Relaying the message---
I would say if this is what he wants to do to go with 4 X 4 joist at 16 - 18" on center depending on his length of planking. This gives him a way to attach the joists to the steel beam. It would be difficult to attach 2 X stock to the steel beams and they would have a tendency to rotate or flip. Also when installing the planking make sure he staggers the joints like you would when installing random length flooring(I don't think 4" wide is enough to butt pieces together and nail them in, especially with hemlock that is brittle/weaker) It would also be a good idea to run a 4 X 6 along each edge to act as a wheel stop.
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The blue is my thought added in.
You will have to let us know how the hemlock holds up over time. Hemlock is not a strong wood, and there seems to be different opinions on it's resilience to rot. I certainly would make sure those 4x4's are strong and out of something else besides hemlock. I don't think it will last 50+ years.
