Vishalvp09:
Irrespective of exactly what the code says you must do, or tells you what to do; why don’t you tell us what a ‘Top Wind Girder’ is, why it’s needed, preferred shapes, and generally how they work? Then we can help fill in some of the details.
To exactly answer your question, read 5.9.1 of API 650. So if you are designing an Open roof or floating roof tank per App. C, you need Wind Girder, this API 650 says, which means the committee feels there is a necessity to maintain the roundness under wind conditions. Beyond this if you may check if required, but it may not require as the cone or dome or any roof and /or structure you use will provide the necessary stiffening which can be provided by adding a Top wind girder.
