Water tank ribs dimensioning

[cast16]

Hi all,

Rectangular tank with flat bar ribs. I remember reading somewhere that there is a limit in the ratio height/thickness of the flat rib you can use, but I can't find it.
Does such a thing exist in any guideline and what is the value?

 

[Compositepro]

The problem is tortional lateral buckling of the ribs, just like floor joists in a building. This is prevented by cross-ribs or using angle iron rather than flat bar.

 

[cast16]

Hi Compositepro,
It is a customer design. They are using a 150x10mm flat bar. It is working fine with FEA and calcs. I think you are right about the reason you don't see usually high flat ribs.
I was going to flag this to the customer, but I would like to back it with a statement from experts "general rule of thumb". As I said before I remember reading it somewhere but can't find it anymore.

 

[SWComposites]

so this is not a student project?

ask the customer if they have performed a buckling analysis, and to provide the results; or are you performing the FEA and hand calcs?

 

[cast16]

The customer specializes in surface treatment tanks and has submitted an initial tank design. Our role as fabricators is to complete the design, verify its integrity, and assume responsibility for the project. While a structural engineer will become involved later in the process, we are currently in the initial phase, and I will be meeting with the customer soon.

I have conducted a linear static Finite Element Analysis (FEA) using Nastran in CAD on their design, which indicates a deflection of only 0.7mm (the tank's thickness is 6mm). I am uncertain about how to perform a buckling analysis in Nastran considering the hydrostatic pressure, but I believe there should be no buckling in the ribs due to the internal pressure. Correct?

The primary concern may be accidental collisions with the ribs, which is why RSA is typically employed when substantial stiffeners are necessary or horizontal ties are used for flat ribs.

 

[lucky-guesser]

There are endless ways you can reinforce these ribs, which if there is any chance some random employee on the forklift (or anything else) can bump these and fold them I certainly would want these to be. One or two sure, but in 10 years when half or more of these have folded it could be catastrophic.

No idea on your ratio, just a sketchy design in general.

 

[BridgeSmith]

The primary concern may be accidental collisions with the ribs

In that case, you should consider using rolled channel sections for the ribs - gives you a stiff flange on the outer edge, and more separation between the welds to the tank than with a plate.

 

[rb1957]

you, as a student, are taking design responsibility for this tank ?? wow.

the option is that your company is taking this responsibility, but then leaving you to do the work, using the internet "experts" as a reference resource. wow.

"surely" there a people near you (mentors, supervisors) who can help you more directly.

How much pressure is in this tank ? or is it just hydrostatic ?

There are myriad design considerations for any sort of design. Will the stiffeners get damaged by people bumping them with fork lifts ? Sure, but you can mitigate that many ways (put a set of bollards around the tank, inspect it annually and replace damaged components ... can the tank be taken off line and emptied, or does it work 24/7/365 ?)

Sorry but IMHO this is not the place to learn structural design.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.