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

 

[medeek]

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

http://www.medeek.com

 

[medeek]

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

http://www.medeek.com

 

[medeek]

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

http://www.medeek.com

 

[medeek]

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

http://www.medeek.com

 

[XR250]

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.

 

[jayrod12]

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.

 

[KootK]

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.

 

[jayrod12]

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.

 

[medeek]

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

http://www.medeek.com

 

[medeek]

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

http://www.medeek.com

 

[KootK]

@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.

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.

 

[medeek]

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

http://www.medeek.com

 

[medeek]

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

http://www.medeek.com

 

[KootK]

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.

 

[medeek]

Something along these lines possibly for the bracing:

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

http://www.medeek.com

 

[XR250]

@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.

 

[structSU10]

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.

 

[KootK]

@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.

 

[XR250]

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.

 

[wannabeSE]

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.