We have a prototype steel frame. 1x1x16ga.
No test data yet.
Check out Wellnuts for attaching things.
http://www.hansonrivet.com/w71.htm
We are also using a Flowdrill for tapped holes. My 1.5hp bridgeport will work a 10-32 flowdrill, if I spot heat with a torch just before drilling.
To get equal durability from aluminum we would have had to heat treat after all the welding. Didn't seem practical.