its mostly a comfort thing, so i am attempting to match the specification for common use floors so that mentally i can imagine doing the same thing on the floor and feel at ease. the specification here is L/360 for floors.
my attic joists are 2x6s, span is 13 ft, and im loading is about 4 feet from one support. based on my mechanical engineering level of analysis (at L/360), that puts each joist able to survive about 250lbsf. so to hold up 1000lbs, id want to attach to at least 4 joists. my plan is to use 2x 2x8s as a beam to cross the joists, and attach to 6, including one of the joists being at the load bearing wall, and the far other side being on top of a non-load bearing wall (i figure thats better than nothing). main reason im going with 2x8s there is that because they are so stiff, they will more evenly distribute the load the joists, ie, even the joists 2 away will see significant load. the cross-joist span is 7ft.
actually computing the load on any joist in such a system would require a system of equations, i think 6 equations and 6 unknowns. so im taking it basically as 2 different 2d problems. the joists strength, and then the strength of the cross beam.
by all accounts this is way over engineering the issue, but by the math its only "reasonable" in my mind. this makes the 40% swing in things like the elastic modulus of wood ranging from 1,000 kips to 1400 kips for example freak me out. 40% is like a lot, and thus why i dont actually feel like i trust the numbers.
in order to trust my numbers more, i took to measuring my floors, and found out they often do not meet L/360, and the only reason my refrigerator doesn't go through my floor is that there is flooring on it, to spread the load paths. this did not inspire confidence.
i am considering now the analysis to link the joists together via top nailed 2x4s at some interval parallel with the cross member, in order to create some of that load sharing and more complex load paths. i would really rather not put actual flooring type coverings up there, because all the insulation etc.