Load bearing wall - load distribution of sub floor

[jdubs88]

I am looking at a site where the homeowner is adding a second story to part of an existing one story house. The new ceiling joists in the 2nd story run perpendicular to the floor joists in the second story, making the wall between bedrooms load bearing (supporting the ceiling joists / attic load). The subfloor is 3/4" Advantech structural panels.

My question is will the subfloor distribute the load from the wall across multiple floor joists, or will the wall load transfer straight down into the single floor joist beneath the wall. This will dictate the need for a load bearing beam or not depending on how many floor joists can be taken into consideration to support the load. The floor joists are a 2x10 sistered to a 2x8. There is not a wall directly under in the 1st story. Any advice or previous experience here?

Green Lines = Ceiling joists
Red lines = Floor joists
Blue line = load bearing wall

 

[XR250]

I would add a beam.

 

[phamENG]

Add the beam.

Will it distribute? Yes. How much? Depends on stiffness. That means that for the load to distribute, you have to have deflection in the floor system. I've been in new construction houses with this situation where they added double joists below the wall and there was a noticeable slope toward the wall within about 3' of it. And pay close attention to the calculated deflections of the floor joists and the beam you're putting in. You can just pass L/360 for the beam, but if the floor is L/780 then there will still be a perceived "problem" in the final structure.

 

[jdubs88]

Thanks for the reply and input, I appreciate your time. I agree a beam would be the best solution here. I did a site visit and unfortunately the area in question is above the untouched first floor (so sheetrock ceiling installed), and they already have the subfloor and walls framed in the second story, so there is currently no open access to add the beams in, so I am trying to figure out a solution with the existing setup if possible.

The load bearing walls are only supporting the attic load (no roof load), but it will have partial storage, so looking at 20 psf LL. I could see the 3/4" Advantech panels helping distribute the load out to some of the adjacent joists, helping resist deflection, but you definitely have a valid point with there being possible excessive deflection depending on how the stiffness plays out.
Running some calcs considering just a single joist (sistered 2x8 and 2x10) it is passing the deflection check (L/528 for Live Load), but failing the bending check. So if I were to consider the subfloor spreading out the load across 16"-32" and loading up 2-3 joists, the strength check would clear.

@phamENG - in your experience, have you seen anything that would justify this approach?

 

[SJBombero]

I don't see how one could justify that the floor sheathing would spread out a concentrated load. The relative stiffnesses of the plywood and the joists are just not comparable. Maybe in real life something actually happens with non-linear effects but I think that will be a deep search.

Can you install a ceiling beam to carry the ceiling joists and remove that load from the wall?

 

[XR250]

I get about 1/2" live load deflection (200 plf E = 1.4E6 I = 150 +/-, l= 12'-6"). L/286
Are you sure you did your math correctly? You can also put a beam at the top of the wall instead of in the floor system.

 

[jdubs88]

I measured the clear span on site and the floor joists have a clear span of 10'1", the Drawings were showing an incorrect value it seems, so that was helping with the deflection #, but I will double check the calcs as it is possible it is not handling the sistered joists of different size correctly for the I value.

Definitely agree, a beam at the top of the wall would be a great solution, would also allow them to knock the wall out in the future if they wanted (it is in between two small bedrooms). Could easily support it on both ends with a stud pack in the 2nd story (2500lb point load), but run into issues getting the load through the first story since all the walls are closed up down there (existing bedrooms). However that may not be avoidable at this point, and they may just need to be open to some sheetrock repair.