ACI Guideline on Differential Settlement of Concrete Footings

[allaboutsoil]

Hi,

I searched the forum but could not find an exact answer to my question. I hope I am not duplicating a previous question.

I am designing a footing for a ground storage tank with a diameter of 110 feet. Bearing pressure will be around 1,800 psf. It looks like we can go with shallow footings, only if we can satisfy the differential settlement requirement of such tanks. One of my collegues told me that ACI (332?) have some requirements on differential settlement such that for every 10 feet of radius, one fourth of an inch of differential settlement is allowed.

This seemed kind of high to me and I could not find the reference to support it. Do you have any information regarding this reference?

Thanks.

 

[BigH]

Why are you putting a a 30ft high 110ft diameter storage tank on foundations? Is it the ring wall you are talking about (which I don't think you need). I would just use a granular pad extending some 10 to 15 ft beyond the wall of your tank, about 5 ft thick raised up above the original ground. Assuming you have reasonable soil beneath. Typically you would camber the centre of the tank since the edge settlements will be smaller than the centre settlement. If in clays - and they are "dessicated" - check out Bjerrum's article in the 1957 ISSMFE Conf from London on edge failure of storage tank.

 

[darthsoilsguy2]

Try posting in API Code Issues too... Since, any differential settlement has implications on the tank structure. I don't recall tolerances in API 650 and API 653 but foundations are definitely discussed as well as settlement.

 

[Ron]

Agree with BigH...why do you need a "foundation". At most a ringwall, but more likely a stable drainable base.

 

[allaboutsoil]

Thanks for the responses.

All the storage tank designs (water, chlorine, etc.) I have been involved so far included some type of foundation, either it is mat foundation or deep foundations based on the soil profile. A stable base never came to my attention as an option. How do you design that; just install a 5-foot-thick granular pad and do not worry about total or differential settlement? How do you calculate the settlement for such a base? Do you treat as a shallow footing in settlement calculations?

I will check into API codes, too.

Thanks.

 

[BigH]

Steel tanks settle. No question - but the flexible base can take it - as indicated, you can camber the centre so that after a while, the bottom "evens out". To be honest, and I've been involved with some 50 tanks or so - some of 150ft diameter and a bit larger - none have had mat foundations nor ringwalls (Ontario refineries). If settlement (after the camber build-out) is uniform, what's the problem? You should use flexible connexions in any event.