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Large Garages
6

Large Garages

Large Garages

(OP)
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
www.medeek.com

RE: Large Garages

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.

RE: Large Garages

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.

RE: Large Garages

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.

RE: Large Garages

(OP)
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
www.medeek.com

RE: Large Garages

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.

RE: Large Garages

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.

RE: Large Garages

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

RE: Large Garages

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.

RE: Large Garages

(OP)
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
www.medeek.com

RE: Large Garages

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.

RE: Large Garages

(OP)
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
www.medeek.com

RE: Large Garages

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.

RE: Large Garages

(OP)
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
www.medeek.com

RE: Large Garages

(OP)
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
www.medeek.com

RE: Large Garages

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.

RE: Large Garages

(OP)
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
www.medeek.com

RE: Large Garages

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

RE: Large Garages

(OP)
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
www.medeek.com

RE: Large Garages

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

RE: Large Garages

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

RE: Large Garages

(OP)
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
www.medeek.com

RE: Large Garages

(OP)
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
www.medeek.com

RE: Large Garages

(OP)
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
www.medeek.com

RE: Large Garages

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.

RE: Large Garages

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.

RE: Large Garages

(OP)
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
www.medeek.com

RE: Large Garages

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

RE: Large Garages

(OP)
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
www.medeek.com

RE: Large Garages

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.

RE: Large Garages

(OP)
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
www.medeek.com

RE: Large Garages

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.

RE: Large Garages

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?

RE: Large Garages

(OP)
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_...

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_bookle...

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

http://support.sbcindustry.com/images/publication_...




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

RE: Large Garages

(OP)
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
www.medeek.com

RE: Large Garages

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!

RE: Large Garages

(OP)
The out of plane wind loads under consideration would be:

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

RE: Large Garages

(OP)
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
www.medeek.com

RE: Large Garages

(OP)
From MWFRS wind calculator and from previous diagram above:

WA = 40.6 psf
WB = (38.7 x 6 + 40.6 x 1.5)/7.5 = 39.1 psf

AreaA = 83.3 sqft
AreaB = 300 sqft

Wtotal = (40.6 x 83.3 + 39.1 x 300) x 0.6 = 9,067 lbs (ASD)
wavg = 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
www.medeek.com

RE: Large Garages

(OP)
Based on Simpsons F2 values I'm going to assume about 450 lbs of lateral resistance per 2x4 brace using their GBC connector. This neatly works out to a diagonal braced every 24" o/c. At the other end of the brace I've got two 16d nails connecting it to vertical blocking between the truss top chords:

Z = 141
Z' = 141 x 1.6 = 225 lbs

Capacity = N x Z' = 2 x 225 lbs = 450 lbs -----> OK



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

RE: Large Garages

(OP)
Here is my analog in Risa3D of the braces interacting with the roof diaphragm:

https://www.youtube.com/watch?v=9FnB5Y0qgHw

The uplift on the roof diaphragm is not very desirable but I suppose the trusses can resist that load in some fashion or another since this seems to be the common method handling this.

I do like the horizontal truss and ceiling diaphragm method however both of these method are not transferring the load into the roof diaphragm but rather directly into the sidewalls and bypassing the roof diaphragm altogether. My roof diaphragm in this direction is very strong (60 ft deep), to not tie into it and replace it with a partial ceiling diaphragm 12 ft deep and 40 ft in length is hardly a optimal solution either. There is also the issue of the diaphragm aspect ratio limits of SDPWS Table 4.2.4, for an unblocked diaphragm I technically need 13'-4" of OSB/PLY on the ceiling.

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

RE: Large Garages

(OP)
If you install kickers at the gable end what you are really doing is creating an invisible hip roof, at least this is how it would appear in the limit.

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

RE: Large Garages

(OP)
Which makes me think how does a hip roof resist the lateral loads. There would probably be a certain amount of uplift at the girder truss supporting the hip sections. However, without a doubt a hip roof is definitely stronger that the gable roof.

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

RE: Large Garages

Yup, hip roof is much easier. I generally don't even check it.
How are you going to get those kickers threaded thru the common trusses? Also, that is a substantial uplift of the trusses that the truss mfg. has to deal with and you have to deal with at the ends.

RE: Large Garages

Why can't you rely on the gyp ceiling?

I usually spec bottom chord blocking in the first few spaces and the way I detail the drywall nailer at the perimeter provides shear transfer into the shear walls.

RE: Large Garages

Relying on the gypsum is tempting. The arguments against it go something like this:

1) If your blocking only goes two trusses deep then your diaphragm is only 4' deep at the side walls where you dump the reactions.

2) If there's an interior partition in the vacinity of your diaphragm, you'll lose sheathing continuity unless some rather unconventional detailing is provided.

3) Layfolk think that they can mess with drywall at will. This doesn't prevent us from using gyp shear walls however.

As far as I can tell, there are only two mechanical clean options:

1) Full height wall.
2) Horizontal truss.

And neither of those is generally acceptable to builders/developers.

A while back, in a Meedek thread, I proposed a horizontal truss that was actually a pre-eng truss in place of XR250's plywood. That gets around the aspect ratio limits but would be even less popular with builders because the truss heights wod need to be trimmed 1.5".

@XR250: good on you for even attempting the horizontal diaphragm in the real world, yet alone actually getting one built. You're a structural engineering martyr!

@Medeek: keen observation on the hip roof comparison. That had never occurred to me.

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.

RE: Large Garages

Why is my diaphragm only 4' deep? There's values for unblocked gyp diaphragms. If I block the first few 2,3,4 spaces to get the load evenly into the diaphragm. The current aspect ratio of his is favourable (1.5:1).

I don't see too many people in residential applications removing the ceiling gyp without replacing it. I guess it depends on the level of resistance you need, I have put in kickers up to the top chords on a few projects they're a bitch to get in but it's generally doable.

I am with XR250 though, my standard gable end detail is balloon framed so the ceiling is a moot point.

RE: Large Garages

(OP)
Even if one was to balloon frame this gable end wall to the roof the studs would be so tall that there deflection would impart some load into the ceiling diaphragm, not nearly as much as the current situation but a certain amount. At the ridge my roof height is 8'-4" plus a wall height of 15' = 23'-4'. The DF No. 2 2x6 studs @ 12" o/c would never work, I doubt even a 2x8 stud @ 16" o/c would barely work given my wind loads.

As much as I would like to use a horizontal truss or a 14' deep ceiling diaphragam (324 plf unit shear) I don't know that either option would gather much support from the owner or builder.

Also looking at the unit shear this would have to be a blocked diaphragm at this depth (324 plf > 237.5 plf). To get the unit shear in the ceiling diaphragm below 237.5 would require 20 ft. of sheathing coming in from each end wall. That is 40 ft. of the 60 ft, might as well sheath the entire ceiling. I don't know maybe my wind forces are too high or my wind trib. exercise above is somehow awry, but this is a lot of force to get into each sidewall (4534 lbs).

I just did a sanity check on those shearwall forces and my numbers coming straight out of Woodworks is 6569 lbs per Wall A and Wall B (sidewalls), this of course does not include the internal pressures and does include the leeward wall pressures. My total windward force if I include the tributary area going straight to the roof diaphragm is approx. 5548 lbs per shear wall. The ratio of the leeward + windward to windward w/ neg. int. pressure is approximately 5:4 so my numbers are not too far off. The Woodworks software also uses a less conservative and more accurate method of trapezoidal wind pressure loading for the windward side, which explains why my numbers will be slightly (10%) higher.



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

RE: Large Garages

(OP)
I think my best option at this point is to use kicker braces with the first two spaces blocked (4'ft). The kickers will be a pain to put in, no argument there.

I've also seen some details with an X-brace configurations and a C-Brace configuration, not sure what is going there but I'm looking at all possible solutions right now and trying to develop an appropriate solution not only for this job but a standard detail I can utilize for future work as well.

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

RE: Large Garages

@Jayrod: I agree, your diaphragm is exactly as deep as your blocking. If you're taking it back 8', then 8' feet it is. Whenever I've seen the gypsum diaphragm detail by other engineers, it's always been two spaces for some reason.

I agree that people generally restore gypsum that is removed but real trick is whether or not Johnny DIY reno guy will restore it using the special details required to maintain diaphragm continuity.

Because of the manner in which the diaphragm load is delivered (diaphragm edge tension/compression rather than shear), I don't think that unblocked gypsum diaphragm capacities will allow the effective diaphragm depth to be extended any further than the blocking extends.

The relationship between the extent of the blocking and the effective depth of diaphragm is really the unspoken Achille's heel of all of the systems that we've discussed, even the balloon framing option.

Quote (Medeek)

My roof diaphragm in this direction is very strong (60 ft deep)

I believe this statement to be incorrect for the reasons mentioned above. Unless you extend blocking into the diaphragm over the entire 60' length, which would get you shot, I think that the effective diaphragm length is only 40'/2 = 20'.


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.

RE: Large Garages

(OP)
KootK I'm not sure I follow you with the effective diaphragm depth. In the longitudinal direction for the 60'x40' building I have 60' of roof diaphragm depth. Every book I've read allows me to take this full depth as the depth of the diaphragm, explain further.

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

RE: Large Garages

(OP)
With some bracing contribution from the gypsum ceiling I figure I can install diagonal kickers every 48" o/c for a total of 9 braces per gable end. My longest brace (@ 45 deg) would be 11.8 ft approx., this would need to be braced laterally (possibly nailed to vertical webs of trusses) every 24" o/c for it to work well in compression.

Do I need blocking between the truss bottom chords? Wouldn't a CLR (2 16d nails per truss) do the trick?

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

RE: Large Garages

It stems from our relative inability to transfer plywood tension across plywood joints using conventional construction. I need a rather complicated diagram to explain it well. I'll try to post something over the weekend. Don't let this hold up your work. Nobody really pays any attention to it in practice.

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.

RE: Large Garages

(OP)
Something along these lines possibly for the bracing:

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

RE: Large Garages

@Kootk - I actually thought about proposing the pre-engineered horizontal truss to the local truss manufacturers as a stock detail for them. Seems like it is the way to go, but installing a big, floppy truss horizontally would be a difficult erection. On skinnier buildings, I have laid a 24" lvl on edge between the first two trusses (gable and first common truss). Works out pretty nicely. As you can get a bunch face nails thru the LVL into end and side wall top plates and it can be supported by edge-nailing thru the first common truss.

RE: Large Garages

I tend to try and avoid kickers if I can. When you get near midspan, the horizontal thrust from the kickers due to vertical load can be a bit much, at least for long span, somewhat flexible members. I usually opt for a horizontal truss.

RE: Large Garages

@XR250

Having been a truss manufacturer in a past life, I know that the hang-up with the horizontal would be the loss of repetition in the trusses and the need to possibly increase all of the truss heel heights to accommodate the handful that would sit on top of the floppy horizontal truss.

I've heard of the LVL method, love it and was thinking of proposing it. Is it a problem that it's 24" rather than 22.5"? If one could get their hands on a 22.5" LVL, they could infill two truss spaces and shear connect the two LVL's across the intermediate truss with Simpson connector plates to form a composite, 48" beam.

Another alternative that I've considered is an extension of your plywood suggestion. One could build plywood box beams 22.5" wide and fit them between the trusses just like the LVL concept. You could make them as deep as needed.

@Medeek: I'm not sure that you need the ceiling blocking unless you're counting on load transfer to both diaphragms.

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.

RE: Large Garages

I did a small apartment using the ceiling diaphragm to brace the gable end. Much shorter and skinnier than this and I recall I had to block and strap 3 trusses deep just to get it to work. Can't imagine it being easy with this tall of a wall and the relatively high wind pressures.

RE: Large Garages

WARNING: I do very little wood design, so take my comments with a grain of salt

If the garage is in seismic design category C or higher, the diaphragm to wall detailing requirements in ASCE 7 §12.11 should be met. This includes continuous ties between diaphragm chords and restrictions on using toenails and nails subject to withdrawal.

RE: Large Garages

Quote (wannabeSE)

This includes continuous ties between diaphragm chords

This is related to my point above. I've always found it strange that it's not required in low seismic areas. Obviously, it's more important in high seismic zones but, at the end of the day, load is load. Do people actually follow that requirement for houses in California? It's hard to imagine.

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.

RE: Large Garages

(OP)
I want to try and leverage a little bit out of the ceiling diaphragm since the client is drywalling the entire interior of the structure with 5/8" gypsum. Even with the kickers at 48" o/c I'm also afraid that with all of the force concentrated at this hinge point I'm still going to get enough deflection to possibly crack the drywall connection or compress it right at the end wall/ceiling interface. Blocking the ceiling diaphragm at least 4' into the space will help redistribute some of this load into the ceiling diaphragm and hopefully eliminate any localized buckling or cracking, at least so my thinking goes.

Double kickers in three places (where roof height exceeds 6'), with a horizontal brace attached to the gable truss at 4' height.

The question: Will they build this as I draw it up? I'm going to put a note somewhere stating that the EOR is required to inspect the installation of the kickers and blocking prior to them closing up the ceiling with gypsum. I love to do on site inspections but so far my clients that I have had the opportunity to inspect their work have not been very pleased with my findings or some of my comments.

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

RE: Large Garages

@Kootk- they can rip the LVL to 22.5" on site. If they use 5/8" roof decking, they can leave it 24". I like you box beam idea. Might have to use screws or straps for the wind suction case..

RE: Large Garages

(OP)
Your right boo1, totally missed something else.

At the garage door in the SWL2 detail (sheet 2) I don't show any holdowns. I'm actually more worried about lateral here since the garage doors will transfer all of their out of plane wind load directly to the two trimmers on each side, half of that force needs to be resisted at the trimmer/stemwall connection. Currently I have a 5/8" anchor bolt there, probably not enough considering my lateral load is about 1250 lbs.

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

RE: Large Garages

Yeah, I'm starting to like it too. I might draw up some details. I guess I'd still be subject to aspect ratio limitations though, wouldn't I?

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.

RE: Large Garages

(OP)
My 5/8" anchor bolt with a double sill plate is good for about 736 lbs (using weakest lumber species from NDS Table 11E). I will probably put a STHD14 there for good measure, even though there is no rating for lateral loads only tension.

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

RE: Large Garages

(OP)
Technically if I am considering the ceiling as a diaphragm I would be required by IRC Section R702.3.7 to limit the diaphragm aspect ratio to: 1.5:1, which means I would have to block 27 feet in from the gable ends.

My idea with the blocking is not to really treat the ceiling as a diaphragm but to prevent localized cracking and buckling, whether this actually does what I intend is currently in question.

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

RE: Large Garages

(OP)
Okay, I might be reading/interpreting this incorrectly. Section 2508.5 does not specifically state that all panel edges need to be blocked only the diaphragm perimeter edge.

But then in section 2508.3.1 it appears that blocking/nailing at all panel edges is required but this particular sections wording is confusing to me.

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

RE: Large Garages

@Kootk - I didn't think there was aspect limitations for plywood box beams (if that was what you were referring to)

RE: Large Garages

Kootk -> why are you insisting on blocking the gypsum board ceiling? For unblocked you wouldn't need any blocking at all, right?

EIT
www.HowToEngineer.com

RE: Large Garages

Right. But there are limits for plywood diaphragms. And I think that we'd be kidding ourselves to think that these box beams are anything other than diaphragms by another name.

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.

RE: Large Garages

Quote (RFreund)

Kootk -> why are you insisting on blocking the gypsum board ceiling? For unblocked you wouldn't need any blocking at all, right?

There are two distinct kinds of blocking in my opinion. The first kind improves shear transfer between sheathing units and is optional. The second kind transfers the applied load into the diaphragm and is not optional. It's this later kind of blocking that I'm getting at.

The sketch that I'll post for Medeek will deal with this too. Stay tuned.

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.

RE: Large Garages

(OP)
I'm not entirely happy with the connection of the diagonal brace to the blocking at the ceiling. In compression it would work fine but if the wall is sucked out and the brace is in tension the nails connecting the block to the gable truss are subject to withdrawal.

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

RE: Large Garages

I'd recommend attaching your braces and your gable end truss to the sill plates, separately.

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.

RE: Large Garages

(OP)
Yesterday I walked the structural documents over to the client. I just called him to let him know I need to add a few details and upgrade some connections (garage door), back to the drawing board on this one.

I think I will continue to develop the bracing solution using kickers and blocking similar to what is shown above but I will also include the option to sheath the entire ceiling with OSB if that is easier.

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

RE: Large Garages

(OP)
Okay, tell me if this makes sense.

I'm assuming, based on my most recent interpretation of IBC 2508.5, that I can use the gypsum ceiling diaphragm as unblocked diaphragm. Given that I'm in seismic D cat. my max. shear capacity will be 50% of the 70 plf allowable => 35 plf.

My ceiling diaphragm load that I'm allowed would be wD = 2vB/L = 105 plf

I have a diaphragm load at the top plt. of 226.6 plf, if the ceiling diaphragm takes its share of 105 plf I am left with 121.6 plf to be picked up by the kickers.

At 48" o/c that would be 486 lbs per kicker, the center ones would be a little more since the loading is actually trapezoidal but this would be averaged out since the three central kickers utilize a two brace method shown above.

At 486 lbs I am overloaded by 14% when the brace is in tension, however based on footnote 1 of the Simpson GBC connector and also on the fact that the loading is trapezoidal and will diminish as the braces approach the sidewalls I can probably give this a pass.



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

RE: Large Garages

(OP)
I forgot to add that I'm assuming the kickers are attached to the top plt. using the Simpson GBC (Gable Brace Connector). This allows me to connect directly to the top plt. and not to the gable truss with my kickers.

The secondary kickers will connect to the gable truss at 4' above the bottom chord as previously noted.

The only problem with the GBC bracket is that it gets in the way of any blocking I may want to insert between the gable truss and the next common truss.

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

RE: Large Garages

How are you going to get 226 plf into the diaphragm?
Also, are you sure the trusses have vertical webs at 48" o.c.?

RE: Large Garages

(OP)
XR250, I'm not.

105 plf of the diaphragm load will be picked up by the ceiling diaphragm.

122 plf of the diaphragm load will be picked up by the kickers and transferred to the roof diaphragm.

Of course the problem with all of this is I don't really know how the load will really be shared between the two elements so I'm kind of stuck. If I design the kickers as if they take all of the load then I'm probably at 24" o/c with them which is absurd.

I could theoretically create 2 ply kickers (sister two 2x6 or two 2x4) together, but the problem really is the connection capacity on each end. If I want to connect directly to the top plate of the wall I don't really see any other option except to utilize a Simpson GBC.

I really don't want to go to a balloon framed wall since the client has already purchased his studs and he is pretty dead set on using them. Also for reasons given above some bracing at the ceiling line will be required either way.

I've added in the holdowns at the garage door at least, as you can see from the updated image above. At least I can figure that much out.

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

RE: Large Garages

(OP)
I guess the board is caching the images or creating smaller versions, here is the update sheet 2:

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

RE: Large Garages

oops, my mistake.
As far as the load sharing - who knows. Make sure the truss designer knows about the new loads.
You are lucky people use concrete foundation walls in your area. In my area, they would have built a 4" brick/4" CMU wall and expect me to do something with it.

RE: Large Garages

(OP)
I wouldn't even know what to do with a CMU foundation, I've never had to deal with one yet.

Available vertical webs are going to be a problem though. The center brace will probably have a center web to brace against (assuming Howe truss and not a Fink) but the next one will not. I can see why people shy away from kickers, its messy. By the time they are finishing installing the diagonal braces (kickers) and the braces for the braces this whole thing is going to look like a rats nest up in the ends of the attic.

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

RE: Large Garages

(OP)
The hinge effect is clearly shown here:

https://www.youtube.com/watch?v=N-jog8SrDpg

The kickers appear to eliminate the hinging but there is still some pinching in of the space between the gable truss and the next truss:

https://www.youtube.com/watch?v=jtlBB-kPqz4

https://www.youtube.com/watch?v=FWAh5OVmqyc


Without any secondary kickers (those above the main kickers tying into the wall top plate) my calculated lateral force to each kicker is 226 plf x 4 ft = 904 lbs, if the diagonal is 45 degrees the compression force in the brace is then 904 lbs x 1.41 = 1278 lbs

The axial loads in the kickers given by RISA3D:



The wall studs loads are:



The RISA Model:



Closeup of the gable truss with kickers:



Planview:



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

RE: Large Garages

You can put a triple truss - say 6 ft back and ladder frame the rest in with 2x4's. That would give you an un-ubstructed kicker path and you can use the ladder framing for attachment of the kickers. Also, do you really need two rows of kicker? Seems it would be cleaner with one.

RE: Large Garages

Here's the sketch that I promised above showing why, in the absence of drag struts connecting the diaphragm chords, the effective diaphragm length should be taken to be about half the diaphragm width. The crux of it is my belief that you can't transfer significant tension across plywood joints without initiating perpendicular to grain splitting. Additionally, when we calculate our nail spacings, we're generally only designing for the diaphragm shear and uplift, not the tension forces that would be required to drag applied load through the depth of the diaphragm.

To get any load spread at all within the diaphragm, the plywood sheets require some internal tension capacity. That tension capacity would be unreliable in gypsum sheathing which is why I'm even more skeptical of diaphragm load distribution with ceiling assemblies in the absence of chord to chord drag struts.

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.

RE: Large Garages

(OP)
KootK I'm going to need a few hours to roll your diagram around and give it some thought.

I wish I knew more about FEA and plate modeling. I think if one could establish a realistic model that accounts for various factors such as nail slip and tension etc... it would be interesting to see really how accurate our simplistic assumptions are.

Here is the Von Mises Plate Stress from the model above:



I never did take an FEA class in school, wish I had. My other electives (robotics, jet engine design, aerodynamics etc...) are pretty much useless to me now. I guess it is time to find a good text book on FEA and understand plates a little better. I would also like to utilize some FEA models to validate my new FTAO and Portal Frame calculator that I'm working on.

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

RE: Large Garages

FEM book recommendation: Link. Presents the theory about as simple as it can get and has a great practical chapter on how to model and check.

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.

RE: Large Garages

@Kootk:

I like your shear model. I imagine a significant amount of tension does occur at the plywood butt joints in reality even if the edge distances are small. I have seen too many instances in practice of plywood acting in pure tension in this manner without failure.

RE: Large Garages

KootK -> nice diagram and I appreciate you putting time into this. I think it is correct but I think in reality the plywood and the gypsum board are able to carry quite a bit of tension. The cross grain force caused by the nailing joint is the weak link but when you consider the joints are staggered the cross grain bending is spread to all joist at many nail locations. This is probably why the ceiling performs so well in reality.

EIT
www.HowToEngineer.com

RE: Large Garages

While a nice exercise, I feel FEA is pretty worthless for wood structures. Too difficult to accurately model the non-linerarity of the the different synergistic systems. Also, way too many variables in the construction process.

RE: Large Garages

medeek: A couple of comments.

1) I attached a gable end wall brace detail. I have others that utilize (2) 2x6 diagonals up to the roof with upside down LU26 hangers connecting the blocking to the trusses. You really only need to figure out the force on the brace and then detail your connections to take the force. Sometimes the Simp. Gable End Wall connectors don't work and you need to use steel angles.

2) The hold-downs at your garage opening will more than likely cause spawling in the concrete. I would recommend using an HDU or something along those lines to avoid the splitting. I have seen many, many straps installed at angles, missing studs when nailing to the studs etc to the point that I don't even look at including them in my drawings any more as well.

3) How are you transferring the out-of-plane force from your garage opening king studs into the roof diaphragm and the stem wall? This can be a critical connection that is generally missed and should be looked at...just like the gable end wall braces.

RE: Large Garages

(OP)
CBSE, thank-you for the detail and also your additional comments. I particularly find the comments about the doubled up braces to be interesting, this is along the lines of what I was thinking. I've determined that I've got about 900 lbs of lateral load on some of these braces which is approx. 1275 lbs tension or compression. The GBC connector is just not going to work. As for steel angles connecting the brace to wall top plt. I'm considering two HGA10 on each side of the (2) 2x6 brace, I'm not really finding anything in Simpsons catalog that fits the bill.

My next best option is to simply require braces at 24" o/c, this will get my forces down to more manageable levels.

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

RE: Large Garages

(OP)
CBSE, I typically stay away from the strap holdowns for the same reasons you mentioned however in this case the amount of wood utilized for trimmmers and king studs gets me such a distance away from the opening that I though it would be better to use the strap holdowns to help eliminate this eccentricity. I am counting on the STHD14 straps for both uplift and also some lateral (out of plane forces at stemwall). The 5/8" dia. anchor bolts will also pickup some of the lateral at the stemwall at 16" o/c.

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

RE: Large Garages

(OP)
Here is my first draft at the Gable End Wall Brace Detail:



The overall structural plan for reference:



I've decided to try 36" on center spacing since 48" on center has to high of point loads at the connections to the top plate and also at the connection to the roof diaphragm

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

RE: Large Garages

(OP)
Now that I've got that more or less worked out back to KootK's diagram.

Take a flat ceiling diaphragm, flip it up vertical, support it at both ends and you have a really tall, thin beam. However the way I see it is that the direction of the rafters or girts play into it. From my recent work on the pole frame structures it can be shown that the diaphragm chord forces are not just at the perimeter as is typically assumed with wood framed diaphragms but rather shared by the internal girts as well.

I would be really interested to read more on the methods used to come up with the diaphragm allowables found in the SDPWS and what their assumptions and simplifications were. Are these numbers from purely empirical testing?

The to further complicate the matter you pitch the roof. Low slope roofs are probably pretty close to the classic diaphragm but what about a 12/12 pitch roof. Does the higher pitch make the diaphragm stronger or weaker? Where does this change in pitch show up in the SDPWS tables? Maybe it is a insignificant factor.

My thinking with the cross grain splitting is what is the failure mode. Will the nails heads pull through the panel first or through the edge before the framing splits. My suspicion is that the nails will pull through the edge of the panel before the framing members splits when the nails are not closely spaced (ie. 6" o/c at edges).

I actually think modeling diaphragm action, nails included would be a good candidate for FEA. I'm just not sure how to model the nails and sheathing effectively. What I am interested in visualizing is how the nails load up in a diaphragm. Do the nail loads mirror the classic diaphragm shear diagram (zero load at the middle) or are there other effects or factors at play that are overlooked by the classical diaphragm model?

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

RE: Large Garages

Thanks for reviewing my sketch guys. As follow up, I submit the additional sketch shown below. Given Medeek's loads, and the assumption of distributing those loads throughout the entire length of the diaphragm, the first plywood joint would need to transfer about 225 lbs of tension. With a 6" nail spacing, that's equivalent to a 12" strip of plywood with two nails in it holding up a standard olympic barbell loaded with four 45 lb plates. Does our collective intuition still tell us that this would be okay? Ditto for a drywall option?

Note that I'm just having fun with this as an arcane technical discussion. I'm still handling this as everyone else does: full diaphragm engagement, blinders up.

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.

RE: Large Garages

ever consider storage trusses?

RE: Large Garages

I'd stand under that for a short duration (especially if beer was involved as shown)
You have a potential career as an artist :>

RE: Large Garages

(OP)
As a follow up I went out yesterday and took a look at the construction. If the roof was a bit higher of a pitch I definitely would have suggested storage trusses. I could really use a garage this big.





Picture of the framing of the shearwall next to the man door:

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

RE: Large Garages

(OP)
Looks like the framer and the building inspector missed the note that all 15 ft. walls are to be 2x6 DF No. 2 @ 12" o/c spacing.

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

RE: Large Garages

Looks like truss bracing and gable end truss bracing are missing, I only see rat runs.
Are truss straps correct?
Can't see the outlooker straps/clips.
Is the exterior water drainage being followed? tall exposed OSB sheathed walls needs more then 15# roofing paper

Consider checking the nailing of the 2x8 on the inside of the garage door opening.
The garage door track attached to it and is a critical elopement we have seen failures during high wind events of the door blowing in.

RE: Large Garages

(OP)
He only has one (center) gable end truss brace installed, I let him know that the other gable end braces needed installing. Its still under construction so I expect some of the elements to still be missing.

@boo1
The exterior cladding will be hardiplank over the felt. What is your thinking on the 15# felt, what do you suggest?

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

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