Large Detached Garage Raking 3

[Simba13]

Greetings all,

Finishing up the design of a largish detached garage (64'x30' w/ 14' tall walls, roof peak at 19'). I've gone through the tall wall calcs, designed some 'shear walls', foundations etc. But my boss is concerned that since the long dimension is fairly long there could be raking away from the gable walls since there are no interior walls (trusses frame front to back with the garage door side being the front). I've been using RISA 2-d for most of my non-hand calc analysis at this firm but I feel like this is a 3-d problem. The trusses with sheathing on them will form a diaphragm and so it should be fairly stiff in that direction right? For those that have designed wide garages, is this something you considered to be an issue and if so how did you solve it?

Thanks,

 

[dhengr]

Simba13:
Remember, scissors trusses have a fairly significant deflection, in their in-plane, thrust force direction, at their reactions, because they do not have a very effective horiz. bottom chord, tension tie member. Just as they tend to spread at the wall top pls. under normal gravity loading, they will tend to hump-up due to wind loading. They act more like a beam, with a center moment problem; they are not as stiff as a normal horiz. bot. chord truss, w.r.t. length change, due to their geometry. They are a PITA and the truss designer works mighty hard to make their centerline moment and connection details work out. Why not use a std. gable truss, with a horiz. bot. chord and a 4/15 roof slope? With the wall top pls. at 16’ high and the ridge at 20’, you would end up with about the same ceiling height as you have now. And remember, that the gable end wall kinda have a hinge at the double top pl. unless they are braced back into the roof truss system. That joint/connection, btwn. the gable end roof truss and the wall top pl. needs some special attention since you are also running significant lateral loads (shear) through it. Now, maybe the single plane, horiz. diaphragm on the bot. of the trusses makes more sense. Also, the entire roof truss system needs proper bracing. Most truss suppliers/designers furnish some std. sheets with their approval drwgs. on these bracing issues.

 

[Simba13]

dhengr I certainly have no issue with standard trusses, but I think the Client is going to have an issue with it since they want the headroom.

 

[BAretired]

Using a standard gable truss at each end does not affect headroom and makes framing of the end walls easier.

If the concern is about the adequacy of the diaphragm, use cross bracing in the plane of the roof.

BA

 

[jayrod12]

Why not just make the walls taller and use conventional trusses?

I'm now sharing your mentor's concerns more and more. I don't feel that scissor trusses will provide the same confidence as others.

Also, with a sloping bottom chord, then the bottom chord sheathing really doesn't benefit you from the concerns regarding shear transfer at the ridge.

You could likely reduce deflections by increasing the thickness of the panels on the short walls and the roof. Instead of 1/2" plywood, what about 5/8"? 1.18" total deflection isn't horrid, but you actually need to compare it to H/400 I would say as opposed to L/120. Which therefore it really doesn't work from a full building sway point.

What r13 is proposing is similar to pole sheds where a deep truss is more rigidly connected to the built-up wood posts creating mini moment frames at a regular spacing (often 4ft o/c). This may or may not be a good solution to your problem, with scissor trusses I'm less confident as he had intimated.

 

[jayrod12]

Using a standard gable truss at each end does not affect headroom and makes framing of the end walls easier.

Except now you've got a knuckle at the top of the wall/bottom of the gable truss and the only way to brace it is to provide a 30 foot long girt (obviously not reasonable) or diagonal braces from top of wall up to the scissor trusses, which to me is no easier to construct than a sloping wall.

 

[Simba13]

jayrod12 I can bring it up, but the Client has already ordered the truss profiles from a manufacturer so it might be hard to talk him down. The panels are all 7/16" OSB by the way (at least at this point, the Client is a custom home builder, building this for himself, so there is some flexibility but it also helps to use what's on hand I guess).

As an aside I had the exposure category on the wind loads as C to keep it consistent with the provided truss profiles. My boss was surprised by this and we took a look at the site on satellite and there are trees all around so I'm bumping everything down to B, so my total deflection is down to .83" without changing anything else. Still probably too high though.

 

[jayrod12]

Of course they want to use 7/16" osb, it's the cheapest board good that can be purchased. He also won't want to increase the wall height because that's more cladding, and non-standard studs etc. etc.

I never would use OSB on a roof. Ever. Didn't even do it for my little 8x10 storage shed at my last house. OSB doesn't have the ability to get wet once before it needs to be replaced. Roofs leak at some point. At least plywood can stand up to a bit of wetting before needing to be replaced.

I allow it on the walls, but not the roof. Plywood only on roofs.

Sounds like one of those nightmare clients, not because they're difficult, but because he's knowledgeable enough to be dangerous. He has a vision and the industry connections and always is further along in the process than you are, e.g. ordered materials before the design is actually completed.

Good luck.

I'm of the opinion that even 0.83" deflection is likely a bit high. The walls are what? 10feet, 12 feet? so you're getting H/173 at the 12ft height. Not ideal when the recommended value is H/400 (granted that's for protection of finishes but still to cut that in half is a bit unnerving).

 

[Simba13]

jayrod12 Yeah, he's a nice guy, no complaints there. But I definitely get what your saying. Didn't know that about the plywood vs OSB though. Walls are 14' high, that's why he can't just go prescriptive like usual. And yeah, still too high, I'll continue thinking...

Thanks for all of your help by the way everyone, learned a lot today.

 

[dik]

Ya... Doug Fir ply (I still think it only comes with waterproof glue) or some other kind with waterproof adhesive...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[DaveAtkins]

My concerns would not be so much with the diaphragm (the aspect ratio is a little over 2:1, but not by much). I would have two other more critical concerns:

* The "knuckle" at the top of the gable end wall, mentioned by jayrod12. Does the client know you will have diagonal braces from the top of the wall up to the roof diaphragm? They won't interfere with headroom.

* The height of the wall studs. I am not sure you can get studs that are 14' long. You might need a splice detail.

DaveAtkins

 

[XR250]

14 ft. 2x6 studs should not be a problem to source. I would balloon frame the end walls with 2x8's rather than deal with the hinge. I probably would not have even checked the diaphragm deflection on this job.

 

[Simba13]

XR250 When you say balloon frame, do you mean to just take 2x8's up the bottom chord of the trusses on the gable ends? I've heard of balloon framing before but I'm not 100% on the differences from a standard stud wall.

DaveAtkins I think the 14' studs should be okay here, we've got a couple good lumber yards in town and they've yet to not carry something I've asked about. As for this knuckle I think I get what you and jayrod12 are talking about, is it basically just due to the scissor trusses not being supported throughout their length, and only a small segment on the ends? Does the balloon framing XR250 mentioned take care of this?

 

[phamENG]

I would be VERY surprised if they provided a scissor truss at the end wall. Unless you spec'd it to get a very interesting window in that end wall. The end wall with either be: 1) balloon framed as XR suggests where the stud goes from grade to the roof with no interruptions, 2) a site built gable end wall with a hinge at the wall to gable end transition that needs to be braced or, 3) a prefabricated gable end ladder "truss" from the truss manufacturer that creates the same hinge that needs to be braced.

Down here in Hampton Roads, I'd say #3 is by far the most common. It allows the builder to put top of wall a the same elevation all around, and then set the trusses straight across. Where it doesn't need to be a truss, they just do a prefab wall in the shape they need.

 

[Simba13]

phamENG They actually are scissor trusses on the gable walls as well... And there's no window there no I'm not sure I understand that. I'll contact the client on this. Tried googling gable end ladder truss, does that refer to the 'ladder' over the soffit? or something else?

 

[jayrod12]

Pham's 3 is most common here as well. Depending on the depth of the scissor truss, may just make sense to extend the studs all the way up to underside of roof sheathing, his number 1. Number 2 would be the least common.

The hinge we're talking about is in regards to building the gable end walls to the same consistent elevation as the side walls, Pham's number 2. With scissor trusses, there would be no bottom chord at the first interior truss to brace the wall top plate to and therefore would require diagonal bracing from the 14ft elevation to the roof trusses.

Honestly I'm likely with XR, I may not have even checked the diaphragm with this ratio.

 

[phamENG]

Simba - ladder truss is used more in floor trusses, since it's parallel cord. If you stand it up, it looks like a ladder. They're cheaper to make than an actual truss, so there's no sense in putting a true truss in when it's fully supported by the wall below. Same principal here.

 

[jayrod12]

Around here we combine dropped gable end trusses, which are what Pham's sketch shows, and then we provide either 2x4 ladder framing that cantilevers over the top and backspans to the first interior truss, or ladder trusses which are essentially pre-fabricated ladder framing. Each eave on each side of the ridge comes in as one large piece. Saves a bunch of time.

Much of the difference is just local terminology.

 

[r13]

Interesting pictures of scissor truss roof construction.

 

[Simba13]

Truss profile in case anyone is interested. A bit different type of scissor from the ones r13's pictures showed

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1607537844/tips/Detached_Garage-Model2_pifg8d.pdf[/url]

 

[BAretired]

Finding 14' studs might be a problem, but finding studs reaching from floor to roof would be even more difficult. I prefer to use two glulam, LVL or built-up columns in each gable wall spaced ten feet apart with studs between. Studs could be supported by a horizontal beam (girt?) at Elevation 14' or less, depending on material available.

BA