Can the central support beam hold a 100lb boxing bag?
It should be fine since you have the weight distributed to two different screws in the beam, but its not just one beam it is multiple 2×8 or 10s that are sistered. I work in construction and when I was young I hung my own punching bag from a ceiling joist in my garage. If you hit hard enough and frequent enough, the screws will eventually rip right out. Mine actually broke the joist in half and pulled the sheetrock down a bit, but that was the full extent of the damage which I fixed easily. Worst case scenario I can imagine for you is the screws rip out and split the two pieces you drilled into. It won’t be any big repair since the two pieces sistering the others will be fine. Maybe just use epoxy wood filler where it cracks as much as you can, then use some screws that will reach through the first 2×8 into the second and that would be a more than sufficient repair although if you didnt feel safe you could put in a support beam under. Again that is just in the worst case scenario which
