Lion06
Structural
- Nov 17, 2006
- 4,238
I have a small (but large scale) residential project where I need to sister a lot of existing floor joists. There are two flavors of these joists - the first pockets into a CMU wall at each end, the second pockets into a CMU wall at one end and frames to a stair header at the other.
We will be jacking these joists to relieve the existing DL stresses. How would you typically determine how far to jack them? I've considered doing a DL only deflection check, neglecting creep and using that value (rounded down to the nearest quarter inch).
We will be stopping the sisters short at the ends where these joists pocket. The biggest reason for this is that there is existing HVAC ducts under the joists at one end that we don't want to have to drop, and there are a lot of wires at the other end that we don't want to have to cut, pull, and re-wire. So the fix looks like an existing joist with a sister that stops roughly 2'-3' from each end.
There are several of these joists that we're not going to count on for any strength in bending at center span (this is mostly because of large splits or shakes at midspan - we had a grader come out and grade in-place all of the existing lumber so we have a very good idea of what is ok and what isn't), but the sister is still only coming to 3' from the ends. This clearly requires a connection at the ends of the sister capable of transferring the moment at that location back into the existing joist to take into the wall. What I'm struggling with is is this required where the existing joist has bending capacity but needs to be sistered for strength? In my mind it wants to be just to maintain consistent rotations bending moment, but how important is that really? The moment transfer is critical at the joists with no bending capacity at midspan to remain stable. In the case where the existing joist does have bending capacity at midspan is there anything wrong with just designing the connections (which will be nailed) at the ends of the sisters to dump a shear only (with no moment) back into the existing joist?
Any thoughts?
We will be jacking these joists to relieve the existing DL stresses. How would you typically determine how far to jack them? I've considered doing a DL only deflection check, neglecting creep and using that value (rounded down to the nearest quarter inch).
We will be stopping the sisters short at the ends where these joists pocket. The biggest reason for this is that there is existing HVAC ducts under the joists at one end that we don't want to have to drop, and there are a lot of wires at the other end that we don't want to have to cut, pull, and re-wire. So the fix looks like an existing joist with a sister that stops roughly 2'-3' from each end.
There are several of these joists that we're not going to count on for any strength in bending at center span (this is mostly because of large splits or shakes at midspan - we had a grader come out and grade in-place all of the existing lumber so we have a very good idea of what is ok and what isn't), but the sister is still only coming to 3' from the ends. This clearly requires a connection at the ends of the sister capable of transferring the moment at that location back into the existing joist to take into the wall. What I'm struggling with is is this required where the existing joist has bending capacity but needs to be sistered for strength? In my mind it wants to be just to maintain consistent rotations bending moment, but how important is that really? The moment transfer is critical at the joists with no bending capacity at midspan to remain stable. In the case where the existing joist does have bending capacity at midspan is there anything wrong with just designing the connections (which will be nailed) at the ends of the sisters to dump a shear only (with no moment) back into the existing joist?
Any thoughts?
