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Different joist heights on the same foundation

M

MrWade

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Joined
Aug 22, 2022
Messages
3
Location
Salt Lake City, UT
Hi guys, I'm hoping you can give me some insight on a dilemma I have. I'm working up some plans for a home addition and I don't know if there's a good way to support the new floor joists without getting an engineer to step in. The existing house has 2x8 floor joists, and the addition needs to have 11-7/8 TJI's. So how do I support these new joists with the bottom sitting 4-1/2" below the foundation wall? I have considered building a bearing wall underneath (bottom of the wall would sit on the existing footing). Or can I get away with running a 2x8 across the top of the foundation and using hangers to support the joists? Maybe a double 2x8? I have my doubts that either of these options would get the nod from a building inspector without the support of an engineer's drawing. Any ideas? Here's a drawing of the bearing wall concept:

52303867342_63bbf25d79_z.jpg
 
If existing wall is adequate (probably is) there are tji hangers that should work. It would probably require packing or building out the existing rim flush to wall. Look at Simpson Strongtie catalog. Seems simpler than wall.
 
Where does the IRC address this situation? This suggests that the IBC should be used.

If the TJI was supported by the concrete wall you might prevent the offset of the floor at the wall. Wood shrinks concrete does not. If we recognized that the concrete wall acts as a shear wall and the plywood sheathing acted as a diaphragm resisting wind and earthquake forces the connection to the concrete wall would be different.
 
bill1952 said:
If existing wall is adequate (probably is) there are tji hangers that should work. It would probably require packing or building out the existing rim flush to wall. Look at Simpson Strongtie catalog. Seems simpler than wall.
Click to expand...
This is what I would do, between the joist design and the IRC I would think it is prescriptive and may not require an engineer.
 
Build out the framing to be flush with the face of the concrete. Place a full height wall under the the joists.
 
Thanks for all the replies! I took some careful measurements of the existing stuff, and it looks like I have about 5-1/2" of space between the existing rim joist and the outside of the foundation. So building out the rim joist would be a pretty good option, as several of you have suggested. I could use a treated 2x6 flat on the concrete with 4 new 2x8s on top of it. Then use Simpson MIT3511.88 top mount hangers to hold the joists. I will probably do as ICE suggests, and build a wall underneath just for insurance. The basement will eventually be finished anyway. So the new floor would look like this:

52306504338_65e15d1a93_c.jpg


I think I like this option. For the other end of the joists I think I'll bring the foundation up near the top of the joists and hanger right off the sill plate. I was hoping I could drop it down a few inches and use a conventional rim joist, but the grade is too high. Something like this should work I believe:

Joist-Hanger-Subfloor2(DSC00115.jpg
 
You probably dont need 4 - 2x8s but this is what I tried to suggest. You shouldn't need the wall below structurally.
 
bill1952 said:
You probably dont need 4 - 2x8s but this is what I tried to suggest. You shouldn't need the wall below structurally.
Click to expand...
That’s a lot of lumber. Has to be a way to have one 2x for the new rim joist, and some kind of interrupted blocking.
 
e hilton said:
That’s a lot of lumber. Has to be a way to have one 2x for the new rim joist, and some kind of interrupted blocking.
Click to expand...
The trick is either to secure tie it to the existing anchored plate or install new anchors. I don't see the 4 - 2x8s doing that very well actually.
 
bill1952 said:
You probably dont need 4 - 2x8s
Click to expand...
e hilton said:
That’s a lot of lumber. Has to be a way to have one 2x for the new rim joist, and some kind of interrupted blocking.
Click to expand...
Good point. I'll want to have a full length 2x8 connecting to the existing rim joist so the new subfloor has somewhere to attach. And clearly I'll need another full piece where the hangers attach. The middle 2 pieces could be interrupted blocking though. And that would leave some hollow spaces for the new anchor bolts to live in.
 
My immediate thought was to install a ledger. Ramset in place for initial placement, then make permanent and suitably solid by drilling and setting epoxy-fixed bolts into the concrete.
The other idea of a second wood-frame LB wall also has merit.
 
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