Composite Box Beam

[D90Left]

Proposed Knee wall "Box Beam" to support (e) ceiling joists below. Ceiling joists to be converted to floor use; (e) ceiling joists are inadequate to support roof load at knee wall + new floor loads.
(e) knee wall is 2x4 studs @ 16" oc, unsheathed (5 ft tall).
I've considered cdx ea. face, w/ 4x6 blk'g beneath top pl. (compression), and 4x blk'g between ceiling joists w/ cont strap beneath (tension). Also bracing "top chord" to ea. rafter for buckling. Any suggestions?
Thanks,
Jim B.

 

[ArneG]

Be careful.

1) your top and bottom wall plates are probably jointed or spliced, hence cutting your chords in two. Your strap would take the tension load, but how much give will your fasteners have before it takes load. Your compression side, even with new blocking would need to go into bearing on the existing wall studs - how do you take into account movement due to compression of the joints and compression of the wood perpendicular to grain? (in other words your beam needs to bend alot before the load is where you want it)

2) how does the load get from the existing ceiling joists into the beam? Are you lapping the plywood down into the existing joist spaces? Sounds labor intensive.

3)What does this new beam bear on? If interior walls - do those walls go down to foundation?

I would do a couple things first

1) Stand on the floor in mid joist span on my toes. Drop your heel as quick as you can. Does the floor bounce?

If it bounces - reinforce the floor joists with 2x's or LVL's sistered alongside. This will also allow you to level out the floor without a bunch of shimming.

If no -

2) Check the joists with minimum loading and minimum snow/live loading. Maybe the knee wall was added at a later date so the roof technically doesn't bear on it? Can you make use of the flooring as a composite member with the floor? Do the joists run continuous over the interior wall with a lap - can you assume any moment transfer over that interior wall? If the floor is stiff enough I would also ask the city official if they would let it slide without a PE reviewing it.

You could probably remove and reinstall the knee wall with a large LVL beam easier than piecemealing a beam together anyway.

If your going to fix it make sure you have a PE review it. No sense in fixing something and then finding out you fixed it wrong.

 

[D90Left]

Thanks for the input. My first solution was originally to add a long spanning glb girder to pick up all the floor joists, behind the knee wall- that was frowned upon by P.M./Client. My next solution was to lap (e) joists w/ joists, as you mentioned- all frowned upon. It was suggested that I investigate the box beam analysis. I agree with your assessment, but felt like I needed a second opinion.
-Thanks,
Jim B., P.E.