Span calculator using arbitrary spacing?

Question

My "new" 72-y/o house is... interesting. The garage is much younger but still I just raise my eyebrows at some things.

One of them is the loft/attic area in the garage. I wanted to move all my computer hardware up there, of which there is a LOT. But just walking down the middle of the floor got me uncomfortable. It deflects too much.

The previous owner used 2x6 as joists, nailed (3x) to the side of the 10' stud ~8' up. The span is 15.5' ... since the ends are not on a sill and nailed IDK if the full 16' would be considered as the span. Regardless, SPF 2x6 with 16" spacing are rated for about a 9.5' span.

I'm trying to avoid removing and reinstalling the floor if possible. My intention was to double up the joists in 5 evenly spaced spots (every 3 stud spaces)... I'll do everyone if needed though. I can't find a span calculator with arbitrary spacing or joist dimensions.

Would this work? What is the resulting span from doing this?

My intention is 2x6 joist - 2x4 stud - 2x6 joist. I'm using it for business purposes but want it to remain usable as a garage whenever I leave this place.... so no beams and posts down the center.... if the thought had occurred to you. :)

Update - Picture

Answer 1

There are several issues: 1) true load on joists, 2) true existing joist size, 3) joist spacing, 4) joist bearing/ support,

1) The Code requires a minimum of 40 psf for usable loft/attic rooms. However, the equipment you describe could easily exceed that amount, but that load would be near the perimeter of the room probably. You have about 2’ of head room on the rave sides, but could be substantially higher on the rake sides. (“10’ stud nailed 8’ up.”) of course the sides with substantially more load will require substantially more support. So, a uniform joist installation will probably not be sufficient.

2) The existing joists appear to be 2x8’s not 2x6’s. However, I’ll use your notation.

3) Existing joist spacing appears to be 16” o.c. (To match stud spacing.)

4) Existing joist support is in shear (balloon framing) with 3-16d (I assume) at each existing joist. Additional load will require additional support.

So, in order for the new attic load to be transferred to the footing, the new joists will need to be secured to the existing studs, (because there is no double top plate).

Without knowing the actual load from the equipment, I’ll assume a minimum of 50 psf plus an additional 30 psf along the gable ends.

I’d sister 2-2x6’s to the existing 2x6’s (1 on each side of the existing 2x6). I’d add a 2x4 under each joist down to the sole plate.

Along the gable end, I’d add 2-2x6’s on each side of the existing 2x6 joists for the first 2 spaces (32” or 2 joists) with 1-2x4 under each added joist. (I’m assuming the first joist will be nailed to the studs on the gable end.)

Btw, with that much equipment, you’ll need to consider adding cooling equipment too.