External Stiffening on API-650 roofs.

[michigander]

I have a "thrifty" client that wants a 30' dia 316 SS API 650 tank. In lieu of a SS center pole and SS rafters, they insist on external CS stiffening welded to the tank roof.

The concept includes an 8' dia rolled bar that is located concentrically on the roof. Angles, usd as pseudo rafters would them butt radially into the ring and extend outward to the compression ring. All components would be welded to the roof. Without a center pole, then,they take credit(sic) for the peak of the roof as being a line of support.

API section 3.10.2.3 states that "roof plates of supported cone roofs shall not be attached to the supporting members". I have reviewed all the interpretations and have not found any reference to this question.

Do anyone have any experience with this type of design and fabrication? Are there any rules that can be applied?

Best Regards,

Michigander

 

[JoeTank]

The roof you describe is a self-supported roof, not a "supported roof".

Joe Tank

 

[JStephen]

I would call it a supported roof.

In any case, I believe Addendum 4 has some new wording that allows you to attach rafters to the roof; check and see.

Keep in mind that API-650, while being used for everything from molasses to molten sulfur, is intended for oil tanks. I assume the reason for not attaching the roof to the rafters was to maintain frangibility, and there could be situations where that was not important.

When a roof has a center ring, you can make it steep and design the center ring as a compression ring. Or you can make it essentially flat, and design the rafters in bending. This loads the center ring in a "roll-out" type loading rather than just in compression.

Carbon steel and SS have different coefficients of thermal expansion. When you weld one to the other and the temperature changes, it will induce stresses into the beam that need to be included in the design.

On a normal rafter-type roof, lateral support for the compression flange is provided by the roof plate. When you put the rafters on top, this is no longer the case, and you need to evaluate rafters for lateral-torsional buckling. This will normally require larger rafters.

On a 30' tank, I would try to go with a self-supporting cone or umbrella roof, rather than structural supports. The exception might be if a large mixer needed to be supported.

 

[JoeTank]

Mr Stephen,
The basic definition of a "supported cone roof" is one that consists of the roof plate being supported by internal roof structure. You are right, the issue of frangibility prohibits attachment of the rafters to the plate. The roof system decribed by Michigander is not a "supported cone roof". Since a self supported roof is hardly ever frangible, it's not a problem to attach the rafters to the plate. It's commonly done for self supported roofs whether the rafters are external or internal.

Joe Tank

 

[michigander]

Thanks for the excellent exchange (as usual).

I wished we were having this exchange over noble issues such as maintaining the frangibility of the roof, but we are not. This is totally dollar driven. S.S. center poles costs more than a C.S. center pole. S.S rafters cost more than C.S. rafters. An umbrella roof is more expensive than a cone roof. In their mind, the cheapest out for this tank is a coned S.S. roof with C.S. external stiffening. That's what they want, and thats what it will be.

I wasn't sure that API allowed this but it appears that it is done. I've never done a design like this and am not quite sure how to get my arms around this.

Best Regards,

MIchigander

 

[JoeTank]

Michigander,
SS plate with CS external structurals is very common. That way he'll have a few dollars left over to pay you. Don't fret about it.

Joe Tank

 

[JStephen]

If you are the owner or consultant on the job, looking for lowest cost, just specify all the acceptable styles and let the fabricators pick what they think they can build easiest. It won't necessarily be the same for each fabricator.

I have seen some tanks where the owner paid a large premium for the roof structure because they had a non-tank person design it in a way that no tank company ever would.