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Sizing plywood truss gussets and fastening schedule

MSSUSR9501 | Posted in General Questions on

Trying to figure out best way to build a gable support that will be between a scissor truss and a standard truss.
Scissor trusses provide gabled ceiling for living room.  Ceiling transitions to standard trusses with flat ceiling over stairwell and into foyer.
Original construction had an aesthetic boxed section at the transition.  The boxed section provided a visual separation of the living room from the foyer/hall/stairwell area but had no structural value.
We removed the aesthetic boxed section and I am now trying to figure out how to transition from the gabled ceiling to the flat ceiling.  The distance spans 17 feet and I am concerned if I just hang a two by joist it will sag over time.  Thought about building a truss and did some calculations to determine loading.  Results show loads are pretty small.  The only real loads will be the weight of the truss and the weight of the gable end drywall plus a couple of feet of flat ceiling.  No roof loads as the roof is several feet above the top of the scissor gable.
Having designed a truss, now trying to figure out how big the plywood gussets need to be and what kind of fastening schedule is required.
Loads indicate the bottom chord will be in tension with a force of around 300 pounds, the top chords will be in compression at around 311 pounds or so.  Scissor gable pitch is 3 1/2-12.  The bottom ends of the top chords will exert a shear load horizontally as well as a down load to the supporting walls.

Any suggestions on how to figure out how big the gussets need to be?
Truss is essentially a long shallow triangle, 17 feet across and 28.7 inches high.

Thanks for any comments.

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    User...501,

    Admittedly I'm not very good at visualizing things people describe, but I'm a bit lost here. If you have a truss package with scissor trusses on one area, and standard ones on another, why are you having to free-frame anything? The last standard truss either has a scissor truss butted up to it, or has a 2"x4" following the same slope as the scissor truss nailed to it. There should be no loads taken up by anything but the trusses. I'm probably just not understanding what you are trying to describe.

  2. user-6184358 | | #2

    Use the code shear wall nailing tables for plywood type, thickness, nail type & spacing. Nail spacing gets you to the plywood size.

  3. 730d | | #3

    I for sure could not follow that but it sounds like you want a transition to occur between the last vaulted truss and a flat ceiling. You could plywood gusset / sheath the entire thing ?

    1. ohioandy | | #5

      What Mike says. Although you seem to be confident that this thing is not carrying any load other than itself and the drywall that covers it. In that case, a 2x4 is all you need; the drywall will stiffen it and prevent sag. If there is more loading of this 17' span in only 28" vertical space, you could essentially assemble a custom LVL in situ. The basic answer to your question, even if you have no inspector looking over your shoulder, is to WAY overbuild. It's cheap and easy, then no worries.

  4. CMObuilds | | #4

    Wouldn't your transition to flat ceiling fall mainly over a wall you can frame on?
    Otherwise blocking between the two trusses and framing down from that.

    If the scissor and common trusses don't match at all run 2x4 against the common truss exactly like the scissor truss bottom chord, then you have something to block between consistently and can frame down from that.

    If none of this would work Im lost without a picture.

  5. MSSUSR9501 | | #6

    Thanks Very much for all your contributions, I appreciate it.
    I recognize I tend to overthink things and over engineer. I get curious about information and learn something new every time I overdo it.

    Malcolm
    You said "The last standard truss either has a scissor truss butted up to it, or has a 2"x4" following the same slope as the scissor truss nailed to it. "
    Actually the transition happens in the gap between the last scissor truss and the first standard truss. The first standard truss has no scissor butted to it and it has no 2x4 following the scissor slope nailed to it.

    Tim R
    Any idea if those tables are available online?

    Mike Theis,
    You said "You could plywood gusset / sheath the entire thing?"
    You are correct, that is an option. I am trying to be somewhat efficient with wood and my wife and I need to be able to lift this into position. Not as young as I used to be and nowhere near as good shape.

    Andy CD
    You are probably right but I would rather overdo it, thus my interest in assembling a mini-truss

    T Carlson
    No wall under the transition point, only open space.

    I have added before and after pics and a couple of sketches, perhaps they will help inform. One pic shows the transition area before demo. Two images of the area after demo. The last scissor truss sits about 6 inches inside the open edge of the drywall in the ceiling. The first standard truss is about 18 inches back from the opening.

  6. CMObuilds | | #7

    How was it framed before, it looks like you pulled it apart and are going to rebuild it.

    Assuming there isn't anything odd going on, you’d nail a flat 2x4 to the same pitch as the bottom of the scissor, then install blocking between the two. From this blocking your “gable” framing would be attached and blocking from your new gable framing back to the common truss.

    Some diagonals from the lower corner back to the common top chord will support the new corner.

    We do this with light gauge steel to save weight a lot, if there is condensation possibilities wood would work as well, its not a big cantilever. Looking at your pictures a beam most likely isn't necessary but could come into play with an inspector. In that case field building a gable truss should be discussed.

  7. user-6184358 | | #8

    Here is a code document with the tables. https://awc.org/codes-standards/publications/sdpws-2015
    They have a free read only version link on the page

  8. user-6184358 | | #9

    If the span is only about 17 ft why not just place a 2x12 on edge and span the entire room. You can buy 20 ft sawn lumber at the big box stores.

  9. MSSUSR9501 | | #10

    Thanks for the comments and sorry for the delay responding.
    Regarding 17 foot 2x12, local big box store does not carry anything over 16 foot for 2x12. Pro Lumber yards probably carry longer but it seems like overkill and a pain to get home on top of a sedan.
    I have been reluctant to add blocking nailed to the existing trusses. The trusses are almost 30 years old and are really dry. I don't want any risk of splitting a chord or causing truss damage that requires professional repair.

    In an effort to break out of analysis paralysis, I have decided to build a truss using a design I found online. Instead of using metal truss plates at the joints I plan to cover one side completely with 1/2 OSB. On the back I will use OSB gussets at each joint. I will through nail using 3" x .131 nails and clinch the nails. Online research suggests that the shear force capacity for .131 nails in double shear is somewhere between 50-100 pounds per nail.

    One downside I see to this approach is that now the support for the ceiling transition will fall on two non-load bearing walls instead of being suspended by the trusses. The trusses are supported on one end by the exterior load bearing wall. The other end is suspended from a double bridge truss that is supported on both ends by exterior load bearing walls.

    Dang it, maybe I'll just add the blocking and be careful nailing into the trusses

  10. CMObuilds | | #11

    Predrill and use structural screws.

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