ringwall foundations

huangyhg

ringwall foundations
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.
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.
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.
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.
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.
why a ringwall anyway?
i am using a ring wall because we have limited space and a square or octagonal foundations are too big for the space alloted.
why use a foundation at all? ringwall or octagonal or square.
civilperson - my point exactly.  what is wrong with putting the tank on a granular "foundation pad"? no structural foundation needed.
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.

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??