Tank Foundation Settlement - Api (american Petroleum Institute) Code Issues - Eng-tips

Tank Foundation Settlement - API (American Petroleum Institute) Cod...

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Tank Foundation Settlement thread809-259243

I would appreciate hearing from anyone that has had experience in tank settlements, both uniformly & differentially. API 650 doesn't address this very much that I can find other than: Par. 5.10.4.7 d. "The columns shall be effectively....in the event of vertical movement of the columns relative to tank bottom of up to 3 in.". Par. 7.3.6.6 (hydro test)"...differential settlement greater than 1/2" per 32 ft. of circumference or uniform settlement of 2 in."...... Neither of these paragraphs address differential settlement across the tank bottom. Other paragraphs refer to "excessive" settlement without defining what is meant by "excessive". Case in point - I am designing a tank that is 100' Dia. x 64' Shell Height, 1% bottom crown up from center to outer shell. Single center column-supported roof with rafters. Geotech report says I can expect settlement of 8.4" at edge and 15" at center of tank which equates to differential settlement of 6.6" Question: do I take the 3" vertical movement of "up to 3" mentioned in Par. 5.10.4.7 d, as the max. differential allowable or am I missing something here? Any thoughts, suggestions, past experience, etc. would be really appreciated. I realize that API 653 has some guidelines but this is a new tank and thus API 653 does not apply. Any other industry standards that would apply?

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Can you compact the soil before building the tank? Can you improve the soils so that the settlement will be the same magnitude inside and under the shell? Can you build the tank bottom such that when it settles it does not stress the shell and winds up where you want it? Does the geotech expect any further settlement? You can also refer to section 7.5.2, 7.5.5 and App B of API 650. Essentially what IFRs is saying is correct. You should design and construct settlement.

your foundations to limit differential

If the tank settles uniformly this is not too much of a problem, provided you know what to expect, and your geotech investigation should give you a pretty good idea of what to expect here (usually conservative). We have built tanks on extremely horrible ground and used stone column piles to limit differental settlement. Those tanks still uniformly settled over 600mm!!

3/9/2018 4:26 PM

Tank Foundation Settlement - API (American Petroleum Institute) Cod...

http://www.eng-tips.com/viewthread.cfm?qid=259243

If you cant avoid dif settlement (or are worried about it) you can build pockets into the ring beam such that you could airbag lift the tank after the foundation has settled and re level it. This is a last resort!!

Geotech report highlights a predictible settlement of 8.4" (21 cm) at the edge and 15" (38 cm) at the centre of the tank ; Do never consider a geotech report as conservative or not, as it is a factual report which results must be respected by each party involved ( civil contractor, customer, tank contractor) The difference between the edge and the centre (6.6") does not correspond to what is usually considered as the differential settlement ; for differential settlement definition, refer to API 653 Appendix B API 650 11th Art 5.10.4.7 does not apply in the present case, it refers only to bottom plate local deflection to guarantee supports legs stability ; it addresses locally bottom plate deflection and earth backfill deformation underneath but without giving any acceptance criteria for earth foundation deformation API 650 Art 7.5.5 is the criteria for foundation checking before tank hydrotest only, when the tank is empty. API 650 11th Art 7.5.2 is an additional criteria to be checked before tank hydrotest only, when the tank is empty, after tank foundation checking according to Art 7.5.5 ; Contractor or Customer shall perform a topographical survey to measure shell verticality and roundness ; note that when tank foundation is OK, out of roundness / verticality defects result from tank construction only. If your tank is cone up type, you can easily manage the difference between centre and edge ; the min requested slope at long term for a cone up is 1 %, so you have to built the earth backfill foundation underneath the bottom plate with 1,6 % in order to anticipate the backfill deformation under loading/offloading cycles during 30 years ; with D = 100 m , the centre will be at 1,6 % x 50 m = 80 cm > 38 cm predictible which is still less than 1 % min ie 50 cm More touchy to manage is the edge settlement ; API 650 11th Art 7.3.6.6 gives now acceptance criteria for settlement at hydrotest but they seem to be very stringent and may be unrealistic with no real feed back from construction sites and civil contractors; I mean that a lot a tank foundations will probably be refused even in presence of good soil & foundation conditions ; my personal understanding is that this is to protect tank builders responsibility On the other hand, API 653 Appendix B acceptance criteria, based on tank shell allowable deflection is very less stringent, applicable to existing tanks However you have to perform your design according to API 650 11th criteria ; assuming you have an external floating roof on such a tank ; For a 100 m D tank, the max distance being 32 ft (9.75 m), you should consider 36 measurement points 8,73 m distant and check the measured differential is less than 1,3 mm/m x 8.75 = 11,4 mm; For new tanks, it is very usual to consider Pr De Beer criteria; still referred by Ph. Myers (AST book, 1997) which is ∆Si <= L/450. With : - ∆Si : edge foundation differential deflection calculated at each point of tank circumference, from settlement measurements and analysis, - L : distance between 2 points considered on tank circumference ; in your case, L = 8,75 m - ∆Si = Si – (S i-1 + S i+1) / 2 - Si : variable settlement calculated from total settlement measured (levelling measurements on site) reduced of uniform settlement (definition as per API 653 - App B) A 21 cm predictible settlement at the edge (corresponding to the total settlement value) is not a big deal for a 100 m D tank ; The cheapest recommended solution is the preloading ; lets assume you need about 50.000 m3 earth to build the bund walls around, this is equivalent to 5 m x 1,8 = 9 t/m2 preloading on tank footprint which is 60 % of the total long term load for crude oil ; preloading time is depending on soil nature ; this is an additional geotech study ; More expensive are stone columns and for really bad soils and they have to be dropped at the right depth to be efficient It is better and cheaper to improve the soil at foundation construction, tank lifting by any mean is very expensive and the cost of business interruption must be added ....

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3/9/2018 4:26 PM

Tank Foundation Settlement - API (American Petroleum Institute) Cod...

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3/9/2018 4:26 PM