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Framing a long span

Bob858 | Posted in Construction Techniques on March 19, 2005 04:22am

building a 1 and 1/2 story house. 42 feet deep with covered porch on front. Load bearing walls front to back at points 0 ft (ext front porch wall, 8 ft (front house wall, 25 ft ( load bearing interior wall parallel to front and rear walls ), and 42 ft (rear exterior wall).  The house is two rooms deep the front being 16 x 16 and the rear being 21.5 x 17 ft. Running parallel to the prev mentioned weight bearing walls there will be an upstairs knee wall six feet high running parallel to the the length of the room. this knee wall will run the entire length of the 21.5 ft. of the rear room.

I want to use I joists to span the entire house front to back. The first estimate on the I joists was $22,000, the lumber yard saying that I need extra large I joists with “plates” a minimum of 3 and 1/2 inches wide at the top and bottom of the I joists.

That is a budget buster for me and wondering about steel as a possibility or is there another route? Of course I will get more estimates on the joists.

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  1. User avater
    RichColumbus | Mar 19, 2005 05:29am | #1

    I would check out a steel price... if nothing else than to justify the cost of the I-Joists.

    Steel ain't cheap... and ain't cheap to set either.  But it wouldn't hurt to get specs and see how it prices out.  Don't forget to add the ancillary costs of the change for comparison sake.

  2. Lansdown | Mar 19, 2005 05:34am | #2

    If you have a load bearing interior wall, why not break the span there and reduce the depth (and therefore cost) of the joists?

  3. User avater
    Mongo | Mar 19, 2005 06:26am | #3

    Did you get a look at the CAD drawing that the yard came up with when they submitted the plans for the takeoff? I've seen a few yards miss some details in the framing, resulting in very strange TJI specifications.

    What load are they designing to? Looking for an L/XXX number, something like L/480, L/720, etc.

    I've also had two different yards come up with two very different spec sheets even though they were given identical plans and cover letters.

    Have you considered not doing the porch with TJIs and framing it separately from the house? Then you'd be looking at a 34' depth with two 17' spans. 17' span 16oc is easy. Frame the 8' porch span with smaller TJIs or 2-by lumber.

    I missed the point on one thing...that 6' kneewall, which I'm guessing is structural...is that stacked directly over a load bearing wall below, or is the load from that somehow being transferred somewhere mid-span on the TJIs?



    Edited 3/18/2005 11:29 pm ET by Mongo

    1. FramerT | Mar 19, 2005 02:23pm | #4

      What Mongo said. Without seeing a drawing or something, I'm guessing here.
      Why use expensive TJIs for the porch, main house rafters sitting on porch beam? You said 1 1/2 story home.
      As mentioned,if that knee-wall bears only on TJIs and not a bearing wall, theres the [one]reason why the big i-joists. My 2 cents.
      I didn't do it....the buck does NOT stop here.

    2. Bob858 | Mar 23, 2005 04:24am | #9

      the knee wall upstairs falls roughly halfway out the 17 foot span which is about the middle of the room below.

      1. User avater
        Mongo | Mar 23, 2005 04:25pm | #10

        If that kneewall is structural...meaning that it is designed to carry and transfer the loads from the roof rafters that rest on top of it to the TJIs that the kneewall sits on...and the TJIs in turn transfer the load to the walls that the TJIs rest on...then I see why those TJIs are spec'd out the way they are.

        If the rafters can be sized to carry the roof load by themselves, then the kneewall can be considered a simple partition wall and it's possible that the size of the TJIs that the kneewall sit on could be downsized.

        Sounds like a lot of design issues, and we really seem to be missing something here based on the dimensions of the rooms that you give.

        You might want to go back to your architect/designer/builder and have them hash out some of these details.

        If you designed this place yourself, then for some more solid analysis, consider posting 100kb or less graphics of your floorplan on this thread.

        Don't get scared away from this thread, instead jump in with more feedback.

  4. highfigh | Mar 19, 2005 02:34pm | #5

    You aren't trying to span the whole distance, are you? Steel columns and I beams would be a lot cheaper, not to mention the height of the structural members would be a lot less.

    "I cut this piece four times and it's still too short."
  5. DANL | Mar 19, 2005 02:43pm | #6

    I think I must be missing something, but if the house is only 25' wide, couldn't you run the joists 90 degrees from the proposed orientation? (Side to side, not front to back.)

  6. Piffin | Mar 19, 2005 03:05pm | #7

    You have left some very critical information out of your description here. If the same information is inspecific for the Ijoist designers, they had no chioce but to assume the worst and design accordingly.

    So they probably have called out for a 42' free span that also carries some of the roof load.

    You have some complicating factors opossible there, and they are sidestepping liability by overdesigning. They are only focusing on one design element while you have several that must work together.

    That is one reason it can be dangerous to not have a professional on board in the design process.

    Here are some of the complications - boss Hog should be along to correct me where I'm wrong too and remember that ALL STRUCTURAL ADVICE HERE IS WORTH WHAT YOU PAY FOR IT OfTENTIMES -
    When an I joist runs continuousl;y across various supporting load bearing walls, you can sometimes induce a reflecive "bounce" wherein moving a load ( walking) acrtoss one side of the system creates an uplift force on the other side of the system. some I joist designers will call for a cut in the top chord immediately above the middle supporting wall to eliminate this, while others will design a stiffer joist to compensate by adding the strength to resist, and others stil will ignore that center wall and design as though it were not there as a load bearing structure.
    You mention kneewalls but say noithing to specifiy whether or not they are intended to function as load transferring members. That depends on how the roof itself is designed. - size of rafters and whether you have a ridge beam sized appropriately. If the roof can carry itself, then no load is transferred to the flooring I-joists, but if you have inadequate roof design, the floor joists have to be beefy enough to handle the transferred loads. if these questions are not adequately adressed on your plans, that adds to reasons for the expensive package.
    So, have someone analyse the WHOLE structural design with an eye to how the systems compliment one another, then go to the i-joist folks for a price

     

     

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  7. MikeSmith | Mar 19, 2005 05:36pm | #8

    newbie.. i figure you have a 34' span as your max...

    i'd break that span with a beam and posts.. that will bring everything back into the realm of reality

    Mike Smith Rhode Island : Design / Build / Repair / Restore



    Edited 3/19/2005 11:18 am ET by Mike Smith

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