Sistering to Undersized TJI Floor Joists

[SHEAR_FORCE]

We have a client who has undersized TJIs (older TJI 150s - out of warranty?) in their second-level living room floor. The joists were shown on the plans as being spaced at 12" OC but they were installed at 16" OC. The floor has ~2" of gypcrete with radiant tubes and is now sagging so they want to add self-leveller and we need to strengthen the joists. We are considering strengthening with an LSL on one side of the joist to reduce further sag and floor vibrations. My thought was to glue the top of the LSL to the plywood and screw/glue the side where it will be in contact with the TJI top and bottom chords. I was plannign on using Simpson #10 SD screws to reduce the risk of delaminating the top and bottom chords of the TJI.

Any opinions on this approach are welcome.

Another option would be to add new joists between but it would likely be more expensive as blocking would be needed each end of the joist.

 

[jayrod12]

Gross. That will be a tough thing to do unless you're jacking the floor back up, which I wouldn't necessarily recommend. Those new LSL joists will be tough to get to match the existing deflection.

I also wouldn't directly sister them as you're showing. All you're going to do is split the joist chords regardless of which type of screw you're going to use, or potentially mess up the connection of the web to the chords.

I assume you were trying to connect the new to existing joists to provide lateral support for the new joists? If so, I'd be more partial to putting the new joists halfway between existing, and then solid block on each side at whatever spacing you need for your new joists to calc out.

Still doesn't solve the issue with the existing deflection, but it solves the concern about destroying the existing joist chords.

 

[phamENG]

Oof. Neither of those options are going to be easy. How much is the floor sagging? To get those LSLs in, you're going to need to jack up the floor to level. If you don't, the new LSLs won't fit. They'll be nice and straight, while the existing TJIs are curved in the current shape of the floor. When dealing with dimension lumber this isn't a big deal - just rip the sistering material a little shallower than the joist so it can bear on the wall at the end and just touch the sheathing at the low point. But that's not a luxury you have with I-Joists.

Be careful when jacking, though. In addition to cracking the gypcrete above and possible other finishes, you have to worry about crushing and web buckling in the I-Joists. Probably need to put web stiffeners in on the side that won't be sistered before jacking.

 

[phamENG]

jayrod just beat me. Good point on messing up the chords - OP did say they're 150s. Those will be dinky little things. Might want to design the LSLs (or whatever you use) to take all the load if you can.

 

[jayrod12]

jayrod just beat me. Good point on messing up the chords - OP did say they're 150s. Those will be dinky little things. Might want to design the LSLs (or whatever you use) to take all the load if you can.

Forgot to add that last bit to my answer, I'd be designing the new ones to take it all just for ease of design.

 

[JAE]

Another problem is getting the new LSL's to bear on the existing wall each end???
Or were you planning something else at the ends to allow the new LSL's to simply be raised into position?

If the LSL's are to bear on the existing walls, they will be "too long for the room" so to speak and the contractor would have to lift them up, slide them past the first wall and then slide back onto the second wall - or something.

 

[SHEAR_FORCE]

Thank you everyone for your replies! I called Weyerhauser and came up with a solution that I think is ideal for this situation. To answer some of the previous questions and to help those in the future which may be running into this unique issue, please see the following notes.

Original Approach
- Apparently TJIs have an indefinite warranty. Sistering the new LSL to the existing TJIs would void the warranty .
- The Weyerhauser engineer pointed me to TB-206 (https://www.weyerhaeuser.com/woodproducts/document-library/document_library_detail/tb-206/), which gives max spacing of fasteners to the narrow face of the top/bot chords, so you can go this direction without the risk of splitting the chord, although you can only do it form one side.
- Since the existing TJIs still worked in shear we were going to add web stiffeners and develop the needed support screws through the blocks. The LSL stiffeners would be short of the wall on each end.
- I am confident this approach would work but I would be taking on more liability.

New Approach
- Adding new joists in between will not void the warranty.
- You can add new joists between the existing by hanging them from new bearing blocks at the top plates. The bearing blocks will be installed between the existing joists and will be flush with the inside face of the top plates to allow for the new joists to be cut just barely shorter than the span.
- The new TJIs can be glued and screwed, with 12" oc min up throught the top chord to the bot of the floor ply. We need to size the screws so that they don't pennetrate past the sheathing into the gypcrete.
- Weyerhauser approved this approach.