Re: how far apart should decking joist be using 100mm x 50 mm x 2mm rhs steel floor sub frame
Welcome to the Bunnings Workshop community @gregpergola. It's wonderful that you've joined us, and many thanks for your question on RHS joist spacing.
The best place to source this information would be with the RHS manufacturer. They should be able to provide the relevant span tables and load capacities applicable to their product. If this is a product you've sourced through Bunnings, I'd be more than happy to contact the manufacturer for you. You might find generic span tables online for 100mm x 50mm x 2mm RHS, but the ultimate authority will be The Building Code of Australia.
Generally, when building a deck, a joist spacing of 450mm would be suitable. This ensures that there is no flex or bounce to your decking boards. However, a deck is also required to have a certain load capacity per square meter. Once you've verified the load capacity of the RHS, you'll be able to determine how many joists you need per square meter to carry the deck's required load. This, in turn, will govern your joist spacing.