Loads on shell and top curb angle design due to geodesic roof

[WarFT]

I am designing a diesel tank which will be equipped with a geodesic Aluminium Dome Roof.
The tank is being designed as an open top tank.
And the roof will be welded on a tension ring which is welded on top of the top curb angle so no radial load is transferred to the tank shell.
my question is:
are there tangential loads and vertical loads which need to be taken into account that affect the design of the top curb angle and the top strake design?
I cannot see that in api650, any help or past experience on the matter, guys!

 

[IFRs]

Yes. All of the loads loads that affect the dome are transferred to the tank. Dead, live, snow, wind, seismic, etc. It's up to you to make sure the tank is suitable and can transfer the dome loads to the foundation. Note: while 100% balanced load conditions should in theory not transfer any radial loads into the tank other than those from expansion of the tension ring, unbalanced loads may result in radial loads on your tank. Also, it is unclear what you mean by "the roof will be welded on a tension ring which is welded on top of the top curb angle" - is the tension ring integral to the dome or did a dome vendor help you decide to design your tank for dome loads to keep the dome less expensive? In any case, the dome vendor must give you reactions for all load conditions that you must resist with the support structure ( your tank ).

 

[WarFT]

Thanks for your input IFRs.
What I meant was: the dome supports will be welded onto a tension ring (so fixed supports). The dome vendor has advised me on which tension ring to be used. And he is going to give all the loads and full design report, once he gets a firm order.
However on my side, I am designing the tank as an open top tank as mentioned in Appendix G of API650.
There won't 'theoretically' be any radial loads as the tension ring will prevent these being transferred to the shell.
I was wondering whether other loads could have an impact on the shell design, like dead load due to the roof, which will eventually be given by the vendor.
In API650 I cannot see any section on how these loads need to be treated; and how it affects the shell design.
My question, set otherwise, could be do you think the shell thicknesses, which I have designed following API procedure for open top tank, will be affected by the loads given by the dome vendor?

 

[IFRs]

Generally the shell thicknesses are unaffected by the dome loads but there is always the possibility of unusual situations so no blanket statement is 100% correct. The upper shell will by analyzed as a thin cylinder with point loads and those loads will have to be spread out over some horizontal and vertical distance to remain within allowables. You may have to add vertical load spreading members under the dome connections. I find it hard to believe that there will be zero radial forces from unbalanced loads, perhaps you can get approximate values from the dome vendor who should have results from previous or similar projects. Aluminum domes are usually supported on sliding supports that can not impart radial loads to the tank but yours apparently are welded. There may also be torsion in the tension ring given the typical cantilever connection to the dome. I'm also curious how the aluminum dome will be welded to a tension ring that will be welded to a steel tank. Somewhere there needs to be an interface between dissimilar metals.

 

[WarFT]

Hi,
when you mean : "The upper shell will by analyzed as a thin cylinder with point loads and those loads will have to be spread out over some horizontal and vertical distance to remain within allowables", how would you do that?
it is not in API650 right? so I need to analyse following structural engineering principles, if I understand...
In fact I submitted my storage tank design which is in mild steel (without the dome calculation, which is to come later) to the consultant for the moment and it got rejected with the following comments:

Given that the tank has a geodesic dome, certain loads are applied to the tank depending on type of dome support (fixed or sliding, and tension ring)
- While the weight of the dome is taken into account for the overturning moment, etc, the roof support loads specifically at the top of the tank are not taken into account.
- For fixed supports, usually there are radial loads, tangential loads, and vertical (weight & wind combination) that affect the design/selection of the top curb angle and the top strakes.
- For sliding supports with tension ring, usually only vertical loads are applied.

FYI the dome vendor sent me a typical connection of the roof supports to the shell, which I am attaching so you understand.

in the end, I would like to understand what I have missed.