I am not aware of a single reference that does a really good job of telling you all the details of tank design.
Most of the tanks built are built by specialized contractors who have their own engineering capabilities. So as an owner, you can furnish a pretty minimal amount of information and the contractor can furnish the rest. When you get into some very specific details, you may find those details vary from contractor to contractor as well.
The best reference that I have actually seen is "Steel Plate Engineering Data: Volume 1 & Volume 2" (actually in one volume), published by the Steel Plate Fabricators/ American Iron & Steel Institute. The version I have is badly dated. I see listed here a more recently revised version, but I haven't actually seen it, so I don't know how it differs from the one I have. At $75, it seems considerably overpriced to me. Anyway, listed here:
Go to Amazon.com, put "steel storage tank book", and it pulls up several promising titles. However, out of the list, there is only one I have actually perused, and it relates more to the regulatory end of tanks.
The Structural Engineering Handbook by Gaylord & Gaylord has some information on tank design and foundation design- I'm not sure if it's worthwhile to buy the book just for that, but if you have a copy available, look into it.
A quick rundown of how tanks are designed:
Flat bottom- is specified, not designed.
Shell- checked for hoop tension due to hydrostatic pressure by simple t=2.6HD/sE type formulas in the standards.
Also checked for blow-in using equations for maximum height of unstiffened shell, usually wouldn't control design on that size tank.
Roof plate- will actually be a low cone, never flat- use radial rafters designed as simply-supported beams. Tank codes give maximum rafter spacing. Friction with the roof may be used for lateral stability of the rafters, depending on the code used.
Center column designed as a pinned-pinned column, may include eccentric loading due to unbalanced snow or erection loading.
If the tank is anything but atmospheric, then some complications arise, refer to App. F of API-650.
Overturning of the tank due to wind varies depending on the design code used.
Seismic design is covered in the design codes- App. E of API-650.
Welding procedures are usually ASME procedures, but AWS can also be used. Actual procedures will vary depending on the contractor. Roofs and bottoms are generally lap-welded from the top-side only, shells are butt-welded.
If the tank is storing anything but water, you may need to look into using floating roofs or other emission control equipment, secondary containment, leak-detection, etc.
I'm not sure what you mean about "...also designing sheathing as membrane and not as a flexural item...". If you mean the roof plate, see the provisions in the standards that give maximum rafter spacing as a function of roof thickness, which is how bending or strength of a rafter-supported roof is considered.