Grain Bin Inverted "T" Foundation Design Methodology
Grain Bin Inverted "T" Foundation Design Methodology
(OP)
I am working on the foundation design for a large grain bin (90' dia. x 120' tall). Due to the high loads and the fact that there is a tunnel running through the center of the tank I am going to use an inverted "T" foundation. This will allow me to get the footing down to bedrock where I will have a much higher bearing capacity (6 tsf), and with the taller foundation stem wall (7'-6"), I can pass the access hole for the tunnel entirely through the stem wall without interrupting the ring footing.
I am curious about what is common design practice when it comes to handling the outward thrust on the stem wall from the grain surcharge. From what I see I can handle it two ways, either design the inverted "T" foundation as a retaining wall, or size hoop steel in the stem wall to carry all the hoop tension. Perhaps a combination of the two (belt and suspenders)? I realize the hoop steel would have to pass through the headers over the tunnel on either side of the tank in order to remain continuous, bar laps would have to be staggered, etc. Any references that specifically address this design would be welcome too. Also, if there are any other design nuances I should be aware of feel free to chime in. Thank you in advance!
I am curious about what is common design practice when it comes to handling the outward thrust on the stem wall from the grain surcharge. From what I see I can handle it two ways, either design the inverted "T" foundation as a retaining wall, or size hoop steel in the stem wall to carry all the hoop tension. Perhaps a combination of the two (belt and suspenders)? I realize the hoop steel would have to pass through the headers over the tunnel on either side of the tank in order to remain continuous, bar laps would have to be staggered, etc. Any references that specifically address this design would be welcome too. Also, if there are any other design nuances I should be aware of feel free to chime in. Thank you in advance!






RE: Grain Bin Inverted "T" Foundation Design Methodology
RE: Grain Bin Inverted "T" Foundation Design Methodology
Since these loads are so high I wasn't sure if I should consider some overturning effect on the footing as the hoop steel elongates. These loads are much higher than what I have dealt with in the past. Thanks again.
RE: Grain Bin Inverted "T" Foundation Design Methodology
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.
RE: Grain Bin Inverted "T" Foundation Design Methodology
RE: Grain Bin Inverted "T" Foundation Design Methodology
One strategy that could be used to mitigate this concern would be to use post-tensioned cables for your hoop steel. I fully acknowledge that not every project can afford to pony up for fancy technology like that however.
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.
RE: Grain Bin Inverted "T" Foundation Design Methodology
RE: Grain Bin Inverted "T" Foundation Design Methodology
RE: Grain Bin Inverted "T" Foundation Design Methodology
With your situation at 90' diameter the bridging is not as significant and you do need the outward retention from the stemwall horizontal steel or . At 90' diameter x7.5' high, you will likely not develop full tension from hoop steel all the way around and you will likely model it more like a shear wall. I would design the slab as the top restraint with footing as base restraint and have the vertical steel do the job. Then your slab is more like a metal building/hairpin type design. I would not isolate the slab from stemwall. Also your tunnel sidewalls need to have the same lateral pressure applied.
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MAP
RE: Grain Bin Inverted "T" Foundation Design Methodology
focuseng, I had considered using the slab as a tie, but both the reaction sheet from GSI and the Geotechnical report are recommending isolating the slab from the stem wall. I think GSI is recommending this because of interruptions in the slab that will not make it continuous from side to side and I think the geotech is recommending this because the stem wall footing will be sitting on bedrock and the slab will be on structural fill. You are correct about the embedded beam over the tunnel opening as well.
They are providing two surcharge loading values; "Floor Grain Load" (3,692 psf) at the exterior walls, and "Non-Dynamic Floor Load" (5,420 psf) at the center of the bin. In the process of developing your manual it sounds like you sized the footings for overturning yet still sized hoop steel. I'm assuming you did not consider the wall "propped" by the slab. How did you determine the amount of hoop steel required? In my situation if I were to size the hoop steel to carry all the load I would need to resist a hoop force of 1.6 x 3.692 ksf x 7.5' x 45' = 1,993 kips. This would mean I would need 1,993/0.9/60ksi = 36.92 sq. in. of steel in the stem wall. This is why I was only going to size the hoop steel for whatever forces are left after accounting for the passive strength of the soil and base friction. Does this seem reasonable?
RE: Grain Bin Inverted "T" Foundation Design Methodology
RE: Grain Bin Inverted "T" Foundation Design Methodology
1) Assume poor soil at about 2-3 ksi bearing, maybe we had multiple tables for a few soil types.
2) The footing was based on full bearing and bending. Just a big donut! Some of those big tanks were pretty awesome in the amount of concrete.
3) Stemwall was also just simple. mostly temp and shrinkage rebar
Frost typically did not extend much past a 4' difference between t/footing and t/slab. The footings were thick so it was easy to get a short wall fairly deep. I remember permafrost were the easy projects -just a big mat:). I worked there back in 1999. I do think you will have a lot of trouble developing and maintaining the tension around the tunnel penetration. 98% of your bars will be terminated and you won't be able to bundle enough through the header at the tunnel if you have a big steel beam in the way. The header at the tunnel was not real deep- That is why we used the steel beams there - Just not enough depth.
How wide is the footing? Can you make a buttress/wing walls at even increments to use the footing size to your advantage? Can you use the tunnel walls to act as tie beams? Just trying to think of ideas. I think you could make your footing restrained nicely with grade beams. Can you post your footing layout so far?
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MAP
RE: Grain Bin Inverted "T" Foundation Design Methodology
RE: Grain Bin Inverted "T" Foundation Design Methodology
The radius on the stemwall is 45' so in a way you can think of this as a continuous retaining wall with never ending wingwall (until you get to the tunnel opening) or angled out abutments -design as shearwalls to hold the tension. The footing can be offset or shifted either way to aid in this and you will have plenty of weight and rebar to do the job. At some point the curvature has to work for you right?. Also- you will make a continuous footing/slab to make the bottom of the tunnel which can be used as a tie beam. Since the tunnel opening is an obvious problem, use the sidewalls of the tunnel as tieback shearwalls/counterfort at that location. If you have to put in some wing walls it will be a drop in the bucket -less backfill right
Maybe a bit unusual to model all this but use the geometry and natural weight of the footings and walls. After all you only are putting in 680 cy of concrete for the footing alone (how many trucks is that??
Gosh I really just want to tie the slab to it! It is just so CLEAR
One more odd question, will there be a dryer floor installed? Don't know that it matters much but if you have this floor space maybe there is opportunity there too.
Also apologies for the excessive smileys. it is late and I am trying to avoid real work on a Saturday night.
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MAP
RE: Grain Bin Inverted "T" Foundation Design Methodology
To answer your questions, yes the tunnel does run all the way through the so there will be two penetrations in the stem wall. Like you said, the tunnel walls and slab would make a good tension tie at least along one axis. I believe there is a dryer floor. I'm not familiar enough with them to know what you have in mind.
Maybe a little off the wall (no pun intended) but I had also considered using the PCA "Circular Concrete Tanks Without Prestressing" to model the stem walls. Once again the tunnel kind of throws a wrench in the approach.