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.