According API-650, is it permitted to weld a structural member (rafter), for a structurally supported cone roof? I was searching into standard, but it does not say specifically.
5.10.2.3 Structural Member Attachment: Roof plates of supported cone roofs shall not be attached to the supporting members unless otherwise approved by the Purchaser. Continuously attaching the roof to cone supporting members may be beneficial when interior lining systems are required, however, the tank roof cannot be considered frangible (see 5.10.2.6).
If you are asking if you can splice short rafters together to get a longer one, API is silent on that option but if you can justify it using AISC and good engineering judgement and practice then it should be OK. My opinion only!
Agree with IFR; anything not specifically prohibited may be used, as long as 'good engineering practices' are used. Splicing beams using AISC criteria meets 'good engineering practices'.