How do I calculate if temp shell bracing is required during erection of an API 650 storage tank? 2

[marcleblanc12]

I design many API 650 storage tanks. I have some customers who insist on my crews bracing the tank shell during erection prior to us having the top stiffener and roof in place. Does anyone know how to calculate whether a tank shell will fold in on itself in the wind before the top stiffener is installed? This would be based on tank diameter, shell thickness, shell height and a given wind load. I'd like to be able to calculate this to prove to my customers when it's not necessary to brace a tank shell during erection.

 

[racookpe1978]

Go back to fundamentals:
Thickness of shell,
height of shell,
diameter of shell,
expected wind load during construction/erection,
method (efficiency and extent) of the shell-to-foundation joint,

Use above to prove that the curved shell for this height and this diameter attached to the foundation around this much of the perimeter will not collapse ...

 

[jtseng123]

I don't think it is that easy to run the calc without using FEA. Considering wind blowing on one side(says 30 MPH),
it is very easy to calculate wind pressure, but to calculate deformation for large diameter thin wall, (windward side will deform more than the rest), you most likely shall build a typical PEA model for the analysis. It is not as simple as cantilever beam.

 

[racookpe1978]

I AM agreeing with the proper analysis.

I'm equally sure it will NOT be found in a simple formula from a code-approved rule book for a complete tank with roof attached. .

 

[marcleblanc12]

Yes, I've realized it's not an easy thing to manually calculate. Any which way I've tried to analyze it manually, it gives results that are not realistic at all. Very difficult to predict what that plate is going to do in the wind. On many jobs we don't do anything to brace the shell during erection and we've never had a problem. We get some customers who are extremely paranoid about everything though. I can understand for tanks that are over 100' diameter and so on, but we're only dealing with 30' diameter tanks on this job. Would be nice to be able to prove nothing will go wrong, without doing an FEA.

 

[Duwe6]

Biggest problem is determining what the overnight wind will be like, EVERY NIGHT until the tank is assembled. Symplify your problem; hire a competent tank erection company and don't pay for the tank until it is complete, the radiography has been accepted, and passed hydro.

For your case, it is the mirror of the above. Assure the client that they havn't paid for the tank during erection, and if there are problems, you will fix them for free. To brace or not to brace is WAAAY to dependant on the exact condition [how many tack-welds, how long, placed where?] at the time the crew quits for the night, and WAY too dependant on the windspeed and direction of the overnight winds. Plus, normal overnight bracing is usually only a couple of tacked on lugs, with wire rope run down to anchor(s). Before the floor is welded, an excellent anchor is the pickup truck that the tools arrived on. But it depends on the exact condition and placement of the tank segments, and the magnitude and direction of the overnight wind. If you can preengineer all these conditions, I really want to hire you. You'd save me a lot of money.

 

[MJCronin]

This may be a thread better suited for the "Storage Tank Engineering" Forum ....

I agree with the comments above, I believe that a seasoned tank erection crew can tell when a partially completed tank requires a wind brace.

This seem to me to be a complex problem of elastic stability, not stress analysis.

MJCronin
Sr. Process Engineer

 

[dhengr]

Marcleblanc12:
It seems to me that Duwe6 (8MAY14, 9:48) has it about right. You flips a coin and takes your chances, you tell me which direction the wind is going to blow overnight, and at what velocity. The fact that you’ve gotten buy without much bracing in the past, doesn’t mean it couldn’t happen, it just means you’ve never seen any serious winds, and been very lucky. You haven’t made it clear, are you an engineer, a tank designer, or a tank fabricator and erector. What size tanks, heights, diameters, shell thickness, single piece sizes which finally make up a complete shell ring. There is not a one size fits all formula here and FEA probably won’t help you much on a problem you don’t know how to model. There is a whole lot of basic knowledge, experience and engineering judgement involved in this kind of problem. Maybe you should turn your problem around a bit and look at it from a slightly different direction. Maybe you should study and refine your erection sequence to always leave the tank in the most stable condition. What are these conditions, configurations, and the general merits or disadvantages of each? Maybe you should come up with some clean, simple, bracing systems that are as much a part of your tool bag as your welding equipment is. Instead of a formula for when a tank will buckle under infinitely variable shapes (completion stages) and load conditions, why not think more about what configurations are most stable and why? Now, flip your coin and guess at the wind speed and direction as it relates to your half completed ring, for that day, and a whole bunch of stuff becomes a crap shoot. Maybe at the end of the day you should spend some time doing misc. tasks and stagging for tomorrow rather than placing those last couple pieces which go unsupported, and could pull the whole thing down. Maybe you can tack those pieces end to end, and standing along side of, and supported by, the lowest erected ring, ready to be lifted in the morning. What can you do in the way of some simple bracing systems/equipment, a function of tank dia., height, piece size and thickness, for various stages in height or ring completion. Can several lifting fixtures be made to act as top edge stiffeners until a ring is completed? Can welders staging be made to act as a temporary stiffener or stabilizing feature? Could parts of the top stiffening ring be used as temporary top edge stiffeners as you moved up in the erection?

OSHA only gets excited if you start injuring or killing workers, they really don’t care if some plates get bent all to hell in a wind, except that that could also happen when workers were on site. And, that may be an indication of lack of attention to safety details.

 

[gr2vessels]

Marc, you brought up a very real topic, which has prompted many fabricator/erection companies to develop their own 'proprietary' procedure(s). I have seen few examples of this bracing, combined with a circular welding platform, but I could not get any details, not even close-up pictures. One classic example is CB&I, the fabricator and erector of the huge LNG tanks on Gorgon project, the site exposed to atrocious cyclone winds. Never have had any problems. The bracing is also employed by other similar companies, but they all have their own proprietary concepts. It probably require appropriate experience and most likely an extensive engineering. It appears the system is costumized for each application, depending on the expected site conditions. Hope I gave you an insight of the size of this very important issue.
Cheers,
gr2vessels

 

[prex]

This book deals with the stress induced in a tank without a wind girder under wind load (page 170). However it does not seem to take into account buckling behaviour, but also shows the complexity of the problem.

prex
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