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Re: Vessel or Stack Foundation - Shallow Bedrock

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Bill,

I've done this dozens of times. They have all been on piles or on spread footings. The footings or pile caps have all been either square or octagonal except in a couple of cases. There is quite a bit of trial and error in determining the size.

Pile Foundations
   For any pile group

       P = W/N (+-) M*c/I

where P = pile load.

W = weight of foundation, vessel, and everything else depending on the load case.

c = distance from the axis under consideration to the furthest pile.

           I = moment of inertia of group, = sum (R^2)

           R = distance from axis to each pile.

I'm sure you've done this before for bolt groups with eccentric loading.

Spread Footings (For overturning about major or minor axis)
   Square (This is the easiest of the three)

F.S. = R/e

Where F.S. = factor of safety against overturning.

           R = distance from centre of loading to edge of footing.

           e = eccentricity = M/W

           M = overturning moment.

Part pf the footing will have zero soil pressure under it. If you have a sandy soil this may be objectionable to the geotech particularly if overturning is a result of seismic loading. The soil pressure will be triangular; width of the loaded area will be 3*(R-e); and the maximum soil pressure will be twice the average.

Working about the diagonal axis it's a little messier but I think you can handle it.

Octagonal Footings.

For octagonal footings the geometry causes it to get quite messy. I happen to have a couple of papers on the subject of octagonal footings which I will send to you separately. I would attach them here but seaintlist doesn't accept attachments. They are both about 50 years old but arithmetic is still arithmetic so they are still useful. Feel free to distribute them to anyone who asks.

       Hope this helps.

Regards,

Daryl

----- Original Message ----- From: "Polhemus, William" <wpolhemus(--nospam--at)craworld.com>
To: <seaint-seaosc(--nospam--at)mail-list.com>
Sent: Wednesday, August 28, 2013 12:50 PM
Subject: [SEAINT-SEAOSC] Vessel or Stack Foundation - Shallow Bedrock

Has anyone here got experience with a foundation for a tall vessel or stack
e.g (standalone, not guyed) in a condition where there is fairly shallow
bedrock?

I'm looking at the possibility of such a thing. Typically I use deep pier
or pile foundation, but In this case there is assumed to be bedrock at 8 to
10 feet depth below ground.

I assume I could then use a "chunk" foundation bearing directly on the rock
in such a case, recognizing I might need a spread there to deal with the
overturning.

In that case what do I need the geotechnical to tell me?

William L. Polhemus, Jr. P.E.

5

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