I've taken down the wall in our entryway to do a stair rail but ran into a situation on how I would anchor one newel post at the top of the stairs. There are four 2x4s sandwiched together that go down to the bottom floor. There is a joist to the right of the 4th 2x4. Should I anchor with a zip bolt system directly down into these vertical 2x4s or is there another idea or system I should use?
If your newels are strong enough at the bottom of the stairs and at the top beyond the corner, I have in many cases like this, used to 2 newels at either side of the corner newel to hold the corner newel steady. All the corner newel needs is to be anchored well enough so if and when the lower newel gives a little, which they all do, so the angle of the handrail coming off the starting newel, will not lift the corner newel up off its curb.
Joining wood end to end is a hard problem and there are few good solutions, none that I am prepared to recommend.
Use your post to replace those studs. So the post needs to be 12 feet long or something like that.
It's going to be messy installing the post, but if you want best results that's it.
Second best is you bolt it to the side of the floor joist still messy, but less messy.
Any type of surface attachment is not not going to work well unless the balustrades are themselves braced in two directions.
When I was building custom homes we had a stack of 3/16" steel plates made up for just this use case. They had one corner cut out to create a fat L-shape to match the floor outline, and they had opposing countersunk holes for mounting the plate to the newel and then to the floor.
I suggest that you find a way to make such a plate. Mortise the floor with a router, chisel, etc. so the plate ends up flush with the subfloor, then set the plate in construction adhesive after dry-fitting and plumbing the newel. Remember to square it as well with the railing axis so a 90° railing cut fits well.
Note that the newel screw holes must be pre-drilled properly or you'll break screws (or split the post). We actually had a larger center hole and used a 5" x 3/8" lag screw there. A recess was counterbored into the subfloor for its head.
It looked something like this:
BOTTOM VIEW
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| | * floor mount holes (countersunk other side)
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| | () countersunk newel mount holes
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