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Controlling Rafter Thrust

Discussion in 'Residential Structural Codes' started by jar546, Mar 1, 2019.

  1. jar546

    jar546 *****istrator

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    The beam is a nice touch to create a clear-span, open space and so is the ribbon strip as a ledger. However, where is the rafter thrust control? In this case, other than the collar ties which are not for rafter thrust there is nothing, therefore this is not a compliant installation.

    IMAG0104.jpg
     
  2. Sifu

    Sifu Gold Member

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    Agree. Have seen it many times. I have seen and approved coil strap used to span across the beam as well as additional framing members above to create a raised rafter tie.
     
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  3. steveray

    steveray Sawhorse

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    Straps under are a nightmare for the drywaller, Simpson does make a strap that goes overtop....Of course...Seat cut on the rafter looks suspect too...
     
  4. Ty J.

    Ty J. Sawhorse

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    Good call. Could use coil straps. Have seen an engineer use two Simpson HDUs (one on each joist) with all thread run through the beam to create the continuous tension path.
     
  5. rktect 1

    rktect 1 Gold Member

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    Just install a raised rafter tie directly over the ceiling joists. Within bottom 1/3
     
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  6. Ty J.

    Ty J. Sawhorse

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    Similar to the following image
    [​IMG]
     
  7. Glenn

    Glenn Corporate Supporter
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    ^ Dude that's nuts. That's nuts for the deck application seeking 1500# load resistance. The IRC Table R802.5.2 gives you the nailing requirements for the heel to tie, and footnote e applies the same nails to the ceiling joist to ceiling joist tie. I admit that this table is nuts too for when you get to low slopes, long spans and raised ties. Way more nails than can practically go in that connection. The photo posted doesn't look like much of a span.
     
  8. jar546

    jar546 *****istrator

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    It was a much longer span because that is not the center of the room, it is offset by a hallway and rooms on the other side.
     
  9. Sifu

    Sifu Gold Member

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    I have seen so many creative ways to counter the rafter thrust. Some I had to reject without an engineer, others not. Usually, a raised rafter tie, sometimes kickers tied across the diaphragm with structural sheathing glued and nailed. In the photo I see I would probably accept strapping up and over the beam if they couldn't put in a raised rafter tie. In a lot of cases the span was too far to put in a raised rafter tie or they had tray ceilings or other obstructions. Back in another AHJ stick frame was the norm, and we battled this issue a lot, especially in the ubiquitous bonus room. (We ended up with a lot of structural ridges.) Glenn is right though. Try explaining how 15 or 20 nails in the heel joint works.
     
  10. fatboy

    fatboy Administrator

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    Bingo! Every 4'.
     
  11. ICE

    ICE Sawhorse

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    Sometimes purlins can make all the difference.
     
  12. FLSTF01

    FLSTF01 Sawhorse

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    I once nailed 18 to 24-inch rips of 3/4 Advantech to the bottom of the ceiling joists to span under the beam, then furred out the rest of the ceiling to match. It worked fine. I have also used and inspected Simpson metal strapping over the top of the beam.
     
    #12 FLSTF01, Mar 25, 2019
    Last edited: Mar 27, 2019

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