Large Garages 7

[medeek]

I'm feeling pretty comfortable with conventionally framed structures as of late and most of my residential work falls with typical parameters that I've dealt with on similar projects.

Today a large garage landed on my desk, conventional light wood frame construction. The only thing that really jumps out at me is the size of this structure. 40'x60' rectangle with 40' trusses on 15' high 2x6 walls. I haven't ran all of the numbers yet but I'm sure I can get the stud walls to work with a minor upgrade as well as the shear walls, roof diaphragm and garage door header. The stemwall foundation and slab have been upgraded by the owner and my experience tells me already that bearing loads won't be a problem either.

Nothing really special going on here other than size which causes me to wonder if at some level I may be overlooking something with respect to a structure of this size. As we begin to scale things up are there other codes or factors that come into play that would not otherwise with a smaller structure. There are no internal walls, just the 4 exterior walls with a 18' wide garage door at one of the gable ends and a 3' man door. From a prescriptive standpoint I know this structure breaks all of the IRC braced wall line rules so the IRC is out of the question on this one.

The reason I have a concern here is based on a conversation I had with an architect on a personal project about 10 years ago. At the time I was involved with my brother in a roofing materials distribution business. Being very young and inexperienced I figured I would design our next warehouse. Ultimately we had to have a architect take it over after my initial attempts. His first comment at my conventially light woof framed structure (100'x60'x20' box) was that it was simply too flimsy at that size and we ended up going with CMU for the 20' high walls.

Also in a recent thread on tall walls I am left wondering if an upgraded wall is necessary on the side walls of this structure what would be easier and more "contractor friendly", 2x6 walls spaced at 12" o/c or 2x8 walls at 16" o/c. I typically don't try and specify over DF No. 2 for studs since I think the expense of No. 1 or SS would be unwarranted, easier to bump up to more studs or deeper studs, at least this is my current thinking which may or may not be correct.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[jayrod12]

I would push for 2x8 studs but if the numbers work for 2x6 then so be it.

In Canada I think that span of roof trusses triggers commercial loading over the residential loading, might want to check that with your local code.

As far as lumber grades I would specify what you need, get on the phone with a local supplier and get some price lists for different lumber at different grades. IF going to D.Fir No. 1 or SS is less than 30% more expensive but will allow you to go at 16" instead of 12" then I would do it in a heartbeat. Less plywood nailing, less insulation batts (same square footage but less to handle) etc.

Here we spec SPF No1/2 for most stuff. They are the same grade, there's also SS and stud grade. The stud grade is normally such garbage that the contractors don't even want to use it.

 

[XR250]

I would think you would require 2x8's in your area carrying that much truss. Also, might want to align the studs with the trusses or you may have a top plate bending problem.

 

[BadgerPE]

Before going any farther, I would contact the owner or contractor to determine who their lumber supplier will be. I have noticed that often the longer the lumber is, the higher grade it is. So the lumber yard might have DF #1 or SS for those lengths and then you can keep your standard 16" o.c. spacing.

One other issue that I deal with on these large clear span buildings with tall walls is the tension transfer from the roof to the base requires additional detailing.

 

[medeek]

BadgerPE: With regards to tension transfer are you talking about uplift?

With the snow load and dead load I've got 790 plf at the top of the side walls. I've read through some discussions about top plate bending before in other posts but I can't remember if one should consider both or only one of the top plates due to splices. I'll have to search back through some of the posts. To align the studs and trusses would require moving the truss spacing to 16" o/c or moving the studs to 12" o/c.

I have never called out DF No. 1 or SS for studs but it seems it may not be out of the question.



A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[BadgerPE]

Medeek,

Yes I was referring to net uplift from wind loads applied to the top plate. My most recent building (60' clear span 4/12 roof ASCE 7-05 Exp. C) had approximately 335 plf of net uplift at the truss/plate interface. I ended up blocking all of the splices per SDPWS requirements and used APA SR-101C for guidelines on nailing and anchorage requirements. Initially, I required A LOT of nails at my splices/plates, but due to the relatively low base shear (~50 plf) I "re-purposed" some of the shear nailing for uplift nailing. A good conversation with an APA engineer put my mind at ease that this was an appropriate method.

 

[mike20793]

I was told once by a contractor that, for southern pine, the difference between stud grade and #2 is about 5% while the difference between #2 grade and #1 grade is about 15%. I'm not sure how accurate that was, but he made it seem like it's a no brainer to go to #2 grade. It certainly helps with bottom plate crushing. The top plate may work for roof loading at that span, but I would be hesitant to rely on it. I'd consider using 2x8 studs (maybe stud grade) at 24" to help with that problem. When I do rely on the top plate to transfer load, I do not consider composite action between them and we have a detail to specify the location of top plate splices away from truss bearing locations.

I think BadgerPE might have been referring to wind uplift on the trusses and their connection to the studs and, subsequently, the foundation. A stronger Simpson hurricane strap may be required at that span.

 

[DaveAtkins]

This is more means and methods, but I know of a project in my area where the stud walls were tall, and the contractor built the walls in two lifts, creating a hinge at the midheight of the walls. The structure collapsed during construction, causing one injury.

My point--make sure it is clear these are 15' stud walls, not 7' stud walls on top of 8' stud walls.

DaveAtkins

 

[jayrod12]

Dave brings up a good point.

I now specifically put full height studs directly on the framing plans as I got bitten by this twice on gable end walls and vaulted ceilings.

The fix ended up being steel columns at a spacing to make horizontal wind beams at the 8' mark work out.

 

[medeek]

With a point load of 1581 lbs and stud spacing of 16" I get the following for bending (assuming 2-2x6 oriented flat, adding their section modulus):

I'm going to give this a pass, for a number of reasons:

1.) I'm treating this as simple supported, in reality it is more like a continuous beam so bending moments would be less.

2.) I'm not considering any additional stiffness from wall sheathing or gypsum wall board.

3.) If you consider a partial composite action of the two top plates the section modulus would be greater than simply adding the section modulus of two 2x6 plates.

4.) In my notes I specify that all top plate splices be positioned over a stud.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[XR250]

I would not count on the splices ending up where you want them. For a double 2x6, you are probably OK, however.
Might be better to go 2x8's @ 24" o.c. - stiffer wall, less material and the ability to tie the trusses directly to the studs for uplift.

 

[medeek]

I haven't checked the studs just yet, but given the axial and wind loads on a 15' high stud, I'm guessing some sort of upgrade will be required.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[CBSE]

Your garage door king studs tend to get a little obnoxious with these large garages with big garage doors. Do you have a stem wall and footing or is it a turned down slab? I have had to do miniature pilasters at the edges of openings to account for loads before.

 

[medeek]

With 2x6 DF No. 2 studs I get:

With regards to strength it will work but what I am not liking is the 1.02 in deflection at 70% C&C wind loads. Even though the IRC gives you a pass at L/120 for flexible finishes and L/180 for exterior walls with gypsum on the interior side it seems to me that a deflection exceeding 0.5"- 0.75" is excessive. Does anyone else consider a hard cut off in this respect?


A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

I just spoke with the owner of the project and he has already purchased the lumber (2x6 DF. No. 2) for the studs. I could probably let him get away with 16" o/c based on these numbers but I think given the high winds of this locale and the overall size and height of the structure I will require him to go with 12" o/c, it is only 33% more studs.


A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[XR250]

That is more than I would use if I had the choice on new construction. A 2x8 @ 24" o.c. is going to have about 65% of that deflection.

 

[medeek]

With the same C&C wind load I am getting 0.67" of deflection for a DF No. 2 2x8 stud @ 24" o/c.

With a DF No. 2 2x6 stud @ 16" o/c = 1.02"
With a DF No. 2 2x6 stud @ 12" o/c = 0.77"

Interesting how the 2x8 does so much better in bending, the depth squared in the section modulus is to blame of course.

I don't know if the customer can return all of those 2x6 studs but it makes me wonder if I shouldn't present him with the option to go with the 2x8 studs. 24" o/c is a lot less nailing of sheathing.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[XR250]

Actually, it is the depth cubed in the moment of inertia

 

[medeek]

You're right, I was mistakenly thinking of the bending moments and not the deflection.

I was talking with a Huttig rep yesterday about a lumber package for a previous job and she mentioned that the max. deflection they like to see is 0.5" (above and beyond the code require L/360, L/240 etc...). I'm guessing this hard cutoff for a deflection is somewhat arbitrary. One of my mentors suggested .75" for the wall of windows I previously posted about. With these tall walls I'm seeing a trend where the strength is not the governing factor but the deflection is.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[XR250]

I usually just limit mine to L/240 on new construction if there is no brick or stone