Gable Wall “Hinges”
I’ve always been confused by the ‘gable wall hinge’ discussions. The issue is the (alleged) hinge in a non-balloon framed gable wall where the wall top-plate meets the truss bottom chord. It is often brought up in reference to scissor trusses—that one must frame studs to the bottom of the scissor and not to a flat bottomed gable. But if I understand correctly, there is supposedly no problem with the hinge point if flat trusses are used throughout.
My question is: What is providing the resistance to this hinge with flat trusses / common rafters with rafter ties. There is usually talk of a diaphragm. But what diaphragm? Are we talking the ceiling drywall? Drywall is a structural item now? I’m not arguing the drywall adds rigidity, but is that what’s actually being *relied* upon from an engineering design standpoint?
Are most gables balloon framed still, even with trusses?
What am I missing?
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Replies
maine_tyler
- Typical trusses with flat bottom chords are engineered to assemble as a completely rigid whole. The required bracing of the bottom chords means the gable truss can't move laterally, so they have no hinge.
- Depending on their design, scissor trusses may or may not be similarly rigid, but usually fall into the same category as rafters.
- Rafters, parallel chord trusses, TJ's, all need to be framed with continuous wall studs from floor to the underside of the roof members. Our code reflects that. If those walls have load-bearing elements in them, then there are height restrictions beyond which the wall is required to be engineered. I wouldn't call those walls balloon framing, which usually refers to walls that extend over more than one storey with the floor attached to the sides of the studs.
"The required bracing of the bottom chords means the gable truss can't move laterally, so they have no hinge."
I'm not installing lots of trusses, but I've seen more than a few. I don't ever recall seeing bracing that would resist the force in question, as far as I can tell. Hence my question.
I see 'ties' that run perpendicular to the bottom chords, which would certainly add some stiffness, but that's not exactly triangulating anything the way a diaphragm would. I've heard framers of large post frames say they've seen these ties get 'blown' out the other side of the building, opposite the gable hinge. Every bottom chord is liable to flex in that direction (albeit all bottom chords simultaneously). Running diagonals would prevent this, but I've never seen them.
Perhaps there is other bracing being specified at times?
maine_tyler,
Each truss package I've used comes with a bracing schedule. It contains the standard ones which apply to every case, but also bracing particular to that design which takes into account the plan below. I've had diagonal bracing on the bottom chords called out as well as from the peak of the gable back to the next four trusses.
That's fairly uncommon because the braces perpendicular to the wall don't need to be particularly strong to resist the forces in play. Does the drywall contribute? Sure, in fact enough that the interior seismic braced wall panels called for in our code can often just be sheathed in it.
The other difference between typical trusses, and TJs, parallel chord trusses, or rafters is that they are designed as a package, not just for vertical loading. If you aren't going to be comfortable accepting their engineering to counter lateral forces at the gables, how do you know the rest is strong enough?
Your truss company is designing it as a package? I just ordered trusses and they don't know (or apparently care) what else is going on in the building.
My sense is that, around here, you would need to hire a separate engineer to engineer the building as a system, with the trusses being engineered by the truss company more or less as a stand alone item (within prescriptive code assumptions). I could be misunderstanding certain assumptions though.
And to be fair, it could be that prescriptive code basically accounts for all this by mandating the studs to the roof line like you say. Either way, I'm still not sure I fully grasp when and where the 'gable hinge' is of concern vs when it's been properly mitigated, save for having an engineer just say, 'yes it has' or 'no it hasn't.' Intuitively, adding diagonal bracing (diagonally across the bottom chords) makes sense to prevent it.
miane_tyler,
Here they definitely take into account the plan. The last set told me the intermediate loading on a girder truss, the location of the wall picking it up, and the size of the support necessary. But when I say "designed as a package", I meant it includes everything necessary to resist the forces applied to the trusses - and that should include the gables, so no additional efforts should be necessary.
You can add diagonal bracing. There are a few guides for use for high wind areas, but you need to get the supplier's okay to add any members to engineered trusses or it voids the warranty.
I can't say I've ever seen a roof failure related to the trusses. It's always something unrelated done by the builder. Inadequate attachment to the walls below, over-driven fasteners, missing bracing...
Maine_tyler, I deal with trusses often, usually from one of the two truss plants in Maine that you are probably ordering from. When I have worked in other places the truss engineers provide more detailed bracing schedules than the Maine companies do, but somewhere in their literature there should be a narrative on how to handle bracing.
In Maine we almost always strap ceilings with 1x3 spruce, and drywall provides a diaphragm; with our low levels of seismic activities, that's probably enough to prevent the gable hinge effect. But I usually see 2x4s or strongbacks on the bottom chords to keep things aligned during framing.
Here are the notes from the last roof trusses I reviewed. It's lame that they don't provide more info but at least they mention it.
In addition to the bracing, one important thing to consider is to have the exterior wall sheathing overlap some of the truss, the attic floor rim board (or top plates) and the first floor wall studs. This will help tie all three sections together so that the truss (and roof) doesn't get ripped away from the lower walls in a high wind event (like a hurricane or tornado) and may possibly help in an earthquake.