Hello all,

I am designing a storage tank with dome roof.

As per Annex F, the participating area at the junction shall exceed the required area as per F.2.

My question is, there is an another formula in clause 5.10.6.2 stats that the participating area as per F.2 shall exceed this requirement too..

Is it really required to calculate the required area twice (as per F.2 and 5.10.6.2) to make sure the junction is safe..

Or the formula in 5.10.6.2 is only for self supported dome roofs..? If so, in tank software, I specified as self supporting roof. But in the output reports, there is no such calculation??

Please clarify above points..

Appreciate your help..

To answer only to the first part of your query, 5.10.6.2 is a part of 5.10.6 Self-Supporting Dome and Umbrella Roofs. By API 650 definition, a self-supporting dome roof is a roof formed to approximately a spherical surface that is supported only at its periphery. I would consider that having structural elements welded to roof plates (the plates and structurals supported only at roof periphery) does not make the roof as self-supported (for me the "spherical surface" is structural supported and the structural assembly is self-supported), however you need to clarify this aspect with your Client.

I would add that by note under the title of 5.10.6 (NOTE Self-supporting roofs whose roof plates are stiffened by sections welded to the plates need not conform to the minimum thickness requirements, but the thickness of the roof plates shall not be less than 4.8 mm (3/16 in.) when so designed by the Manufacturer, subject to the approval of the Purchaser.) it seems that the committee tried to make a difference between constructive aspects, but strictly speaking, they don't mentioned in note the area required can be excepted.

As for the differences between the various paragraphs requirements as you've mentioned, it is worth to say that basically both formulas start by considering the failure pressure of a membrane- roof which is
pf=8*Fy*A*tanθ/D^2 - units in [Pa]
5.10.6 considers a safety factor of 0.6 vs pf and gives 5.10.6.2 formula.
Appendix F started by recognizing that the gravity helps and considered DLR; in that section pf is rather:
pf=8*Fy*A*tanθ/D^2+4/PI*DLR/D^2 - units in [Pa]
and in other section of Appendix F it appears that they considered a factor of 1/1.6 for the term containing pressure (1/1.6 is 0.625, close enough to 0.6).
I tried to explain the math derivation in an old post, http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=44909#Post44909
Anyway the above is just giving you some guidance to understand why the 5.10.6.2 is more conservative than Appendix F when consider the area requirements. In my opinion is more related to the history of API 650 development- what various committees considered in a particular moment.

As a conclusion, my advice is to clarify better with your Client the degree of understanding of "self-supported roof" wording.

Thanks for your suggestions.

But I am in a position of deciding this. Actually my problem is, My designed participating area passes the Annex F (Required area), but failed with 5.10.6. Because of with more external loads (Gravity loads P) are acting on roof (live, vacuum, insulation, nozzles, platforms, coils etc.). The more you apply loads, the design fails with 5.10.6.2.

So let me share some more details of my tank design and you can get any other idea..

Dome roof of radius 1.2D, with sections welded to the roof, both roof and sections are supported at periphery (Roof plates at top angle (detail-d) and sections at the shell).

As per NOTE of 5.10.6 and my understanding, the above details comes under self-supported roof category which means no need to check the minimum thickness requirement but the check for roof-shell junction is needed..

Hence, my question is before increasing the participating area by increasing the Top angle thickness, please clarify my understanding is whether correct and the check for 5.10.6.2 is really required or not??


Thanks in advance for your reply..

Dear sivastatic,

1. Appendix F refers to Design of Tanks subject to Small Internal Pressures. Under the internal pressure, the roof is subject to lift tendency (and a component is a lift-off tendency in shell) but the upper ring is also subject to a compression load (tries to collapse inside). That's why the title of F.5 is Required Compression Area at the Roof-to-Shell Junction.
Comparing to the general requirements of API 650 in 5.2.2 it appears that F5 formula address the roof-to-shell junction under 5.2.2 Load Combinations point a) Fluid and Internal Pressure: DL + F + Pi (heavy fluid is not in contact with the junction,so equally the case is DL + Pi).

F5 formula does not differentiate the cases in which you have or you haven't roof rafters attached to the upper shell and/or to compression bar. F5 formula is a simple one counting a dome or cone membrane attached to a cylinder.

2. In my engineering understanding, one still need to demonstrate that the roof and the roof junction have been checked to all points of 5.2.2. This is not usually happening in practice because also API has been changed several times and the loads definition was in Appendix R previously (and the designer had the alternative to say that the Appendix was not mentioned as mandatory). But now is a requirement of basic API 650, probably the Clients will strongly ask for. On my side, I cannot argue why there is no need to comply with all points of 5.2.2.

3. A part of 5.2.2 is to count the Gravity Loads points e1 and e2. Now the effect is an axial force down in the upper shell but also a tension in the upper ring. There is no indication how could be counted- because e1 and e2 are explicitly reffed under the titles Self-Supporting [...]. Let's say that under your efforts to comply with e1 and e2, you are able to perform a FEA analysis including the junction and the rafters connection, hence you are able to demonstrate the mechanical tensions in junction are within allowable- that solved this point in analysis. In case you haven't such tool, one alternative is to count the necessary area similarly with the case described in F5; the difference is that the Dead Load is not acting against pressure but it is rather the load generating the mechanical tension stress in junction. This approach (that does not make difference you have or you haven't roof rafters, exactly as F5 approach) leads to a formula similar with 5.10.6.2, regardless the roof is defined as self-sustained or not.

4. In my opinion, including 5.10.6.2 under the title 5.10.6 Self-Supporting Dome and Umbrella Roofs is not the best choice of API 650. The original intention of API was to define the self- sustained dome type only as a membrane with spherical shape sustained by the upper shell. Having rafters you may say it is not your case, your Client may say it is, etc. But the engineering logic remains as I tried to describe in my point 3. And note that by API calculation anyway you are not addressing at all the local buckling in upper shell and roof, so a prudent approach is to consider 5.10.6.2 anyway, even it gives you the need to increase that area vs. what is mandatory, i.e. formulas in F5. If I were you, I would consider it. If I were your Client I would ask for compliance with all 5.2.2 Load Combinations checking.

As you said, you must decide.

Thank you very much.. I am gonna consider 5.10.6.2 requirements..