Jeff C wrote:

I recently bought at 100 year-old Cape-style house. The floors were
sagging and the culprit appeared to be the center beam, which was being
supported by 3 6x6 posts that had rotted from the bottom up over the
years. The worst sagging was probably about 2".

I've jacked up the beam and re-supported it with adjustable jack-posts
and all seems well except that the joists don't seem to have moved with
the beam. In other words, the beam is far more level and the floor
directly above the beam is just as level, but the floors simply have a
"speed bump" shape in them where the beam is supporting them but they
quickly sag off to either side of the beam.

The joists appear to be in good condition, though they're certainly
original and they are supported both by being mortised into the beam
and additionally by bulky (1/4" thick?) 90-degree angle braces. I'm
trying to figure out why the beam moved but the joists apparently did
not.

N.B. - I did jack the beam up relatively quickly. Over the course of
maybe a month. Could that have something to do with it?
Help appreciated, thanks.

Concur w/ others, but...I'd suspect perhaps the angle braces aren't
original but were added to try to solve an earlier problem. How large
are they and are they really mortised in or simply setting on a ledger
(and the angle is an attempt to keep them there)? 100 yrs isn't that
old so could be this is dimension lumber structure? I'd get another
jack and support the end near the beam of two or three and do some
serious exploring of the joint technique and condition of the joists by
taking one or two of the angles down (if necessary, that's the reason
for the additional support, just in case it's the only thing left really
holding anything) and see what's actually going on. If you raised the
beam but the joists didn't move and that's anything close to 2", you've
got a problem. If, otoh, they went up but not quite as far, it's
probably not too serious. Fixing it may not be easy, however.

Have you put a line on the joists themselves, do they have pronounced
bow? Did you check this before lifting so you can compare?