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

 

[boo1]

If the trusses are 24" oc, can you use double 2x6s at 24" oc too match?

 

[medeek]

I will give the doubled up stud option to the client as well.

Interestingly with the one gable wall of this structure I only had 12' of shearwall. Given the deflections and the eccentricity of rigidity of the structure I opted for a rigid diaphragm analysis and distributed the shear wall loads accordingly. Even with that the load to the garage door wall was approximately 6432 lbs (ASD wind), with a unit shear of 539 plf and uplift of 8080 lbs. I always conservatively neglect the counteracting dead loads because I don't think you can rely too much on a distributed dead load to fully counteract a single "point" upload.

The 8" x 24" stemwall below this wall required two #6 rebar top and bottom to deal with the bending moment imparted into the stemwall foundation and footing. I'm not very experienced with concrete so I'm not entirely confident that my calculator for this portion of the design is not too conservative but there will be a significant moment at the edge of the shear panel as shown below:

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

http://www.medeek.com

 

[medeek]

I needed to create a detail for the concrete at SWL2 however in the process I decided to draw up the entire wall simply to get a better feel for the size of the structure and what a fully blocked shearwall really entails. With the 12" o/c studs it does look a little out of control. Note that most of the detail notes have yet to be added, I'll finish it out tomorrow. In order to get enough moment/bending capacity out of the stemwall I dropped it low enough so that I have at least 24" of depth at the door cut outs:

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

http://www.medeek.com

 

[medeek]

I was thinking that if the contractor sheaths the wall with the panels vertical then on the remainder of the walls I would only need to call out midspan blocking at 8' height and there will only need to be one row of midspan blocking. The problem with this is if there is no Gyp. Board on the interior of the wall when the wall gets sucked out by neg. pressure my unbraced length of the compression edge of the stud is then only 8' and my calcs for the studs don't work. Can I assume that the walls will be gypsum on the interior and go with 8' midspan blocking or the safe bet with midspan blocking every 48" o/c. Building this thing is going to be a lot of work and materials.

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

http://www.medeek.com

 

[jayrod12]

Never assume the contractor or client will sheath the interior walls. Ask them. If you need the sheathing on the inside face, then make it known on the drawings.

If you have gyp on the inside, do you still need blocked sheathing on the outside? I would think the contribution from the gyp might be enough to allow unblocked shear walls. For studs at 12
o/c and minimum plywood nailing it's only a 20% reduction for unblocked walls. if you tighten up the spacing to 6" everywhere there's no reduction.

Alternately you could show strapping at 24" o/c on the inside face, I would then consider the studs braced at 24" for suction as you won't have all of the studs roll over at the same time when there's sheathing on the tension face.

 

[BadgerPE]

For a 2x8 wall, would bracing even be "required" for bending? Per NDS 4.4.1.b, when d/b < 4 based upon nominal dimensions, "the ends shall be held in position by.....nailing.....to other framing members....". In my mind, that means that compression flange bracing is not required for bending forces, but axial bracing may be needed.

 

[medeek]

On the two gable walls I need blocking due to the high shear loads, however if I am allowed an unblocked shearwall for a perforated type shearwall (Co = 1.0, some very small pict. windows for min. natural light)then unblocked will work, the unit shear in the longitudinal direction is only 109 plf for wind and 80 plf for seismic, I'm well within an 80% reduction.

I still like the idea of some midspan blocking, I think I'll call out mid span blocking at 8' o/c if gyp board is applied or 4' o/c if no gyp is applied. There needs to be a penalty without the gyp.

I also think grouping the studs into doubles and then putting them at 24" o/c would probably be easier to build but either way it is the same amount of wood .

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

http://www.medeek.com

 

[jayrod12]

doubles at 24 do make more sense generally. Note that for members at 24" the reduction for unblocked gets larger.

 

[medeek]

Here is the final detail, the critical part is really only the concrete stemwall:

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

http://www.medeek.com

 

[XR250]

Is your ceiling diaphragm gypsum? How are you getting out-of-plane wind on the gable end into the ceiling diaphragm? That can be tough with high winds and tall studs. I usually end up balloon framing the gable end in these situations to avoid the issue.

 

[medeek]

Yes, the ceiling will be gypsum with standard 0/12 bottom chord mfg. trusses. In these situations I treat the gable truss like a drag truss and in the notes I specify to the truss manufacturer the lateral loads these gable end trusses can expect as well as sheathing requirements. I've had a couple truss manufacturer's actually call me a couple times requesting a decrease in lateral loads for these trusses, but how else can you get the roof diaphragm loads down to the shearwalls unless the truss above can transfer this load?

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

http://www.medeek.com

 

[KootK]

The gable truss above is normally plywood sheathed and acting as a vertical diaphragm, right?

I generally do this kind of thing for the out of plane forces: Link

Having a gypsum ceiling in addition to the discrete bracing feels good from a belt and suspenders standpoint but I never count on it. Unless you detail it carefully -- and atypically -- it's unlikely to be a complete / competent diaphragm.

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.

 

[XR250]

I am talking about wind blowing on the end wall of the garage. How does that load get into the ceiling diaphragm as it is bracing the hinge between your stud wall and the gable end truss?

 

[medeek]

I have never provided a detail for this before (relied upon the truss manufacturer for their standard gable end frame detail) even though I have discussed it at times in varies threads on the forum here.

Being that the span and size of these gable end trusses are so large it is something requiring a second look.

The SBC summary sheet (B3) has a very brief treatment of the subject:

http://support.sbcindustry.com/images/publication_images/b3.pdf

I contacted the owner and requested a copy of all truss documents that will come from the truss manufacturer. If I do not see a bracing method specified within these construction documents that is satisfactory I will create a detail specifically for this bracing situation.

This PDF provides more samples of bracing details:

http://support.sbcindustry.com/docs/06_BCSI_booklet_FINAL.pdf

This document utilizes a horizontal truss to transfer the gable out of plane loads to the sidewalls:

http://support.sbcindustry.com/images/publication_images/pbrace.pdf

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

http://www.medeek.com

 

[medeek]

Simpson also makes a gable brace connector specifically for this application:

http://www.strongtie.com/products/connectors/gbc_pt.asp

Does anyone have any experience using these?

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

http://www.medeek.com

 

[medeek]

The more I think about it the more I'm realizing that this is actually a very critical location. Half of the out-of-plane end wall wind load is being transferred by the studs to the top plate of this wall. Somehow all of the load has to get transfered either into the roof diaphragm or into the side walls (shearwalls). The gypum ceiling cannot be relied upon for this. The continuous lateral restraints at 10' o/c attached to the bottom chords of the trusses will help spread this load into the other trusses but without sheathing they offer no diaphragm action on their bottom chords.

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

http://www.medeek.com

 

[XR250]

Exactly.
This detail is usually overlooked by most engineers (in my state anyhow)
That is why is makes sense to balloon frame it. There is no free lunch. You will need at least 2x8's if not more to do that.
I have tried the GBC's, but it is about impossible to get the kickers fed thru the trusses - much easier if it is stick framed. They also put a huge uplift on the end wall and the truss that they connect to (depending on the wind direction) This load is also about impossible to deal with.
Might want to try a plywood ceiling for the first 12 ft or so.
I have done this by specifying that the first 6 trusses be made 1/2" shorter in height than the rest. Then they can sit on top of the plywood and you still have a smooth GYP ceiling.
The contractor hated it!

 

[medeek]

The out of plane wind loads under consideration would be:

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

http://www.medeek.com

 

[medeek]

This may be an oversimplification of what is going on at the gable end but I think it is good enough to give me some ballpark numbers.



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

http://www.medeek.com

 

[medeek]

From MWFRS wind calculator and from previous diagram above:

W_A = 40.6 psf
W_B = (38.7 x 6 + 40.6 x 1.5)/7.5 = 39.1 psf

Area_A = 83.3 sqft
Area_B = 300 sqft

W_total = (40.6 x 83.3 + 39.1 x 300) x 0.6 = 9,067 lbs (ASD)
w_avg = 9,067 lbs / 40 ft = 227 plf (ASD)

That is a substantial load that needs to be transferred into the roof diaphragm, Wow.

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

http://www.medeek.com