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]

What is the material the wall made of, reinforced CMU, concrete, is there interior pilaster/column?

 

[Simba13]

r13 Just standard timber framing 2x6 stud wall, there are no internal columns as it stands, the foundations aren't pilasters, slab on grade with continuous stem wall and footing around the perimeter.

 

[phamENG]

Simba - are you talking about the joint between the bottom of the gable end truss and the top of the wall? If these are standard gable trusses (sloped top chord, horizontal bottom chord at top of wall elevation), then the end should be braced. Some strongbacks in the trusses along the bottom chords with diagonal braces down to the end wall ought to hold the wall in line.

 

[DaveAtkins]

I am not clear about where your boss thinks racking can occur. In the 64' direction, the diaphragm will span between shear walls, and in the 30' direction, you will have a three sided diaphragm, cantilevering off the back wall.

DaveAtkins

 

[Simba13]

phamENG It's actually away from the gable ends that my boss is worried about. I'm not explaining it very well but if you have the 64' x 30' the distance between braced wall lines in the longitudinal direction is 64', so at the middle of the garage it is 32' from a bracing on either side, which is a fair amount of distance, so he's worried that this might result in raking. Does that make sense? I feel like with the diaphragm of the trusses + sheathing it can't really go anywhere but maybe my intuition is off. I just can't envision any kind of connection that would deal with this. (As an FYI they are scissor trusses all the way across)

DaveAtkins Maybe there isn't an issue, I guess it's the large distance between the longitudinal shear walls that sets off a red flag for him.

 

[Simba13]

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1607447992/tips/Detached_Garage-Model_oa8fzy.pdf[/url]

Something like this I suppose.

 

[phamENG]

That's a fair concern. You can't use braced walls for this - needs to be engineered shear walls. Braced walls can be no more than 60 feet apart. I think you can get by on a technicality (it's 60 feet between braced wall lines, and the braced wall and braced wall line can be up to 4 feet apart), but I wouldn't try it - especially if your boss is concerned about performance.

Do a wood diaphragm analysis and see what kind of deflection you get. To make your life easy, assume it's a flat roof.

 

[jayrod12]

For something this long, I'm partial to bottom chord diaphragms. Have plywood installed on the underside of the trusses, provide a 2x8 nailer between each truss to fasten the sheathing to at the outside edges. This simplifies the load transfers and also allows for ease of diaphragm deflection calculations. If you did those diaphragm deflection calcs, your mentor may be proven correct, or have his concerns lessened depending on the outcome. I'd expect a diaphragm with a ratio 2:1 to not really have an issue.

The issue with one this long and only relying on the sheathing on the top side, is the shear transfer at the peak. If you provided the bottom chord diaphragm instead of the top, you could just strap the roof with 2x4 depending on whether the finish roofing requires a board sheathing below it. If metal roofing, often they prefer strapped roofs.

That being said, I've seen a significant number of these types of structures constructed without anything bad happening.

 

[r13]

I share your boss's concern. Suggest to stiffen the frame in the middle (by add columns and kickers) to help for stability. But I am not familiar with wood construction, so see how others suggest.

 

[Simba13]

phamENG yep, I used the braced wall term but I've actually designed shear walls like in Breyer's book. They should be all good. Yeah I think that would give me what I want, looking at the calc in Breyer's (PG 489) the deflection due to chord slip (delta_c) is a little tricky, but I'll noodle it over. Thanks I think this'll get me there, is there a general criteria on maximum diaphragm deflection?

jayrod12 I think it's a good idea but I might have an issue with the Client on this. You'll laugh, but this garage isn't for cars, it's for basketball, so I assume the reason they went with scissor trusses was for the increased ceiling height, not sure if they'll roll with a flat ceiling. Would it be enough you think to do the sheathing on the underside even if they're scissor trusses?

 

[Harbringer]

I think this is a non-issue. The span to depth ratio is plenty less than the allowable maximum in the SDPWS. Your image shows diaphragm deflection, not what I would call racking. You can check diaphragm deflection using Eq 4.2-1. What's your unit shear of the diaphragm?

 

[Simba13]

r13 Yep, your picture is it exactly. Since this garage is for basketball I think columns are gonna be out. That's kind of my conundrum, but I'll do the deflection calc and see what shakes out.

 

[dik]

I would think with the bracing in the roof trusses and the roof sheathing and bottom chord bracing (don't think it needs sheathing, but doesn't hurt), the roof will act as a fairly deep diaphragm... 30' ish...

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

-Dik

 

[Simba13]

Harbringer Unit shear is 333 lb/ft in that direction.

 

[r13]

I meant adding columns along the long walls, and make the connection rigid to minimize the rotation. I don't know any good detail as you have scissor truss above.

 

[phamENG]

Max deflection will depend on your finishes and loads on the wall. Remember, your end shear walls will deflect some distance - that becomes your new zero datum for diaphragm deflection. Then the diaphragm deflects. Whatever the sum of those two deflections is will be the distance the top of the wall moves in relation to the bottom of the wall at the middle of the diaphragm. This will flex finishes, and impose a second order eccentricity on the wall that could be significant in a building this size with walls as tall as they are.

 

[r13]

Maybe you need a very sturdy eave/edge beam to minimize the lateral deflection.

 

[Simba13]

phamENG Hadn't thought about the new datum, makes sense though. I did the diaphragm deflection (.56") so I'll do the shear wall deflection now. There won't be any drywall on the inside, so I don't need to worry about the finishes. Second order effects huh... This is turning into quite the science project but I'm down.

 

[Simba13]

Ok, so making some assumptions about the roof sheathing I end up with a total deflection of diaphragm + shear wall = 1.18" It's less than l/120 for walls with flexible (or in this case no finish) but it's still fairly high and I haven't taken second order effects into account. I wish there was some kind of relatively easy to construct way of stiffening it up. I like jayrod12's idea of putting sheathing on the bottom chord, although not 100% sure how easy it would be to actually build, on this point if there is sheathing on the top and bottom would I be allowed to just halve the shear deflection component of the diaphragm?