RINGWALL FOUNDATIONS

[MiguelD]

I am designing a ringwall foundation for an above ground tank. I am not sure how to find the max bearing pressure on the soil due to the weight and moment of the tank. There is an equation in the api 650 which is what i am using for the design. The eqn is as follows (1.273Mrw/D^2)-(wt(1-.4Av)

where Mrw is the moment at the ringwall surface.
D is the diameter of the tank.
wt is the line load from the tank shell and roof
Av is vertical seismic pressure.

is this the correct equation to use to find the max soil pressure? Is there any other approach? Why doesn't this equation take into account the wight of the product inside the tank?

Your help is appreciated,
~Miguel Diaz E.I.T.

 

[civilperson]

I reread my API 650 looking for the formula cited without success, (API STANDARD 650, TENTH EDITION, NOVEMBER 1998, ADDENDUM 1 & 2, NOVEMBER 2001). The soil pressure beneath the ring is: weight of wall and roof divided by the area of the concrete circumfrence ,pressure of the concrete foundation , area of floor over the foundation times the fluid pressure when full, the moment due to wind or seismic divided by the anular moment of inertia times the distance from the centroid.

 

[MiguelD]

THE EQN CAN BE FOUND ON API 650 TENTH EDITION ADDENDUM 1,2,3,4 PAGE E-13 JUST BELOW SECTION E.6.2.1.2 THIS EQN IS TO FIND THE TENSION IN THE ANCHOR BOLTS SO I FIGURED THAT IF INSTEAD OF SUBTRACTING I ADDED THE PRESSURE DUE TO THE SHELL LINE LOAD AND THE PRESSURE DUE TO SEISMIC EFFECTS THEN I WOULD GET A NUMBER FOR COMPRESSION AND TO THIS NUMBER I WOULD ADD THE PRESSURE DUE TO THE FOUNDATION WEIGHT.

 

[kslee1000]

I couldn't comment on the equation as I haven't used it before.
However,unless your tank botom is specifically designed to carry the load to the wall/foundation (a rare case), the ring wall foundation usually only supports the tank shell and its roof. The weight of the content is carried by the fill/soil material directly below (usually sloped to account for settlement).
You should consider the following load combinations:
1. Selfweight + Wind (Tank empty)
2. Tank full during earthquake.
Now you can perform simple hand calculation to get the max/min bearing pressures.

 

[IFRs]

Using API 650 (by the way the 11th edition is out) you can determine the pressure at the bottom of the shell from wind and earthquake loads. These would add to dead, live and liquid loads to get toe pressure. API 650 also has load combinations in Appendix R (I think it is R) to help you combine load cases.

 

[BigH]

Why a ringwall anyway?

 

[MiguelD]

I am using a ring wall because we have limited space and a square or octagonal foundations are too big for the space alloted.

 

[civilperson]

Why use a foundation at all? ringwall or octagonal or square.

 

[BigH]

civilperson - my point exactly. What is wrong with putting the tank on a granular "foundation pad"? No structural foundation needed.

 

[JStephen]

In some cases, the design can get rather involved.

The anchor bolt equation does not include weight of the contents because they figure that you'll load the anchor bolts without significant movement of the shell, which will not allow any uplift resistance from the contents on the bottom plate. When you're figuring loads on a ringwall, the liquid directly over the ringwall will be included in the design, though.

Ringwalls can usually be sized using P/A +/- Mc/I. The design either needs to preclude uplift or include it in the design, and needs to meet allowable bearing criteria.

Why to use a ringwall- for a more stable foundation, basically the same reason you'd put a concrete slab under your house instead of building it on gravel.

 

[emmgjld]

I agree with BigH. Why use the ringwall??
I can think of many instances where a ringwall is useful or required. So far, there has been nothing in the problem description which would indicate to me that a ringwall would add anything to the structure, except cost.

There are thousands of large, significant tanks without concrete foundations. There are thousands of smaller tanks here in western Colorado alone without concrete foundations. I know of cases where the ringwall contributed to distress after differential settlement. Actually figuring the applied loads to a ringwall and making the system work properly is a real chore, as this post illustrates.

Why use the ringwall??

 

[BigH]

The analogy to putting a house on gravel is not correct - in a large tank, the steel plate is in fact a mat. As emmgjld indicated, I have been involved with quite a few above ground storage tanks (some up to 50 to 60 m in diameter and none had a concrete ringwall. We used a 1.5 m thick layer of well graded sand and gravel (usually crushed) topped by 3/8 inch crushings for leveling. This has been confirmed by a colleague (quite reknowned soft ground geotechnical engineer in India) where he has similarly designed tank foundations for over 1500 tanks.

 

[IFRs]

The ringwall also serves to contain the fill under the tank from spreading outwards under the liquid load. I see a lot of tanks on ringwalls (recently 225' diameter) and quite a few on full slabs (recently 145' diameter). Geotechnical analysis of the soil conditions are required to determine if a ringwall is needed or if a simpler (or more complex) foundation is reaquired.

 

[BigH]

The tank pad foundation usually extends about 10 ft minimum beyond the end of the tank. If the soil is so soft or loose that you have "edge" effects, then I would think that the bearing on the ring wall would overstress the soil as well. As it is, we are all prisoners of our own (and company experience) - it is just that I don't ever remember seeing a tank that I have been involved with with a concrete ringwall.

 

[kslee1000]

Without concrete foundation, how to place uplift anchorages, which may be need for strong wind event (while tank is empty)? Soil anchorage? Cable tie-downs? I have never involved in a tank design without concrete foundation. Maybe cost more, but I sleep better.

 

[engphila]

If you can find it, there's a great design guide for foundations supporting vessels called PIP STE03350. It also provides a decision tree towards evaluating whether a ringwall foundation is appropriate for your tank.

engphila

 

[edwin17th17]

tank foundation is actually in PIPSTE03020