Roof Framing Input 1

[bookowski]

I don't do a lot of residential/wood framing, this existing condition strikes me as sketchy - just wondering if this is something that is common or not. This is a residential house that is relatively new (< 15years old). This was a pre-fab structure brought to site in panels and assembled, the drawings were stamped but I didnt' see any details for this (I may not have the full set). At first glance this struck me as possibly unstable under unbalanced loading unless you count some degree of fixity at the gussets. Am I missing something and this is typical/fine or is this a bit daring?

To clarify my sketch, the mid-height 2x10 collar ties (not exactly) hang at their midpoint from a 4 ply LVL that is end supported on posts. I couldn't get in there enough to see much of the posts.

This occurs over 1/2 the house, other 1/2 is not a vaulted ceiling so it does not have the hanger LVL and does have attic floor framing + subfloor.

 

[jayrod12]

I would say that's very interesting. I've never seen a detail like that before,

however... I actually think it's quite clever. It's this weird psuedo-site framed truss. It's unfortunate that there isn't more documentation on staples. They actually provide quite a significant attachment (ever tried taking off a piece of stapled plywood? it's a nightmare... source: I've personally done it).

I think there would be a degree of fixity at the gussets. Don't count out your drywall on the ceiling.

I know a lot of people will disagree with me. But considering some of the other site framed stuff i've seen I would consider this acceptable but extremely daring.

 

[bookowski]

So I take it this is not typical. Here's a photo - not great but you get the idea. The framing in general appears as if someone thought about it and was well executed. Definitely strikes me as daring though. You need to consider those gussets to provide some moment capacity for unbalanced conditions.

 

[jayrod12]

Yes, atypical. But not wholly incorrect. like I said, the drywall will contribute significantly to the lateral stability to such a point that there may not need to be any fixity in that joint for it to perform in service (don't ask me to calculate how).

If someone were to pull the drywall off and there were then an unbalance loading scenario then I can see there being issues.

 

[Archie264]

They've hung the whole thing from that 4-ply LVL beam, that's all that's going on. The 4x4's making up the "teepee" at the top of the frame don't really contribute anything to it, other than to allow for sloped roof to continue on to a ridge thus keeping rain/snow from ponding/piling up on top, which, granted, is not insignificant. I don't like to see anything framed with 2x4's but neither is it really doing much.

As long as the end walls can support the reactions from the LVL beam and assuming no ridiculous level of snow loading, it should work. It has the benefit of keeping the 2x10 "rafters" from being overstressed.

 

[KootK]

I vote unstable and unacceptable in an engineered design. I think that your initial assessment was spot on Bookowski.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JAE]

KootK - something like this:

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[Archie264]

Yikes!

Agree that it's unstable if whatever's supporting the LVL beam is unstable. For some reason I had assumed it was a CMU wall; I now see that it's a post. So, it would depend on how stable the post is, to include whatever lateral system (or internal partition walls) might contribute to the post's stability.

In other words, since it's all hung off the LVL beam if that's stable the rest of the framing's along for the ride. Or staying put and not going for a racking ride, rather.

Something's kept it up for lo those 15 years.

My $.02 worth.

 

[KootK]

Exactly JAE. Thanks for stepping up with the sketch.

I'm a former truss guy. A big plate will resist some moment. Nothing you'd want to bet your livelihood on however.

If your plywood and drywall sheathing is properly connected to the end shear walls, you might be able prevent the upper triangle from rotating as Jayrod has suggested. That's a pretty big if in the case of the drywall however. Typical drywall detailing wouldn't support it.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[haynewp]

Referencing the collapse diagram JAE posted; if either a legitimate roof diaphragm or if the ceiling drywall can keep the eaves from shifting horizontally, I think that would stabilize this hung framing. Kind of weird roof system though.

 

[bookowski]

Here's a photo of the other 1/2 of the house, the non-vaulted portion. Also was interesting to me but I don't see a lot of these. On this side the LVLs are dropped low (to attic framing level) and the 2x10 rafters are on propped 2x4 struts from the LVL. It seems like these details are all based on being pre-fabbed. Looks like the lower 2x10 portion is brought as a panel, flipped up, propped on 2x4, then the top 2x4 cap goes on.

The reason for this in the first place was a question about removing the 2x4 struts to partially finish the attic/add a dormer etc., and of course the question came up 'why can't we do what they did on the vaulted side'.

 

[bookowski]

One last photo - where the 2x4 struts hit the 2x10 to 2x4 rafter intersection.

 

[4thorns]

The roof is a 12/12 pitch. Vertical load should be quite small. At 25' wide it is 12 and a half feet tall +/-. Basically it's a giant sail! As far as vertical loading I agree with Archie:

As long as the end walls can support the reactions from the LVL beam and assuming no ridiculous level of snow loading, it should work.

With that said I've attached a drawing that i think is fairly close to the existing structure.(please chime in bookowski) I had a lot of fun with this so if it's not quite right then I'll get over it!

I don't think that the original drawing quite does the gussets justice. (no offense book) If my interpretation is even close then they are quite large. My guess is that they were designed (in most part) to handle the horizontal loads imparted on them.

Gussets that size, on both sides of each rafter pair @ 16" o.c. (along with lateral resistance from the LVL girder) are apparently enough to keep this building standing.

 

[RHTPE]

Definitely not typical in my region.

Certainly an interesting way to frame a roof. I suspect it had a lot to do with how it was preassembled (or precut) in the shop.

What is the thickness of the plywood gusset?


Ralph
Structures Consulting
Northeast USA