Dome Roof Rolling Practice

[SEJohn]

I have a 40' diameter atmospheric tank designed per API 650. The roof needs to be self supported, so I am anticipating a dome roof. I am owners rep and will be specing the tank and assisting in the owner in purchasing and construction.
I am unclear on how these are typically built. To make a perfect dome, plates would have to be cut into triangles and then radiussed in both directions. In practice though, are they usually radiussed in both directions, or are they only radiussed from the wall to hight point and then just fit up to each other - I am guessing the latter.
If it is the single direciton roll, then are their rules of thumb on how wide the plates can be (compared to the radius) to get a good fitup?
Thanks for any help.

 

[JStephen]

In API-650, they address "dome" roofs and "umbrella" roofs. A dome is pressed so the plates are curved in two directions. Umbrella plates are curved in one direction only, except for some minor field fit-up out at the edges. For a 40' tank, plates would typically be 5' to 6' wide, although there is no set rule, and that could vary from one manufacturer to the next. While there is a distinction in the terms "dome" and "umbrella", they are often used interchangeably or incorrectly.

It is possible to build dome roofs where the plates are not pie-shaped, although I don't know how common that is (or was). But I've seen tanks like that, at least. The plates were more or less rectangular, just pressed to shape.

 

[IFRs]

A 40 footer can also probably be a self-supported cone depending on the loads, etc, which would be simpler and less expensive.

 

[SEJohn]

Thanks for your responses.
IFRs - you are right, when I run the #s, I get a 1/4" pl at dome and 3/8" at cone (assuming a 4.5:12 slope which produces similar total roof height). A cone may be more economical.
JStephen - in the API 650 Roof Definitions - it says 'A self-supporting dome roof is a roof formed to approximately a shperical surface that is supported only at its periphery.' and 'A self-supporting umbrella roof is modified dome roof formed so that any horizontal section is a regular polygon with as many sides as there are roof plates that is supported only at its periphery.' I am picturing an umbrella as effectively being a dome with a flat center section. Is this correct? Why would this allow plates to be rolled in one direction only?
Final question - if a dome or umbrella is used, can plates be lapped, or whould they always be butt welded?
Thanks again for your responses.

 

[IFRs]

An umbrella roof looks like an umbrella - one of those things that pedestrians use to keep them dry on a rainy day. The plates are usually roughly triangular ( look for a wiki on "gore shape" ) and curved in one direction only. They are not spherical. Domes are spherical. Either can be butt welded but most that I see are lap welded for a variety of reasons, no the least of which is cost.