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Thread: Steel Defection Over '

  1. #1
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    Confused Steel Defection Over '

    Greeting everyone, I am new this forum and i am hoping to get a hand with a bouncy floor solution. Because the 2 joists in question have wires and pipes going through them I would like to make flitch plate of steel to one or both sides of each joist if needed.

    Joists are 8" and wires and pipes are on the bottom, and as such have been compromised.

    My thoughts are 3/8" x 4" x 8"6" hot rolled steel, held in place with 1/2" threaded rod, flat & lock washers, and heavy nuts.

    Joists are 16" on center under a small bathroom. They are not sagging.

    I hate to admit it but I'm struggling with this unorthodox approach.

    Am I underkill, overkill, and of course I am open to suggestions and appreciate your help with this. Thanks.
    Last edited by docfletcher; 02-25-2014 at 11:09 AM.

  2. #2
    Lead Engineer Cake of Doom's Avatar
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    Re-analyze the joists to see how much they are falling short of the required size, for the deflection limit you need. This should then give you enough information to size your plates. Or (cheaper option), if simply sistering the joists in that area will be enough to cure your woes.

    Just for clarity; what size are the existing joists? Picking through the dimensions in your post it looks like they're 4" x 8" x 8' 6" long.

  3. #3
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    Floor Joists are 1 1/2" x 8" x 8' 6"

    There are holes with wires and pipes along length of the joist within an inch of the bottom. I would like to use a piece of 3/8" x 4" x 5' hot rolled steel over the pipes/wires in the middle of the span to stop the floor from being bouncy. It's a residence so the live load is only people.

    I think 2x10's should have been used when this home was built instead of the 2x8's. So in effect I want those 2x8's to act like 2x10's.
    Last edited by docfletcher; 02-26-2014 at 08:16 AM.

  4. #4
    Lead Engineer Cake of Doom's Avatar
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    Are the joist deformed? Reason I'm asking is because over that distance, under normal loading, they should only be deflecting ~2.3mm; which is nothing. The limit is 6.94mm so its well within normal boundaries (or should be, which is odd).

    Anyhow.

    Normal rule for a flitch beam is that the steel depth and length should match the timber it's being mated to, which should be more than enough to act as a bounce limiter (using original thk of 3/8).

  5. #5
    Technical Fellow jboggs's Avatar
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    Cake of Doom - aren't you getting into a local codes issue here?

  6. #6
    Lead Engineer Cake of Doom's Avatar
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    Depends on the cause of the bounce. L/360 for deflection limit is pretty much accepted all over, as is a 1.4 DL FoS and 1.6 LL FoS.

    If the bounce is due to shoddy installation/timber grade being to low for the job/the holes for services being over 1/3 the depth of the timber (or just huge)/damage, then yes; get a qualified local engineer to come and inspect. I did forget to add the caveat of "don't take advice from the internet" though. Too late for an edit.

  7. #7
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    Joists are not deformed and not sagging. Code is not an issue. Just want to get floor deflection reduced for large ceramic tile install. I believe the holes so close to the bottom cause the joists perform like 2x6's. Piece of steel in the middle of the span will reduce deflection, but is it enough? I say 4" steel because I don't want to pull the pipes and electrical.

  8. #8
    Lead Engineer Cake of Doom's Avatar
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    You would have saved me a lot of bother looking at the existing joists, if you'd have mentioned it was for a DIY job.

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    So sorry. I did not wish to cause trouble or problem. I did not realize DIY would at issue. Please accept my apology for the inconvenience.

  10. #10
    Lead Engineer Cake of Doom's Avatar
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    No worries.

    Generally, holes through joists for domestic services, are not a great concern and only have a nominal effect on the beams performance. If however, your joist looks like Stallone used it for a set piece in the Expendables, you should get them checked by a professional before imposing further loading on them.

    Out of curiosity: what are the vital statistics for these tiles, that you're worried about your joists?

  11. #11
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    3/8" thick 12" x 12" . I'm going to use a single layer 1 1/8 inch thick Advantec engineered sub floor with a isolation membrane to tile to.
    Once the Advantec is down it might stiffen things up a bit. If not I will need to do something. 3/8 x 4" weighs about 5lbs per foot. So the steel would at 25lbs per joist. Since I will be using 3/8 thick steel maybe I won't need to place wood over it.

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