Guard Rail Posts
If you look close, you can see I cut the vertical guard rail posts with tails, then cut a little 2"x4" slot in the floor, and dropped the tails down through where they will be bolted to the 4x12 joist butts.
I also set a piece of scrap at 38" for the guard rail height (black 1" piece clamped to the posts). Minimum is 36" (IRC/IBC for residential), and there is no maximum height for guard rails. Stair rail max height is 38". Going to try to make them match or be close. There is also a 4" max space between uprights; I'm thinking rebar, painted black, and epoxied into the floor. I'm figuring three 20 foot sections of rebar - two upper overlooks + stairs. Which, at 4" spacing, means I need 180 rebar pieces at 40" each, or 600 feet of rebar. I'll get there.
For the guard rail itself - it is oak, but is still drying out back.
I have 4 more posts like these to make for the stairs, but they will be 5"x5"'s, instead of 4x4's. I decided this because the stair stringers are 3" thick, and that would leave me 2" overhang to bolt to the side of the stringers.