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,

 

[r13]

The OP's truss is quite sturdy due to the arching lower chords. It can be further strengthened to improve wall-roof connection. I was more interested on the end gable though. Seems you shall use the same truss type to avoid transverse tie in problem.

 

[jayrod12]

Must be area dependent, I can go to home depot at the end of my street and buy up to 175 14 ft 2x6 right now. That may not be the same everywhere.

Although you have a point about the studs going all the way to the eave. Based on some quick calcs, you'd be 22'-change" to underside of truss at the centre. Would need engineered lumber. I have seen 22ft 2x6 before, but they're rare and likely ridiculously priced nowadays.

 

[Simba13]

r13 This is exactly where my mind has been at that last day or so. Some kind of braced connection to stiffen it up. Are those rods you're proposing?

 

[r13]

Yes. It was the idea to use "kicker" to strengthen the straight corners.

 

[phamENG]

I'd be VERY careful about doing that. It might be fine in a timber frame, but prefab trusses on a conventional light frame wood structure might get a little interesting. You'll be introducing quite a bending moment in your already rather slender studs.

 

[r13]

Good point.

 

[XR250]

Simba13 - Balloon frame would go from the slab to the underside of the roof deck.
2x8's should be good for at least 20 ft. in height or more. They are readily available in my area.
I did a church a few years back balloon framing 2x10 studs up to 29 ft. They were finger joined material.

 

[Simba13]

phamENG Yeah, my boss brought that up. that's the problem I'm struggling with I guess.

 

[DaveAtkins]

Simba13,

When we talk about bracing the gable end wall, it is not what r13 is showing. We are talking about bracing perpendicular to the top of the wall, where it meets the bottom of the gable end truss. Since the wall studs do not extend continously from foundation wall to roof sheathing, there is a "knuckle" or pin at the location where the top of the wall meets the bottom of the gable end truss. It is unstable for lateral loads perpendicular to the wall.

DaveAtkins

 

[Simba13]

DaveAtkins No, I got you, those are two separate problems, r13 is looking at the raking in the (let's call it) transverse direction since the gable end walls are so far apart. You're talking about the longitudinal direction at the gable end. If the guy hasn't bought the trusses I'm kind of hoping I can talk him into changing to a standard gable end truss, the balloon framing is a good idea but I think it's going to mess with my shear walls since there wouldn't be a top plate (and so one edge of the panel won't have nailing) in that case right?

 

[gte447f]

Simba13, to the topic of your original question, I would not be too concerned about the lateral deflection of the roof diaphragm unless you have a brittle exterior finish like brick veneer. What wind speed are you using to calculate the lateral drift? You can use a lower wind speed, corresponding to a lower MRI (mean recurrence interval), to check for serviceability. For your building, I would use a 10 year MRI wind (ASCE 7-16 Figure CC.2-1) for serviceability in lieu of the 700 year MRI wind (ASCE 7-16 Figure 26.5-1B) used for nominal wind design. For most of the country the serviceability wind speed would be 75 mph instead of 110 mph. The serviceability wind speed is about 70% of the nominal wind speed, and the serviceability wind pressure will be about 50% of the nominal design wind pressure since wind velocity is squared to calculate wind pressure. If you have already taken this in to account, then just ignore this suggestion.

Other than that, there doesn't seam to be anything unconventional about your simple structure, and it sounds like you have gone through the necessary analysis and design steps. This is practically the epitome of the rectangular box that appears in text books to illustrate the concepts of roof diaphragms and shear walls, but rarely appears in real life in such a simple, straight forward, "text book", fashion. Design the stud walls for axial and bending, design the diaphragm, design the shear walls, make sure the gable end walls are braced at the hinge (or balloon frame so you don't have a hinge), specify the design criteria for the delegated design of the roof trusses... you're done.

 

[r13]

I suspect the kicker will reduce the thrust at the lower chord, as well as other truss members. Just for fun, you mat create models to compare. If you do, please let me know the results. Thanks.

 

[Simba13]

gte447f Me? Complicate a simple problem? That doesn't sound like me at all. I played around with the numbers a bit, changed exposure from C to B (no idea why the truss guys had it at C in the first place) Since there will be continuous sheathing around the whole thing, for just the serviceability check I spread the shear through the whole sheathing rather than just my shearwall with the hold-downs, that brought the unit shear down a fair amount. I'm now at .37" total deflection of the diaphragm and the walls together which is less than H/400. I also like your point about the wind speeds, I get in the habit of just using the same one for strength level. At this point I think I'm ready to bug my boss with what I have. I'll bring up the gable ends, I think the trusses either need to be changed or I'll ask about balloon framing. Simpson also has a gable end connector I'm looking at.

BTW: Thanks for your patience everyone, I'm not long out of school and despite getting my masters and undergrad at a good school, diaphragms in particular are one of those topics that just weren't covered at all. I still remember in our capstone senior design course they brought it up as a vague qualitative concept and that was it. Still had no idea what a drag strut or collector was until I got Malone's book (even though one of my professors used the word, that was it).

 

[gte447f]

Simba13, good for you for working your way through this problem to gain a better understanding and increase your skills as a design engineer. That is commendable professionalism.

0.37" max lateral deflection sounds very acceptable for the type of building you have described.

I encourage you to make a note to use lower MRI wind speed to check lateral drift in the future. The commentary to the serviceability chapter of ASCE 7 states that it is exceedingly conservative to use nominal design wind speed to check lateral drift.

 

[r13]

Just food for thoughts - bottom chord bracing.

 

[phamENG]

r13 - I'd avoid anything like that. Prefab trusses can be temperamental. The truss manufacturer should be designing the bracing for the trusses as part of a complete engineered system design. Simba should review it and learn from it, but playing with the design isn't going to be beneficial and could be detrimental. The diaphragm is the roof sheathing. Even on a slope, it still does the job.

 

[phamENG]

I agree with gte about 10 year MRI for serviceability checks, but it's good to check it at strength level wind, too. You still need the studs to support the roof if they're leaning out an inch or two. Do you care about the drywall cracks at that point? No. But it does have a direct impact on strength. This is only going to be an issue when you get above the 2:1 diaphragm aspect ratio, or have REALLY tall studs. You're right at the line where second order may be an issue - probably would be down here right on the beach.

So check your H/400 (or whatever your firm standard is) against 10 year MRI, but check second order (if appropriate) at the deflection caused by the code mandated load combination just to be sure.

 

[r13]

How you guys provided loads to the truss manufacture to design bracing system, especially lateral loads? Does the manufacture then calculate the in-plane forces? Do you select the truss per manufacture's data sheet, or provide the loads, geometry, and let the manufacture to handle the "design"? I am not familiar with the design of wood frame buildings, just curios.

 

[phamENG]

We spec' the loads and bearing points. Architectural drawings usually have enough information for the truss designer to put together the profile, but on jobs where it's important I've done profiles showing open space for attics, etc. Then the truss designer provides the truss and bracing and reactions at the bearing points.

If a truss needs to act as a drag truss to get lateral loads down to an interior shear wall, then that collector load is provided on the drawings and the truss designer has to design for that. In plane forces in the diaphragm are for the EOR to design - that and attachment of the diaphragm to the truss beyond minimum code requirements.

 

[r13]

I see. Quite different from the design of steel buildings. Thanks.