Hi, Timber Framers! I am new to this. I have a circa 1840 TF house in Iowa. I am a woman, if that matters, and I have 35+ years experience in carpentry and woodworking, started in the woodshop with Dad when I was 10 or so. This is my 4th old house.

Here's my question: My house is 24 x24 square, with a 6.5"x 6.5"girder down the center. The girder was cut out at 4" to accept the 2.5"x 6.5" joist. The joist notch isquare, which I have learned from other posts is the worst case scenario.
On the north side of the girder, 3 joists were completely broken, several others were cracked and ready to fail. I removed the subfloor, and the joists, lag bolted 2x8's to the girder, and the other beams rimming the floor, then used joist hangers to support new 2x8 joists, 16" o.c. I used .75" OSB for the new subfloor. This has been installed but not finished, because I am concerned that I need bigger joists to make the floor structurally sound. I am still getting some jiggle and bounce.
On the south side of the girder, I currently have one broken joist, and the others are seriously sagging. The floor upstairs dips almost 3 inches in the center. Someone framed a new dropped ceiling downstairs, using 2x4s that is also failing, and must go. I cut out a small part to see what is going on with the joists, and it appears some were already "sistered", but that, too is failing. I'm not as worried about being historically accurate as I am about having a repair that will last at least a couple more generations. I welcome your ideas and comments. Thanks, Katy