension in the bottom, and acts basically like a double canti. under the bin wall, 4' inward and 4' outward. The inner most slab acts as a slab on grade for the load above it, but ends up with potential tension in the top because it might be distributing some wall load toward the center more than the 4' canti. So, this transition of tension in the bot. of the slab to potential tension in the top is important. An important consideration is that you have good uniform soil conditions under the slab so you don’t end up with ‘the leaning tower of Pisa,’ I mean ‘of the barn yard.’ If a GeoTech Engineer says there might be a few inches of settlement under full load, that can usually be designed for in these kinds of structures, as long as it’s uniform.

Anything other than a symmetrical found. about the bin wall system starts to get more complicated, because the stab canti’s. w.r.t. the bin wall get longer and shorter. Drilled piers are certainly a possibility, and they should be symmetrical with the bin wall too, and they may have a ring beam on top of them to deal with their concentrated found. loads vs. the relatively uniform bin loadings, and punching shear in the slab. They would allow you to reduce the outside dia. of the doughnut, but would probably not be less expensive.

 

[BigH]

What's interesting from the OP is that the OP is "assuming" a 4000 psf (200 kPa) soil capacity. As a geotech, I would like to know (1) has there been an investigation done to determine what soil(s) the structure will be founded on? (2) and based on (1), is there any facts to back up the assumption of a 4000 psf soil capacity? Might it be 4500 psf? 5000 psf? 3000 psf?(3) is the soil capacity based on shear or on serviceability considerations?

I think it a good idea to have this information before going off on various support systems any of which might be appropriate depending on the answers above.

Perhaps, too, and I am sure that most are aware, there have been some very classic silo foundation failures in the past - the Strathscona failure being a prime example. One may also see reference to another failure in Tschebotarioff's Foundation Book (1951) where a silo was placed on a stiff clay overlying a softer clay. Bozozuk also talks about silo failures (see National Research Council of Canada).

 

[dhengr]

I agree with BigH, this is not a light structure, and its foundation soils conditions and found. design should not be taken lightly either. Some time and money spent with a GeoTech Engineer on this project would be a worthwhile expenditure as compared to the total project cost, and particularly when compared to a bin failure. These types of bins are actually fairly fragile w.r.t. differential settlement, tipping or unsymmetrical loadings.

 

[lindsaylu]

Thank you very much for the posts. They've had a geotech for a previous bin, which will abut this location. They are definitely intending to get a geotechnical report completed, but wanted to know whether this site is a possibility before investing in an expensive report for that location.

Thank you all very much for your help.

Lindsay

 

[chrislaope]

Get the soil boring test report first. If the soil really can reach 4000 psf net allowable soil bearing capacity (or by some earth work, such as excavate certain layer and refill and compact to make the soil reach 4000 psf net allowable soil bearing capacity), then, the foundation diameter might be reduced to 52'---53', and the footing width might be reduced to 7'.