Uplift In Storage Tank Foundation

[fahedjavaid]

hi i fahed
i have to design the Foundation for 77.5m dia 17m high storage tank foundation,
The problem is
Tank self weight = 1900 MT
Internal Pressure = 0.175 kg/cm2
which translates into abt 15000 MT of uplift force as per API 650.

also API 650 Appendix F 7 requires 1.25 x Uplift against empty weight of Tank...so i cant use weight of liquid advantage .....

i have tried all thinkable solutions to counter this uplift force , Ring beam Padded foundations, piles etc etc .....

can any body help me in this ????

BR
Fahed

 

[BigH]

Are you sure your uplift number is correct? I don't have API 650 so can't check . . . but the height of the tank is quite small to the diameter. We used to design all our tanks on gravel foundation pads extending 5 to 8 m beyond the edge of the tank . . . Are you in a very highly seismic zone???

 

[fahedjavaid]

Thanks for reply;
no this is riyadh area, very low geological activity.. but this uplift is coming due to internal design pressure of 0.175kg/cm2 ....
n for the same reason i have to provide large number of anchor bolts .....but is the foundation i cant figure out uptill now ...

due to very large diameter of tank this internal pressure translates into immense uplift forces approx 15000 MT ....i have checked n double checked ... Pressure x Area of roof = Uplift

 

[Ussuri]

Does the internal pressure not also act on the base of the tank? In which case balancing the pressure on the roof?

 

[JStephen]

The problem is that it is an unreasonable pressure for that size of tank. You need to contact the tank designer and tank specifier and let them know what is going on. In most cases, when a too-high pressure is specified for a tank, they are able to reduce that pressure once they realize it has a major impact on the tank and foundation design (and cost).

API-650 includes allowances for pressures up to 2.5 psi, which is what is used here. For smaller tanks, the uplift is manageable, and may result in a larger ringwall. For larger tanks, you come up with unreasonable amounts of top angle area required, unreasonable anchorage, and unreasonable foundation sizes.

Note that the test pressure and failure pressure must also be considered as a design case, and may control rather than the design pressure.

The tank and foundation could likely be built for that pressure, it would just be very expensive. One thing to note on foundation sizing is that as you extend a ringwall inside the shell, the tank design pressure is also acting down on that part of the ringwall, which will offset some of the uplift. Using an inverted-T foundation will allow you to use soil weight instead of just concrete weight. If you hold the capacity and make the tank taller and narrower, uplift problems will be reduced.

 

[fahedjavaid]

Thnks JStephen
same conclusion i has come over too... the only solution to remove the condition of this irrational pressure requirement ..for this size of tank ...however if client does not do that i will surely share the a

http://wwwwww

some design i will have to come up for th design of this tank foundaion

already tested extension of foundation to use soil weight but 15000 MT of uplift .......no soil weight can match that

 

[dhengr]

77.5m dia. and 17m high steel tank, with 2.5psi internal pressure.... wow. Isn’t there any pressure relief system for this tank? And, if you can’t design a foundation and hold down system for this tank, at that pressure, how are you ever going to design a roof system spanning 254' in dia. for a 360 lbs./sq.ft. pressure? I’m no tank guy, but this sounds kinda crazy.

 

[SlideRuleEra]

fahedjavaid - Ussuri has offered a clue to the answer. Appendix F "Anchored Tanks with Design Pressures up to 18 kPa (2'/2 Ibf/in.2) Gauge" is for design of the tank, by the tank manufacturer, NOT for design of the foundation system.

Section 5.12 "Anchorage" gives the requirements for foundation design.

The entire API 650 document, 2007 Edition (all 449 pages) can be downloaded (free) here:

https://law.resource.org/pub/us/cfr/ibr/002/api.650.2007.pdf

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[ztengguy]

Is this a above ground tank? How does the internal pressure cause a overturning force? what am I missing on this post?

 

[JStephen]

The API-650 standard has been updated several times since 2007, including overturning criteria. It doesn't address foundation design in detail, but does have some requirements for foundation design.
Ztengguy and Ussuri- the tank is a flat-bottom tank, with thin steel plate acting as a membrane across the bottom to seal it. Internal pressure acting upwards on the roof is not counteracted by pressure on the bottom, but must be transferred to the foundation if it exceeds the weight of the shell and roof.
dhengr- The roof plate is not necessarily self-supporting, it can be structurally supported in the downward direction and still resist pressure as a membrane in the upward direction. Though, as noted, on a tank this large, it leads to unreasonable results.

 

[dhengr]

Jstephen:
Your explanation is exactly what I was thinking, but that’ll still be one hell of a roof truss system for a 360lbs./sq.ft. uplift, less DL. What is the 2.5psi pressure requirement all about? Don’t they have some sort of a pressure equalizing system (relief valve, breather, some such) on these tanks? I assume it has to do with vapor pressure changes due to heating and cooling of the tank contents.

 

[fegenbush]

dhengr,

The rafter system does not resist the uplift. The membrane stress in the roof plate (which is not attached to the rafter) resists the uplift force.

The 2.5 psi pressure requirement could be caused by many things, here are two possibilities:
1) A nitrogen blanket on top of the product to limit evaporation losses. The relief valves must have a setting above that of the nitrogen blanket, further increasing the pressure the tank must resist.
2) A well meaning process engineer read in API 650 that the limit of the standard is 2.5 psi and wrote that as the requirement since they want the most robust tank for their client. While they may be good intentioned, they are costing their client additional money unnecessarily.

Either way, the only design that is reasonable for a 2.5 psi rated tank is a full slab. If concrete weighs 144 lb/ft^3 (1 psi/ft of thickness), it would take an area of concrete equal to the tank area 30 inches thick (for the case of 1.0x design pressure) or 37 1/2 inches thick (for the case of 1.25x design pressure). This assumes that the roof and shell weight is negligible.

 

[fahedjavaid]

Guys thanks to all
for showing interest .....but as "JStephen" , "fegenbush" said .....
this is really the case in hand ....API 650 does not give detailed foundation outlines but gives min value as per the Tank Design...so foundation must resist those min Uplift forces.

for the foundation design we have consider the tank weight as empty .. to take into account the Low liquid level with nitrogen blanketing ....

After Going through all the Mess i am now trying to use Ground/ Rock anchors n checking if this system can resist the uplift ...fingers crossed

 

[IFRs]

Helical piles?