Thanks for the additional thoughts!
> Can you lose 2" of headroom to sister with 2x12's
Not easily - HVAC plenum and ductwork, plumbing and gas pipes, etc.
Even if that weren't the case, I'd still have to notch the ends so they sit on the existing beam and sill (or not sister the entire joist length). As the provided link confirmed, the strength is directly proportional to the square of the depth, and the stiffness to the cube of the depth - in my case, going to a 2x12 rather than a 2x10 would result in a sistering joist that's 44% stronger and 72.8% stiffer. That would be nice, but I think it's too much work to make it worthwhile - I am hoping that the steps I've already outlined will be sufficient. (Of course, if I'm wrong, I'll be cursing myself later...)
The second link was indeed an interesting read, but it left me wondering about other, untested scenarios. I was surprised that cross-bridging with a 1x4 strapped across the bottom of the joists actually performed better than solid blocking; I wonder if strapping under solid blocking also has benefit. I think it might, since the strapping presumably spans across several joists (I didn't see this explicitly mentioned in the article). I remember seeing something in a Fine Homebuilding article about metal strapping along the joists helped because it spread load along a joist; 1x strapping probably does something similar, except it is across multiple joists. I wonder, too, if the results would be similar for standard dimensional (non-LVL) joists. (of course, I could always use LVLs as the sistering joists! Hmm...) Also, the author was only measuring vertical (vs. lateral) deflection; I still think solid blocking may still be better to prevent lateral deflection / roll - that may be an important consideration in earthquake zones (not a real requirement or concern for me, though). Reinforcing braces with metal (aluminum or steel) brackets seems interesting, but my gut says it might be overkill and unnecessarily expensive. I wonder how the results would have changed had the author used two or more bolts (rather than a single bolt) to attach the bracket to the joist. Also, it would be more "real world" if there were some test cases that used both traditional nailing with the bracket (as would result from a "retrofit" where the metal bracket is added after the fact to "shore up" a weaker floor system).
I confess that I may have a bias against cross-bridging - not necessarily in the method itself, but because I have seen a few installations where it was not fastened properly (not attached at either the top or bottom). I suppose improper installation happens for solid blocking, too, but I haven't seen it. (Admittedly, I've personally seen *far* fewer solid blocking installations in general - in fact, only one that I can recall.)
I apologize if I sound overly critical or cynical of any proposed ideas - I do appreciate the feedback/discussion. If any of my thoughts don't hold water, feel free to shoot 'em down!
Does anybody have any experience with really large/long nails and how to drive them? I certainly don't! (I'm assuming it's a bit more complex than simply whacking at them.) Unless I have a solution to that issue, I'm going with screws.
