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.

 

[JStephen]

chrislaope, there isn't any slab.

 

[cvg]

the tank pressure causes tensile stress in the tank shell. Depending on the design of the shell, there will be some strain which will cause a vertical upward force on the anchors. It could not possibly lift the tank out of the ground, it could pull the anchors out of the concrete though or damage the bottom of the tank shell.

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.

 

[dhengr]

Jstephen:

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.

 

[Vivek19]

JStephen (Mechanical),


The foundation is not a Ring wall type.It is Block type.
Almost it looks like chrislaope's sketch but teh thickness is uniform.