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Thread: Floor truss question

  1. #1

    Default Floor truss question

    Hi I have a house that has 16" floor trusses placed 16" o/c, the trusses are made of osb and look like I beams. The span of the trusses are about 25ft across with load bearing walls in the middle. My question is does anyone know what roughly the max load capacity would be for this type of setup......Thanks

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    The only safe way to check is through the manufacturer. There should be stamps on the things identifying the maker and model. It very much depends on the flange size, thickness, and material and the web height. Anything other than from the manufacturer would only be a guess. Sometimes, they make that info available on their website.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    DIY scratch-pad engineer leejosepho's Avatar
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    What you have are called I-joists rather than trusses, and I believe they are stronger than a regular 2x? of the same height.

    Here is a link you might find helpful:
    http://www.apawood.org/level_b.cfm?content=prd_joi_main

  4. #4

    Default I-joists

    I joists are considerably stronger than 2X lumber because loads must shear the laminates rather than overcome the fiber bending strees of a homgeneous piec of lumber. Their real advantage is not that of increased load capacity but rather the increased span length you can utilize. The link is great, but I suggest calling the mfg. When I contacted them for a long laminated beam to go under 3 stories to replace an old timber they were more than helpful. One thing to rember though the load capacity of the I-joist is heavily dependent on span distance and spacing between the joists. My book does not cover your size, but one important point to remember is all 1st floor structures are designed around a minimum of 40 lb/sq ft live load and 10 lb/sq ft dead load. Your I-joists are probably considerably higher, but you would be conservative using those values to determine capacity.

    FRC

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
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    There's a big discussion between the various agencies that make recommendations on floor structures to spec things for tile installations. The current recommendations are to set a limit on deflection, but the newer thoughts are to design for the actual load rather than some arbitrary spec, like the 40/10 mentioned. One thing to keep in mind on that is that the entire subfloor and finished material makes up that 40 pound value. Throw on some cement board, and extra layer of ply and maybe some leveling compound and a stone tile, and you may have used up your entire dead load without any furniture or appliances. One other thing that is part of this whole discussion is long term creep of the structure. While a tiled floor might survive with the current spec for awhile, but, especially with dimmensional lumber, a constant heavy load can cause it to gradually continue to bend and the floor to fail. The Tile Council of America personnel were asked to investigate just this problem and this started the discussion of 'is the current means of specifying floor structure adequate to ensure long-term survival'.

    This may be totally off topic for what your objective of the original question was, but I thought it was interesting, and might be very pertinent.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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