Outward Thrust on Wood Garage Framing Members 1

[mfstructural]

Good morning, for some reason these wood roof framing scenarios rack my brain. I'm currently looking at a garage (48' wide x 34' depth) with scope to provide roof framing design, as the building official is requesting that what the arch has shown can work. I've attached the plan view and elevation. There are no rafter ties, the design as shown is an offset bearing wall with rafters. The client wants to use TJIs as roof rafters. As you can see the rafters on the right side of the garage cantilever to the left over a bearing wall and in turn support the ends of the left rafters at the ridge. My thought is that there should not be outward thrust if the right rafter is considered as a s.s. beam that picks up the left rafters. There would have to be an LVL or members along the ridge to make that connection. There is of course a horizontal component of the loading on the rafters which will induce thrust. I'm thinking that if the connection at the roof is sufficient to accommodate this horizontal component (thrust) then the design will be ok. I think hurricane clips will def be necessary at the exterior walls. My only concern is that if a collar tie is introduced, compression loads will be induced into the TJIs which is not allowed per manufacturer specs. When I look at the elevation though, I feel like there will be outward thrust and the model I created also shows outward thrust at the ridge. I am thinking of recommending a redesign and just having the roof ridge offset and in line with the bearing wall. This way, the TJI roof rafters can bear on the wall, but connections will still have to be made to resolve the horizontal component of the roof loading.

 

[dik]

Have you looked into the prices for engineered wood trusses? They even allow you to do a ceiling and add insulation, readily.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[mfstructural]

I suggested that during our initial conversation and he wants to stick with TJIs, if possible. I think if this get overly complicated and the connections are cumbersome/tedious, I would ultimately recommend to use trusses.

 

[ChorasDen]

Looking at that front elevation with the large window on the left wall, and the short 2ft wall on the right side, what's your plan for resolving lateral on this structure? You could try a 3-sided diaphragm, NDS allows L' up to 35ft.

Normally for the situation you describe, we wouldn't consider lateral thrust, only the vertical component of the offset load, since I would require the left rafters be attached to the LVL/LSL ledger with hangers, and then have the right rafters provide resistance to the LSL/LVL ledger with inverted hangers. This all assumes the right rafters have the capacity to resist the load from the left side at bearing. I'd normally assume the roof diaphragm resolves any supplementary axial load, but then again, assuming a 3-sided diaphragm may challenge that consideration....

 

[Eng16080]

Normally for the situation you describe, we wouldn't consider lateral thrust, only the vertical component of the offset load, since I would require the left rafters be attached to the LVL/LSL ledger with hangers, and then have the right rafters provide resistance to the LSL/LVL ledger with inverted hangers. This all assumes the right rafters have the capacity to resist the load from the left side at bearing.

Exactly this. I'll reiterate the importance of the right side rafters to the ledger connection using an upside-down hanger. Basically, the right side rafters are designed to cantilever over the wall, supporting the ledger. One other thing to add: be sure to add solid blocking between rafters above the wall (like you would for any other cantilever).

 

[KootK]

I imagine that the thrust effect that your seeing really just arises from the deflection of the right side cantilever. I would be inclined to accept that given that:

a) This is little different from the kind of thrust that one gets in a ridge beam setup which is common.

b) Given the short span of the cantilever, I would expect that a little lateral give at the left bearing wall would probably have the thing acting as you would want it to.

 

[RontheRedneck]

Every I-joist manufacturer I've worked with will not allow I-joists as rafters without a ridge beam. So you're probably going to go it alone on designing this.

I suspect web stiffeners will be required at both sides of the ridge.

Be sure you call out I-joists that are readily available locally.