Tank Foundation Uplift Force
Tank Foundation Uplift Force
(OP)
Hi,
I am in need to design the Foundation for a API Tank. The Uplift force calculated for that tank is too heavy.The details as follows.
1. Empty Load = 11,000 kg
2. Test Load = 95,750 kg
3. uplift = 119,000 kg
4. Tank Dia = 4.5 m
5. Tank Height = 5.2m
And I was informed that , the uplift load is the net value. After the deduction of the Gravity Load ,the uplift load is 119,000 kg ( 119 Tonnes).
Which looks like ubnormal.
When I referred the API 650 code , Table 5-21a - Uplift Loads.
As per the Code , for the Design pressure, Netuplift force in N = [(P-0.08th)xD2x785]-W1,
Where ,
P = Design Pressure in kPa
th = roof plate thickness in mm
D = Tank Diameter in m
W1 = dead load of shell minus any correosion allowance and any dead load other than roof plate acting on the shell minus any corrosion allowance in kN
And as per the Clause 5.12.13 : The foundation shall provide adequate counterbalancing weight to resist the design uplift loads .
Then for the given Uplift Load , the Foundation size approximately required is , 5m dia , and foundation depth should be more than 3m.
Almost the foundation depth is more than 50% of the Tank Height.It looks like un realistic.
Is there any wrong interpretation in the Uplift force calculation for the foundation.
Since , I am a structural Engineer , I am not familiar with API codes.
Please help me to resolve the issue.
With Regards,
Vivek.
I am in need to design the Foundation for a API Tank. The Uplift force calculated for that tank is too heavy.The details as follows.
1. Empty Load = 11,000 kg
2. Test Load = 95,750 kg
3. uplift = 119,000 kg
4. Tank Dia = 4.5 m
5. Tank Height = 5.2m
And I was informed that , the uplift load is the net value. After the deduction of the Gravity Load ,the uplift load is 119,000 kg ( 119 Tonnes).
Which looks like ubnormal.
When I referred the API 650 code , Table 5-21a - Uplift Loads.
As per the Code , for the Design pressure, Netuplift force in N = [(P-0.08th)xD2x785]-W1,
Where ,
P = Design Pressure in kPa
th = roof plate thickness in mm
D = Tank Diameter in m
W1 = dead load of shell minus any correosion allowance and any dead load other than roof plate acting on the shell minus any corrosion allowance in kN
And as per the Clause 5.12.13 : The foundation shall provide adequate counterbalancing weight to resist the design uplift loads .
Then for the given Uplift Load , the Foundation size approximately required is , 5m dia , and foundation depth should be more than 3m.
Almost the foundation depth is more than 50% of the Tank Height.It looks like un realistic.
Is there any wrong interpretation in the Uplift force calculation for the foundation.
Since , I am a structural Engineer , I am not familiar with API codes.
Please help me to resolve the issue.
With Regards,
Vivek.






RE: Tank Foundation Uplift Force
RE: Tank Foundation Uplift Force
RE: Tank Foundation Uplift Force
Is this uplift caused by seismic load? which will be possible for this high magnitude at high seismic zone.
Is this uplift caused by wind? which will be unlikely to have such high uplift by wind.
RE: Tank Foundation Uplift Force
Is that net uplift some sort of an empty tank buoyance force with some factor of safety, to keep the empty tank from floating away?
RE: Tank Foundation Uplift Force
RE: Tank Foundation Uplift Force
Your uplift works out to a net pressure of around 8.6 psi, while API-650 is limited to 2.5 psi. If that is the "design pressure", something is wrong, either you have an error somewhere in the calculated uplift, or the tank would fall under API-620 rather than API-650. If that is a "failure" or "1.5xfailure" or "frangibility" load, then it may be correct.
In many cases in which an unreasonable amount of uplift is generated, it will turn out that the specifier can in fact reduce the specified pressure, and did not realize there was a motivation to do so. People that are accustomed to dealing with vessels will specify a "nominal" pressure, not realizing that it may have a major cost impact on the project. So double check there.
Keep in mind that when a ringwall extends inside the tank shell, the uplift pressure is also applied downward on that shelf, reducing the net uplift. If you use a rigid slab under the tank, the pressure induces bending in the slab, but there will not be a net uplift of the slab from the pressure. For a 5m tank, I would explore this option more. In any case, using a ringwall for weight, it will be cheaper to make it wider rather than deeper.
In some cases, it may be practical to use an inverted-T shape ringwall, in which case soil over the slab can be included in uplifting weight.
RE: Tank Foundation Uplift Force
Actually that is an API 620 Tank only. But as per some clause from API 620 , they need to calculate the Uplift as per API 650.
P is the Maximum Design Pressure = 0.07747 MPa. ( something around 11.236 psi ).
In addition to Clause 5.12.13 we need check as per Appendix F of API 650.
Are these information sufficient.
With Regards,
Vivek.
RE: Tank Foundation Uplift Force
The pressure in pressurized tank will not cause any uplift on foundation. It only cause tension on tank structure itself.
RE: Tank Foundation Uplift Force
unless your pressurized tank is filled with hydrogen or similar light air, then it will cause uplift.
RE: Tank Foundation Uplift Force
Where ,
P = Design Pressure in kPa
th = roof plate thickness in mm
D = Tank Diameter in m
W1 = dead load of shell minus any correosion allowance and any dead load other than roof plate acting on the shell minus any corrosion allowance in kN "
your W1 should be in NEWTONS not KN
RE: Tank Foundation Uplift Force
This is presumably a vertical tank with flat bottom supported by the foundation. The bottom acts as a membrane only, and the tank does in fact transfer large pressure uplift forces into the foundation.
RE: Tank Foundation Uplift Force
Without copies of API codes the OP refers to, I am not going to try to determine what the formulas or variables are or mean. But, aren't you guys kinda running around in circles here? When you say foundation uplift to a Structural Engineer, most would assume you mean a loading which tries to lift the found. right up out of the ground; thus my first post, and several others, re; wind and EQ. Because of the pressure in the tank, aren't you actually talking about a tension force btwn. the talk wall and the found. at the connection btwn. tank floor plate, tank wall and concrete found. A distributed hold down force and detail is required btwn. the tank wall and the concrete found. to prevent the tank floor plate from being over stressed in bending. With this proper hold down design, the tank floor plate acts primarily as a closure membrane, uniformly supported by the found., as JStephen suggests.
With a properly phrased OP we wouldn't be guessing at what kind of tank this is, "presumably a vertical tank." And, we would be talking about the general concept and details not some damn 10 variable formula intended to cover everything as long as we can keep our variables and units straight. When you ask your question by referring to a specific formula or code section, you do limit the help you might get to those who have that exact edition in front of them. And, some of us often wonder, maybe wrongly, is he having trouble with the darn formula and all it tries to encompass btwn. several different sets of brackets; or is the fact of the matter that he doesn't understand the basic concept and detail and has little idea what he is actually calculating, and intending to accomplish?
RE: Tank Foundation Uplift Force
RE: Tank Foundation Uplift Force
delagina- If the tank is designed for 11 psi, there will be uplift.
RE: Tank Foundation Uplift Force
[(P-0.08th)xD2x0.785]-W1,
(77-0)x4.5x4.5x0.785 - 107880
= -106655N so no uplift
he got 2 wrongs in the formula
W1 should be Newton
and it should be 0.785 not 785
RE: Tank Foundation Uplift Force
Unless the physics law of this Universe changed since I left school. I can not see how an internal force/pressure inside a tank can uplift a foundation. Could you please sketch a free body diagram to show it?
RE: Tank Foundation Uplift Force
(77,000N/m2 - 0)x4.5mx4.5mx.785 - 107880
= 1116131 N
=115000 kg
yup huge uplift there LOL
RE: Tank Foundation Uplift Force
In the formula above, P is in kPa, not Pa, and uplift is in N, not kN, so the 785 is correct- pi/4 times 1000.
RE: Tank Foundation Uplift Force
RE: Tank Foundation Uplift Force
RE: Tank Foundation Uplift Force
There could also be larger vertical forces generated by wind, seismic and / or flooding or high water table. These forces are generally considered to be "uplift".
The combined upwards force due to tank strain and uplift must be resisted by the weight of the foundation, tank and any liquid in the tank. Doesn't matter if there is a slab or not, except that the slab would increase the weight of the foundation.
RE: Tank Foundation Uplift Force
I see your sketch and can see that your ring wall foundation could cause a net uplift on the concrete ring wall. I did anticipate anchor bolts or straps and some sort of a bottom flange on the tank wall as part of the hold down details. But, won't your scheme cause tank floor plate stress problems, bending stresses and radial tension stresses, in the immediate area of the conc. ring wall to allow the tank floor plate to finally act as a flexible membrane? And, what is the anticipated tank floor plate deflection at the center, and/or alternatively, the equilibrium/compatibility upward movement of the conc. ring wall for this membrane action to take place? No doubt this found. and tank design could be done either way, but I was thinking more along the lines of Chrislaope's sketch to provide a stiffer found., hopefully more uniform movement/settlement, and to avoid the uplift problem by supporting a lighter tank floor plate, just a closure and radial tension plate. In your opinion is one found. scheme significantly better than they other, and why? Does API-650 only allow the ring wall found.? I don't doubt that that can be made to work, but there will be funny (difficult, nasty) tank and found. deflections and movements as the tank pressure varies from max. to min. and the tank changes from empty to full and back again. I would certainly assume that API-650 contemplates and accounts for these.
My comments otherwise, were that a more detailed OP might have laid out a few of these basic tank design details, in a general way, and saved us the need to guess at all the possible variations. Obviously, Chrislaope and I didn't imagine the same ring wall found. you arrived at. Otherwise, that is a clever formula, but they don't always translate well from one word processing format to another forum formatting system, short of writing out 'diameter squared,' so there is potential for confusion. We all have trouble with units and variables at times, the more so as we (old goats, maybe just me) switch btwn. unit systems, one of which we don't have a life long experience with and feel for. It seems to me that the way some of these formulas show up in codes and are taught these days is so clever and convoluted that we lose the step by step concepts of the real problem as we struggle to keep all the convoluted variables and units straight. And, in the process we never end up with a real feel for a reasonable answer or magnitude. Today's codification and the all encompassing formulas, with six different sets of brackets to include six steps into one, hides the real problem thinking process, and turns it into an algebraic manipulation. Just follow the cookbook, and all will be well; never mind if ten or a million is the right magnitude or if you really understand the problem at hand.
RE: Tank Foundation Uplift Force
The foundation is not a Ring wall type.It is Block type.
Almost it looks like chrislaope's sketch but teh thickness is uniform.