ringwall footing

[struct6]

I am designing a ring wall foundation for a tank which is 32 ft tall and 22 ft dia. I have designed the ring wall itself and now working on the footing part of the ring wall. I am planning to use a 4 ft wide footing. Since this footing will have both axial load and lateral moment due ti wind i will have to determine whether the resultant of the forces will fall within the kern limit. how do i find the kern limit for a circular footing whose Outer Diameter is 25' and Inner diameter is 21'.
Thank you

 

[JoshPlumSE]

I just ran some tests with a Foudation design programt that I have (RISAFoundation). Although, I'm sure there is a way to derive this mathematically as well.

It appears that if the footing is assumed to be rigid (a pretty big assumption), then the centroid of applied loading needs to be at a distance of 5.3 ft from the center of the circle before a the footing will experience partial uplift. Interesting little exercise.

 

[JAE]

Be sure not to make the footing too wide as you want to balance the footing pressure with the basic slab pressure under the tank.

You do not want to have different pressures, and thus different settlements between the interior part of the tank floor and the ring wall area around the perimeter as this creates stress and possible damage to the tank floor plating.

 

[BAretired]

A = pi(d^2 - di^2)/4

S = pi(d^4 - di^4)/32d

Solve for P/A - Pe/S = 0

and find e = (d^4 - di^4)/(8d*(d^2 - di^2))

d = 25; di = 21; e = 5.33

BA

 

[JStephen]

It would be unusual for a tank 32'x22' to require any kind of footing under the ringwall.

Trying to balance the bearing under the tank and under the ringwall will generally lead to an excessively wide ringwall, rather than the other way around. Even if you do balance the two, it's only balanced at one specific depth of product, and unbalanced at any other depth.

 

[BAretired]

Maybe you should read these:

http://www.eng-tips.com/viewthread.cfm?qid=219486

http://www.eng-tips.com/viewthread.cfm?qid=219662&page=20

http://www.eng-tips.com/viewthread.cfm?qid=210425&page=31

http://www.eng-tips.com/viewthread.cfm?qid=87532

BA

 

[struct6]

Thanks everybody for your valuable information. I determined the eccentricity and the eccentricity falls within the kern limit when the tank is full but when the tank is empty the eccentricity is way over the kern limit which means I will have to reduce the area of the footing. how do i calculate the reduction in area for a circular hollow footing?

 

[SeanMD]

I wouldnt think you would proportion the size of the footing so that the result falls in then kern. The load always changes, so I would take another approach. I would size the RW so the bearing pressure under the footing and the bearing pressure under the tank (at the same depth of the bottom of the ringwall, so add soil wt) would be the same for the full (hydrotest?) condition. Then add reinf steel to resist the twist from the eccentrictiy so the footing, in theory, doesnt rotate.

PIP has a good design guide for this, tank foundations.

I would think that with a 4 ft wide RW, you might get significant differential settlements at the interior of the RW and damage the tank floor.