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Thread: Locate support for beam in floor

  1. #1
    I&C Engineer (mostly WWTP) Lakee911's Avatar
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    Default Locate support for beam in floor

    Hi all,

    Where should I locate a support under this beam to stiffin my floor? These are the floor joists for my kitchen. It'll be flush with the floor/ceiling. Support midspan or closer to the doubled 2x8 end?



    That's the addition at the very top. My 107 measurement is too small by 6in. Opps.

    Yellow: 2x8's
    Aqua (lower): 2x4's -- joists sit on top of this ledger
    Gray: Concrete Block Wall
    Aqua (upper) : 2x10
    Brown: Supports for addition -- upper story (above addition) load is transfered to these points not via the 2x10's.
    Purple: Some termite damged sections (from late 1970's, no sign of return). May replace these. Has somee wiring going through though.
    Peach: New beam, 2x10...not installed yet. Needs to be flush w/ ceiling below. Already not a lot of headroom. Right side rests on foundation. Somewhere in the middle will be another support. Not sure where to put this yet. Idears? Was planning on supporting the joists, cutting them and plopping this sucker in from the top. Support will be on a 4x6 through basement slab on a sizable foundation --need to start cutting/digging as soon as I know where.

    Floor sags the most around those two joists that are close together. Double 2x10's on the left are at the same height as those that are on the right--no, or negligible sag.

    Thanks for taking the time to look at this.

    Thanks,
    Jason

  2. #2
    Plumber Cass's Avatar
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    Default

    While I'm not a carpenter I would place the support mid span.

  3. #3
    In the Trades Bob NH's Avatar
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    Default Other Options to Stiffen the Floor

    The beam will be stiffest if the support is approximately 25% (detailed calculation required) in from the free end. The point is to make the deflection of the shortened span the same as the cantilever end. The deflection for a point load (someone standing in the middle of the span) is inversely proportional to the span cubed.

    The span is not that great, and the floor can be stiffened by increasing the stiffness of the joists. The stiffness would be doubled by putting a 2x4 flat on the bottom of each joist (glued and screwed) and by gluing and screwing the subfloor to the top in a manner designed to form the top flange of a beam. The bottom 2x4 doesn't have to go to the support because the deflection depends on what is happening in the middle of the beam. The 2x4s would not be any lower than the proposed beam.

    If it is most important to not lose any height, then the stiffness could be achieved by putting plywood on both the top and bottom of the joists to make a box beam.

    Does the existing floor have diagonal bracing? That is important to make the joists act together for a single load such as from someone moving across the floor.

  4. #4
    I&C Engineer (mostly WWTP) Lakee911's Avatar
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    Thanks Bob. The support is cheaper than the plywood method, so I'll prolly go that route. Joists do have cross bracing...I put a lot of them in myself. Didn't help much. Long span for 2x8's ... I'll probably have to put them in (again) half way between foundation and the beam. Think it'll help much for that?

    Thx
    Jason

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
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    Cross braces keep the joists from twistin and a little sharing of the load, but don't really help much for reinforcing the floor from a deflection standpoint.

    BBCamp gave you his suggestions and is a structural engineer...didn't believe him?
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  6. #6
    I&C Engineer (mostly WWTP) Lakee911's Avatar
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    No, it's not that I didn't believe him. It's more of, 'How much can a joist twist in a 6 to 7 foot span?' I know it's essential in a longer span, but I'm sure there is a rule of thumb as to how often to place them. Sorry if I was unclear

    Thx
    Jason

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