Stripped (non-functional) Beam bolt holes.

[evilC]

It may be that the captive nut is spinning in the housing. See if there is a section visible on the outside, it might be flush though and put a blob of weld on it just enough to stop it spinning until there is some tightness after which tightening further will jam the nut solidly. I am guessing that the action of tightening will secure the nut better.

If that doesn't work/apply then either solidly weld a nut in from the outside or use a steel countersunk Rivnut. You will of course have to drill out the existing nut that will no doubt end up floating in the beam. The harder you tighten a rivnut the more secure it becomes (all must be within the limit of the threads though)

Bolting through would seem sensible but you would have to insert an anti-crush tube welded in trom the backside. Any plain holes would IMO dish over time that would mean a sloppy connection thatonce dished gets weaker the more you tighten it.

Clive