Modern joists between 1860's log joists.

[Mediastinia]

I have a two-story family farmhouse built in the late 1860’s. Still square, stone basement walls have not moved much. In time, the first floor received plumbing electricity and central heating. These caused minimal damage to the house structurally.
 
The problem is many of the first floor logs joists are beech with typical powderpost beetle damage. A few of these have severe enough damage that they are crumbling away. In time, I hope to replace these.
 
The log joists do not appear to bounce, but the floor between them does. The main beam runs east-west. The joist logs on the north span 17 feet and have 27 inch centers, on the south 13 foot spans and 24 inch centers. Logs have 8-9 inch diameters. Sills rest on outside of stonewalls, and sometimes the irregular stones narrow space to sill.

First floor main beam straight, sills look good. They are 7 inches tall, 9 inches wide.

I have been planning to put joists between the logs to reduce the bounce. For the joists, I was thinking of doubling 2400 MSR southern yellow pine 1 x 6’s and using hangers.
 
We have been lessening the weight in the house (i.e. modern attic storage, piano and late model cast iron tub were removed), and have no plans for tile floors and can live with squeaks. Many decades ago basement dug deeper and concrete floor and thick sill were poured, clearance a little over 6 feet.

[Mediastinia]

Skipped a query, sorry. Obvious modest goals, but curious if there are crucial reasons not to proceed or is there something clearly better to do?

I have been working on a plan a long time, but kept it short purposely. 

Thoughts? 

[Don P]

Well, it sounds like the beech joists are not in a condition to be counted on structurally. Then the question in my mind revolves around what is the intended span and on center joist spacing, at that point you can size the joists properly. If the span is 17' you are looking at 2x12's more than likely. Deflection will likely control, get an E number off those msr timbers as well.

[Hooterspfld]

I ran into a problem like this in an early 1900’s house my dad and I were remodeling. I don’t believe ours were quite as bad as 24” centers, but it certainly felt like a trampoline when you walked on the floor upstairs. I big portion of the problem was resolved when we removed the thousands of pounds of lathe and plaster and replaced it with modern drywall.  As far as the floor joists went, we ended up using a 20 ton bottle jack to raise the bowing centers of the existing joists and sandwiched new 2x8’s on either side and glued and bolted them all together. We then installed new oak hardwoods upstairs and when it was all said and done those floors/ceiling were solid. That’s been 10 years ago now, with no signs of cracking in the ceiling.

[Mediastinia]

We still have some load reducing to do. All rooms had multiple layers of carpets and flooring added. We were fortunate that when some weight added (i.e. kitchen brought inside with electricity), supports added in basement.

Hand hewed beams under load bearing walls (lathe and plaster) had supports put under them, probably early as they are timbers, some with bark, set on fieldstones. More of these under main beam as well. No problems there.

We do intend to replace extremely weakened log floor joists. These will be replaced with log joists. Spent 4 months before winter repairing 1860's fairly large two story barn. We replaced log roof rafters with log roof rafters. In house, will replace as needed as I work on individual room floors.

Putting in modern joists is a compromise I am willing to make because 24 and 27 centers make floors springy between them. My other compromise I am willing to make with my ancestor's work (their pictures on the walls always looking at me), is put support under 17 foot spans, as either a beam runner across or individual posts. Stairs come down on this side (inside stairs!, another modern convenience added!). Add support to 13 foot side as needed.

We will stain the joists and paint the hangers to reduce their distraction from the logs joist. All electrical and plumbing now on logs will be moved to these.

Am thinking of using 2 x 8's, trimming down to not hang below main beam and clear stonewalls to increase stiffness. I will use heavy gauge hangers that require lower screw counts.

Never found much on plain center cut sawn lumber kiln dried for joists. Any experience with this option?

2400 MSR lumber E# may be 2.0E

[Don P]

That's a convoluted problem. If you run a girder at midspan under the 17' span and the modern joists are at 2.33' spacing with a 50psf total floor load then they can carry the floor load exclusive of the log joists, the logs are then what is keeping the floor deflection down and the modern joists are bearing the load, then you might just borate and keep the existing joists. Don't notch the modern joists more than 1/4 depth and taper that notch out gently, not a sharp re-entrant stress riser corner, those split. The 13' span passes in bending but fails in deflection at about 13/16" under full load. Doubled up they pass.

If the floor is deflected, if it has taken a permanent set, when you tip up new straight joists under there it may just rip the floorboards up off the sagging log joists. Check the floor with a string, if it's bad that is a real possibility. The beetles are more than likely in the floor as well, I've redone it all before, check and think about what you are seeing.

[Mediastinia]

Feeling better about this. 

With a modest goals of making things better, limiting impact on appearance and doing no damage, working mostly solo . . . I'm thinking of this,

Materials
1. Doubling 2 x 6 2400 MSR SYP
2. Simpson LUS26-2 hangers (or HUS26-2, 14 vs LUS 18 gauge,  4x LUS cost; suppose to carry same load)
3. Simpson SD10 screws (#10 x 2 1/2, approved for this use)

Assembly
1. Doubling with screws and adhesive 
2. Will put in pilot holes for hangers
3. With logs in the way and stairs on 17' span side, I was planning on putting in posts under individual joists. I have dense black walnut stand to thin, so post may be this.

I have ideas on screws and adhesive, but would like input on what others have used and suggest. I can keep assembled joists in barn to dry glued joists.