As others here have said, if you are needing to do that sort of work then get someone else who has the gear and experience to do it. As it is structural then insurance and liability come into it if it goes pear shaped and maybe someone gets killed or maimed should it come crashing down. Most of that sort of thickness steel generally gets welded up by industrial type/sized machines and not your typical handyman type machines.
Sure you could do it using multiple passes and lots of prep, but then would you be able to sleep at night wondering if the welds had any penetration, were they any good and not just pretty?
Sure you could do it using multiple passes and lots of prep, but then would you be able to sleep at night wondering if the welds had any penetration, were they any good and not just pretty?