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I will assume it supports the roof and attic. The strut may be necessary support for either the joists or rafters. I will also assume you're in an area with a ground snow load that does not exceed 25 psf.
I will provide a steel I beam size for you. If you need connection details, I can provide that as a premium service, just let me know if you would like me to submit the offer.
I'm also assuming the "main joist" is no greater than 16' long.
You can plan for a W8x15 beam ASTM !992 Gr. 50. You will need to be sure the joists are detailed to brace the beam against lateral torsional buckling.