Tank Foundation Uplift Force 3

[Vivek19]

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.

 

[JLNJ]

Is this an underground tank?

 

[dcarr82775]

Where is 'P' coming from?

 

[chrislaope]

Information not clear.

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.

 

[dhengr]

Vivek19:

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?

 

[delagina]

post vendor drawings of tank if you can. you may have units wrong. idk. i have designed a few tank foundation and never had problems with uplift. or maybe nothing wrong with the tanks, it's just that you have pressurized tank.

 

[JStephen]

A flat bottom pressurized tank can generate large uplift forces. Check your units and equations to make sure you have it figured right, though. And unfortunately, API has been known to have typos, so check that the equations make sense. (The factor on the roof thickness is just deducting unit roof weight from uplift pressure. The 785 factor is pi/4 with unit conversion factors. These equations basically boil down to P/A and Mc/I type equations.)

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.

 

[Vivek19]

JStephen (Mechanical),


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.

 

[chrislaope]

Agree with delagina.

The pressure in pressurized tank will not cause any uplift on foundation. It only cause tension on tank structure itself.

 

[chrislaope]

unless your pressurized tank is filled with hydrogen or similar light air, then it will cause uplift.

 

[delagina]

"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 "

your W1 should be in NEWTONS not KN

 

[JStephen]

"The pressure in pressurized tank will not cause any uplift on foundation."

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.

 

[dhengr]

Vivek19:
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?

 

[delagina]

my guess would be OP's unit is wrong if he used Newton instead of KN for W1 then there would be no uplift.

 

[JStephen]

dhengr, API-650 only covers vertical tanks with flat bottoms. With a ringwall foundation, a pressure in the tank does tend to lift the foundation right up out of the ground. The "distributed hold down" is anchor bolts or anchor straps.

delagina- If the tank is designed for 11 psi, there will be uplift.

 

[delagina]

lets do the math

[(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

 

[chrislaope]

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?

 

[delagina]

oops there must some errata with these formula as units are not consistent.. anyway,

(77,000N/m2 - 0)x4.5mx4.5mx.785 - 107880

= 1116131 N
=115000 kg

yup huge uplift there LOL

 

[JStephen]

Sketch attached if that helps clarify. Note that the bottom plate is considered a flexible membrane in this case, rather than as a rigid plate.

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.

 

[chrislaope]

OK, I think you are playing some games on terminology, like dhengr said, your uplift is not the same meaning as the conception in structural engineering. It is acturally an internal load which need be solved in structural design and you have to take the foundation slab as part of the tank bottom structure to resist shear and bending of the tank bottom. In this case, a minimum of 18" Thickness Slab with #6 Rebar @ 10” BothWay at bottom layer of the slab plus some rebar @ top layer will do. There is still no uplift need be considered. Please see attached sketch.