Raft foundation for flat bottom grain Bin

[rittz]

We have been engaged to design a raft foundation for a flat bottom Westeel corrugated grain bin. Dia. 33 ft., capacity 39,000 bu., overall ht. 60 ft. dl 2l,000 lbs. I was hoping one or more of you fellows could tell me how you would calculate the appropriate slab thickness and reinforcing.

We have a Geotechnical subsurface soils report. The soil is glacial till (undrained shear strength: 60 kPa.. moist & very stiff) overlain by about 3 ft. of moist to wet sand. Suggestion from Geotech: replace the sand with compacted engineered granular fill. No significant water in the till to at least 25 ft. Typical winter frost dept here 5 to 6 ft.

 

[TLHS]

I'd start by quickly running some numbers and seeing if your foundation needs to do much of anything. It looks like you might have a service load around 150 kPa once you factor in structure weight. You might be in the easy range where your applied loads are less than the allowable bearing pressure of the soil. Since it's a flat bottomed bin, which should supply an area load, your foundation just basically has to transfer loads via bearing. You could run into problems if you can get significant unbalanced loads because of piles forming inside the hopper. In that case, you might need to start thinking about it as an elastic foundation, or make a few assumptions. You'll also want to check uplift in an empty case with wind and full and empty cases with any seismic.

Talk to your geotech and get their recommendation. It's better to work closely with them than try to make things up. How to design it really depends on what structural function your foundation is performing and where you need to carry your resisting geotechnical loads from.

 

[dflva]

I would look into a ring foundation or ringwall. This is a concrete ring following the circumference of the tank or bin shell. This could be ideal for your situation since you have to go so deep anyway.