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Thread: Attic Floor Joist Sistering Options

  1. #1
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    Attic Floor Joist Sistering Options

    Hi All, new member and have a question about a project I'm currently working on.

    In my garage attic there is plenty of space and I am converting half of it into a walk-in closet off of the master bedroom. The joists are 2"x12" on 12" centers, with a span of 23.5'. I have been working with a structural engineer, great guy, and he said I have to sister the 8 joists that would be under the attic closet conversion as the load would be too large for the current span.

    The issue I am running into is going to be getting 23.5' SPF boards into the attic and enough space to put into place where the existing joists currently sit. I have read some options online with connecting two boards to act as the sister joist, potentially using tie plates with bolts or additional blocks of wood where the connection occurs.

    Are there any options that exist that would allow me NOT to use a full 23.5' board and instead use multiple (I would assume two would be best, not more) to meet the span to allow me to sister the joists and take some of the load?

    Any and all options are very much appreciated!!!!

    Thanks,

    Adam

  2. #2
    Lead Engineer Cake of Doom's Avatar
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    You'd be better off discussing this with the engineer that first advised you as he/she will have a better understanding of the design limits. They'll probably knock up a construction splice detail to allow to you to connect the SPF without loss of strength.
    Last edited by Cake of Doom; 12-06-2016 at 08:54 AM.

  3. #3
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    Hi - To clarify, I have discussed with the structural engineer that did the analysis. His comment was that he's heard this can be successfully done, but there aren't structural calculations to back it up, thus he can't recommend it.

    I have found some examples online that similar solutions have been used, but not exactly for this example, thus I was hoping to get some additional advice to see if more people thought this was a usable solution before I remove walls or part of the roof to get the full joists in. Obviously I'd prefer to tie two together, but if this is worthless for taking the load then I have no other options.

    Thanks - Adam

  4. #4
    Administrator Kelly Bramble's Avatar
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    What recommendations did the structural engineer give you?

    Additionally, this engineer should be able to do the structural analysis on this concept to provide a formal recommendation.

    Quote Originally Posted by ellisadamm View Post
    Hi - To clarify, I have discussed with the structural engineer that did the analysis. His comment was that he's heard this can be successfully done, but there aren't structural calculations to back it up, thus he can't recommend it.

    I have found some examples online that similar solutions have been used, but not exactly for this example, thus I was hoping to get some additional advice to see if more people thought this was a usable solution before I remove walls or part of the roof to get the full joists in. Obviously I'd prefer to tie two together, but if this is worthless for taking the load then I have no other options.

    Thanks - Adam
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  5. #5
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    He gave me the recommendation of sistering 8 of the joists the full length (23.5'). He sticks with needing the full length joist, which is fine, again just looking to see if there are any other opinions / options out there.

    Adam

  6. #6
    Lead Engineer Cake of Doom's Avatar
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    There are other options. As noted above, the could provide you with a construction splice detail; either a bolted/screwed and glued scarf joint or there are options for resin fixed, steel dowel rods (this option could be costly) etc. Non of these are out of the ordinary. Is your engineer giving you advice as a favour or did you employ them specifically for this job?

  7. #7
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    Asking an on line forum to second guess an engineer that has been on site and made recommendations is a bit like asking us to review your **********s.


    Your engineer must recommend a solution that is safe and meets building codes. Nothing good happens to him if he fails on either score.

    That said, you can look at wood joist span tables just like the rest of us. If you do, you will see that if you have joists rated for 1400 psi (as opposed to 1200 psi) and if your live load is 40lbs/ft^2 or less your 2 x 12 on 12” centers will be right at maximum for bending stress. There are two big if’s here.

    Where the 2 x 12’s fall short, regardless of bending strength rating, is in the deflection which will exceed the 1/360th of the span rule upon which many of the tables are developed. The engineer must either recommend 1) more wood, as has been done, or 2) design a truss like structure that will resist the deflection, assuming your joists are up to the bending stress. The additional wood for this second approach plus the engineering time may make option number one the most attractive.

    We on this forum cannot read the stamp on the lumber, or determine the outline of the rafters above in order to determine if they can be incorporated into truss arrangement.

    The main problem is probably in obtaining 2 x 12’s just over 24’ long.



    Ask about using two 16’, 2 x 12’s, one on each side of the existing joist, with a 90 inch overlap. My guess is that two 16 footers are not much more expensive than something over 24’. ‘X’ or cross-bracing every 8’ is really a good plan with joists that deep. Follow the advise of your engineer.

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