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Thread: Cross Bridging

  1. #16
    Join Date
    Dec 2008
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    Milford, DE
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    168

    Default Re: Cross Bridging

    R502.7.1 of IRC requires bridging for members *greater* than 2x12, though I suppose it couldn't hurt.
    Mike

  2. #17
    Join Date
    Dec 2009
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    Sterling Heights, MI
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    Default Re: Cross Bridging

    Quote Originally Posted by mrd999 View Post
    R502.7.1 of IRC requires bridging for members *greater* than 2x12, though I suppose it couldn't hurt.
    It doesn't hurt if you don't mind squeaks in the floor system.

  3. #18
    Join Date
    Jul 2007
    Location
    Northern Jersey
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    1,283

    Default Re: Cross Bridging

    I had an architect that loved the structural end of the const. We had an over spanned floor system he wanted remedied by putting an upward camber on the joists with braces then glued sheathing and blocking so when the braces were pulled the floor would tighten more under normal deflective conditions. It was a truely stiffer floor.

    Dick you brought up a foggy area of the code with solid blocking over interior supports. If the goal is to prevent rotation of the joists then the floor sheathing & plates of walls above would handle it for us. So only an unfinished attic space would need blocking.

    The confusing part to me is if the framing over interior beams happens to be under a shear wall or greater than 10ft so its blocked for fire stopping. Of course I rarely see these blocks, this area of the house is usually under a hallway where electric and duct work is fed.
    Tom

  4. #19
    Join Date
    Aug 2009
    Location
    St. Paul, MN & Northern WI
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    275

    Default Re: Cross Bridging

    Tom:
    Regarding your blocking question, we have to be able to transfer lateral loads from above through the depth of the floor system and to the beams and foundations. There is a shearing load (lateral load) in the plane of the fl. sheathing, from above, and an equal and opposite load (force) in the sill pl. and found. below the rim jst. At the outer walls, the rim jst. does this transfer as it is nailed to the sill pl. and the fl. sheathing. Then the jsts. are end nailed through the rim jst. to prevent them from rolling over. This rolling over issue also has something to do with walking on the jsts. before the deck is applied. Over an interior support (beam, bearing wall) the only thing that will properly transfer these lateral loads is solid blocking (a 2x jst. piece by 14.5 or 13" long) in the jst. spaces. It is nailed to the fl. sheathing and the plate on top of the beam or wall, and acts essentially like the rim jst. does, it can not parallelagram in between the jsts. The IRC just says there should be blocking btwn. the jsts. over interior supports, and that is to transfer these potential lateral loads. It can be shown that every jst. space doesn’t need blocking to meet this need, the fl. sheathing and the plates will transfer these loads across an unblocked space. So, you can get mechanicals through some of those spaces. But, fl. sheathing and wall pls. won’t do this through the depth of the fl. system, thus the need for the blocking. And, an inspector shouldn’t hassle you about a few unblocked spaces, as long as you don’t leave out 50% of that blocking. Fire blocking is a whole different issue. And, in many older houses this lateral load transfer blocking was not done, usually with little ill affect. An experiment for you is, nail three or four joists down to a deck, nail a piece of sheathing on top of the jsts., and push laterally on the sheathing, in the plane of the sheathing and perpendicular to the jsts. The jsts. will tend to roll over. But, put a piece of blocking btwn. two of the jsts. and now they won’t roll over.

    Regarding floor system stiffness, given a joist size and span length, various systems in order of stiffness line up as follows, least stiff to stiffest: (1) nailed sip lap fl. sheathing; (2) nailed plywd. or osb; (3) screwed plywd. or osb; and (4) glued and screwed plywd. or osb. This stiffness relates mostly to improvements in deflection and goes from individual jsts. with boards atop them to something which is a bunch of tee shaped members which act much stiffer. As the tee member action improves the stiffness, you will finally overstress a given jst. size bottom edge in tension, so there is a limit to each jst. size and its cap’y. The glueing and screwing makes the system stiffer, but only marginally stronger. When you camber the joists up and then glue and screw the osb sheathing on top, it’s akin to prestressing the jsts. You have locked some tension stress in the top of the jst. and some compression stress in the bottom of the jst., by your jacking process When the glue dries and you release the shoring the fl. system will retain some of its camber, a negative deflection. So as you load this fl. it is starting its deflection from the higher arched shape, and also those locked in stresses must be overcome before you see a positive deflection. The first few pounds you put on that jst. are just overcoming the locked in stresses and now you can continue loading the jst. to its full cap’y. Your Arch. friend may have loved the structural end of const., but his understanding is questionable if he speced. the over spanning fl. system, unless he was trying to fix someone else’s error.
    Dick Hackbarth, PE
    RWH&AI, Consulting Engineers

  5. #20
    Join Date
    Mar 2006
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    Seattle, WA
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    Default Re: Cross Bridging

    Quote Originally Posted by jimAKAblue View Post
    It doesn't hurt if you don't mind squeaks in the floor system.
    Huh. I've blocke plenty of floors. Those floors don't squeak. Sorta shoots your "skilled framers" argument down a bit.

    Nonetheless, it is a good point- Dick likes to point out that they uased to wait a year before nailing in the lower end of the cross member if they could. That ensures the lumber is acclimated. I'd say that today we can measure humidity and do that nailing when everything is about right. Using dry lumber certainly helps.
    http://www.lavrans.com

    "He uses statistics as a drunken man uses lamp posts; for support rather than illumination." -Andrew Lang

  6. #21
    Join Date
    Sep 2009
    Location
    Tacoma, WA
    Posts
    36

    Default Re: Cross Bridging

    Is lumber really such an expense that a solid block couldn't be used in stead of crossed members?

    Have you looked at engineered floor joists?

    What about joist spacing?

    Simpson makes stuff for this (of course they do, they have everything and the engineer will wet his panties): http://www.strongtie.com/products/ca...ging_misc.html

    I wouldn't recommend floor trusses, used them once but never again.

  7. #22
    Join Date
    Mar 2006
    Location
    Seattle, WA
    Posts
    11,339

    Default Re: Cross Bridging

    Quote Originally Posted by RocketScott View Post
    Is lumber really such an expense that a solid block couldn't be used in stead of crossed members?

    Have you looked at engineered floor joists?

    What about joist spacing?

    Simpson makes stuff for this (of course they do, they have everything and the engineer will wet his panties): http://www.strongtie.com/products/ca...ging_misc.html

    I wouldn't recommend floor trusses, used them once but never again.
    Solid blocking isn't necessarily better or less likely to squeak.
    I-joists can benefit from bridging (we've gone over this one, rather unfortunately), but generally are engineered to not require it. That's part of the "silent floor" idea, although really it requires a careful install and will squeak if installed sloppy... just like solid lumber.
    Metal bridging is common, and Dick H. already noted that you can even simply strap it.
    Heck- if you're really just wanting it rigid, you can build torsion boxes, or buy them. Tri-Cell makes a variety of panels with a honeycomb cardboard core that are rated for 8' floor spans, and other companies probably make similar (I only have knowledge of that company).
    http://www.lavrans.com

    "He uses statistics as a drunken man uses lamp posts; for support rather than illumination." -Andrew Lang

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