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Octagon Base Foundation 2

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thanhtang

thanhtang

Structural
Jul 5, 2006
8
I have an 1' thick OCTAGON FOOTING down below the grade. On top of the octagon footing is an smaller octagon pedestal. The pedestal is 6' thick and 1' above the grade. There are vertical load, moment, and shear on top of the pedestal. I can calculate the total moment M and vertical load P at the bottom of the footing.
I need to CALCULATE THE FOOTING BASE PRESSURE to compare against soil bearing capacity provided from Geotechnical Report. I also need to CALCULATE STABILITY RATIO.
I know how to get it if the footing is rectangle, square, and round shape, but not octagon shape. My text books did not help.

Please advise me with approach, general formulas, etc.

Footing and pedestal size or details and applied load orientations are not necessary though.
 
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I'd just call it a circle with radii perpendicular to faces and and lose 5% theoretical area, if you feel comfortable with that. the reinforcing may have been done or may be done to act like a circle anyway.
 
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DarthSoilsGuy,
It is a good idea. However, octagon foundations have being used for long time that I guess there are particular approach and formulas to calculate the base pressure for this type of shape.
If I do not have anything when the time comes, I will treat it like a round shape as you suggested. But I still prefer to have it treated with its actual shape.

Thanks
 
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Do you need the shape of the footing to match the shape of the column? It would be easier to have an actual rectangular footing, both engineering-wise and for constructibility.

If you mean with reference to eccentric loads, some of the equations commonly used are based on the stress P/A [±] Mc/I : axial stress plus moment stress. You may be able to use that here with the properties of your octagon. I have not done this myself.
 
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UcfSE

This is an existing foundation that will be reused to support a new vessel if possible. The old vessel is replaced with a new vessel of the same kind.
My task is to check if the existing foundation is adequate to support the new vessel.
The base shear and the moment (due to wind load on the vessel) on the top of the pedestal creates moment on the footing.
The P/A ± Mc/I is good for the case that the footing base area is fully in compression. It may be the case that the footing base area is in both compression and tension.
I need to check the foundation in some worse load cases that may result in both types of pressure distribution mentioned above.

Thanks
 
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To me, I would use a round shape. An octagon is almost round anyway you look at it.
 
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The normal design procedure is to treat the octagon as a circular base with a diameter that of an inscribed circle. Mark Fintel's handbook for concrete structures might have what you want. It's been a longtime since I used his book, but it might just cover the octagon (or hexagonal) shapes

Joe Tank
 
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JoeTank is correct - I picked up Fintel's book in India - not the best quality of printing but acceptable for the most part. He has a pretty extensive discussion on the topic at hand.
 
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Thanks to all. All comments help me in certain way.

The analysis with Octagon shape was done last weekend . I came up with the Net Allowable Bearing of 3 ksf. The Geotechnical Report is not submitted to us yet, but it is likely that the Net Allowable Soil Bearing capacity will be 3 ksf which is just in line. (I have 3ksf on other Geo. Report performed in the vincity of the investigated location)
You can see it is extremly marginal.

Again, I appreciate all of your helps.
 
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