Gable End and Interior Shearwalls 4

[medeek]

I've been mucking around with the Woodworks software and reading through some of its documentation. I noticed that the uplift forces being calculated for the holdowns was different than I was calculating manually for gable end shearwalls. Looking through the help files I noticed that the height being used to calculate the holdown force was not the wall height but actually the average height to the roof diaphragm for that segment (see diagram below):

When a roof like the one shown above is composed of closely spaced trusses (max. 24" o/c) my thinking was they would act like mini shearwalls of their own and bring the diaphragm load down to the ceiling level where it would be transferred to the walls. I suppose the same argument can be made for interior shearwalls as shown above as well. However, I am now having to rethink this assumption.

The exterior shearwalls parallel to the ridge obviously are same height as the wall height but how is everyone else handling the gable end and interior shearwall heights?

A confused student is a good student.

 

[medeek]

So which is better?

PDF and AutoCAD

A confused student is a good student.

 

[KootK]

Mechanically, I like four. I'd expect contractor pushback though and would probably compromise to three. Like woodman88 suggested above, sooner or late, you end up using drywall for stability. Maybe CBSE can comment on how well the pre-fabbed bracing has been received on his sloping roof projects.[pre][/pre]

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.

 

[Brad805]

If the resistance is the same in all four, #2 is the one they will pick in the field. That is very easy to implement. Your personal preference is meaningless unless it is your house. If the resistance is different I don't follow the point of the question.

Many truss companies will sheet a gable end truss in the shop, but you would have to accept a joint at the top of the wall.

 

[KootK]

Has anybody used two and had the opportunity to verify its condition after a few winters? The concept is clever but fatally flawed in my opinion. The wall won't be centred between the trusses, it'll be six inches or some other random value from one of the trusses. When the trusses deflect, if the plywood doesn't snap in flexure, it might just shear clean off.

I should mention that all this stuff is hypothetical for me. Unfortunately, I work in a region where diaphragm design garners little respect so it just doesn't get done for the most part.

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.

 

[RFreund]

I don't think 1 or 3 are that unreasonable, weather it comes sheathed or not. Really any of these I could see working. I've never seen #2 implemented but in order to brace a partition between trusses (and parallel) you normally would have wood blocking between trusses to brace the top of the partition. I don't recall seeing this blocking in bad shape but I can't say that I have looked either. I was thinking 4 was being used for the perpendicular case but I see that it helps eliminate buckling stability concerns. I suppose though the gypsum board ceiling will do fine in bracing the bottom chord (given the lack of failures??).

EIT

http://www.HowToEngineer.com

 

[Brad805]

I agree with the premise Koot if the roof is considered to be a combination of 2D trusses that all deflect independently as the truss calcs suggest, but once the roof is complete it functions like a unit. If this were a problem I would expect this problem to exist at all interior walls where they are parallel to a truss. It is very common to screw the edge of the ceiling drywall to an interior wall that has a larger plate attached to the top of the wall. After the ceiling board is in place, the wall board is installed tight enough for taping. This in effect will create a similar situation since the walls are much stiffer than any one single truss. I have also seen many renovation projects where walls parallel to the roof trusses have been framed tight to the U/S of existing drywall and they have performed acceptably. That is not a good practice, but if you have installed any sheetrock you will get why some might choose to do so.

He could add a 2x4 on edge to stiffen the shear element and avoid excessive drwyall cracks if he is concerned. That detail still can be completed with a framing nailer, and some leftover material that was ordered to create all the tee bracing truss guys come up with these days to deal with the long unsupported truss chord lengths. Nails and 2x4 scrap are both cheap. Think of implementing the other details if you had a 12/12 roof where the truss span were say 35'. The total truss height would be 17'-6". With a 10' or 12' ceiling that puts you almost 30'-0" in the air. That is yet another piece you get to fly in with the picker, whereas detail #2 is ladder/scaffold work whenever you have a grunt to do it.

The plywood will not shear off, nor snap, but it might deform. I have watched truckers use 3/4" plywood scrap to offload their loaded trailers (high boy trailer). The plywood is not happy, but it is does not shear or snap.

 

[KootK]

I'm with you on all of that Brad. However:

1) I consider connection to a shear wall to be much more critical than acceptable performance of nearby drywall. For shear wall connections, I'm much less willing to place my trust in anecdotal experience that doesn't calc out.

2) A long truss could easily deflect 2" vertically next to a wall 4" away. That would induce a good deal more curvature into the plywood than your scrap offloading example. Snap, snap, snap I say.

Everything in your post confirms a suspicion that's been building in my mind: the Malone detail is bass ackwards. Designing the plywood to be flexible enough not to draw gravity load is silly. Instead, we should design the element that spans from the wall out to the trusses to be able to hold the trusses up. 2x6 @ 24" o/c with clips or something along those lines.



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.

 

[Brad805]

Yes, I agree the shearwall is more critical from a structural point of view, but drywall cracks are more critical to the homeowner. Homeowners get annoyed by a such a problem and they call to complain or ask you to spend your time investigating the problem.

I doubt the plywood would fail before the nails would start to deform in the example, but it will boil down to a battle of stiffness. I have a vacuum press for veneering and have edge loaded plywood many times by accident when the backer hangs over the edge of the piece being veneered. I have also built curved forms for veneering. The trailer was not a great example since it is sitting on clay, but the trailer was loaded to approx 25,000lb.

The importance of this question boils down to ones belief in the 2D truss deflection analysis. I agree with the 2D approach, but I do not believe it is very accurate at predicting the deflection of a 3d roof assembly. Have I tried to prove that with a detailed analysis, no, but I have looked at enough snow loaded roofs to be comfortable. I was going to share some of my examples, but I found a couple of technical papers that might be of interest to your inquisitive mind.
- ASCE:

http://ascelibrary.org/doi/abs/10.1061/(ASCE)1084-0680(2008)13:3(135)

- Wind Engineering Conference:

http://www.engr.uconn.edu/~wzhang/Pdf/C201306ACWE12FEMLow.pdf

The ASCE paper does not appear to have compared deflections, but they did find up to a 58% reduction in connector forces. The ASCE study did not include a plate element for the roof sheathing.

 

[KootK]

Thanks for indulging me Brad. I worked with the Wood Truss Council of America for a spell. At that time, they were very interested in finding a way to account for 3D system behaviour. Unfortunately, it's a very difficult thing to quantify.

So I've got to ask since we're geographical cousins: do you actually incorporate any of these fancy details into your work? I've yet to see an inter truss drag strut in real life. Of course, I don't do a ton of wood.

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.

 

[CBSE]

@ KootK

I haven't had any contractors complain about the detail...yet. The purpose of the blocking or the diagonal kickers is to create out-of-plane stability for the shear wall. Your detail with flat blocking with drywall attached in theory would work, however, would you use Sheetrock for shear wall sheathing? Some do, not me though. I will say I have used Sheetrock as a last resort in the past, only twice.

Truss manufacturers don't mind the blocking, they get paid to build it. Just make sure it is noted on both the roof plan and details. I will typically put a note on my roof plan for the truss mfr so they don't miss things..."Truss Mfr to refer to details...for additional requirements." Pretty hard to miss.

The Toenails are redundant. Have you inspected most contractors toe-nail patterns? It can be all over the place in regards to adequacy. I like the clips because I know the connection is easy to make.

 

[medeek]

Away for a christmas party and I miss all the fun. Here are the other typical methods that Terry Malone's book considers as flawed:

Detail 5 is problematic in that the nails connecting the flat blocking to the trusses will probably fail in shear.

Detail 6 is problematic in that the 2X cont. plate will be in cross grain bending and/or prying.

A confused student is a good student.

 

[medeek]

I've mostly seen Detail 5 both for interior shearwalls and other interior partition walls.

A confused student is a good student.

 

[jayrod12]

I've seen detail 5 more times than I can count. And I would reject 6 in a heartbeat if I ever saw it.

2 would bother me as well. since the sheathing would only be a single span if the board didn't snap in 2 I would expect the fasteners to rip out of the edges and the whole piece get popped into the attic.

1 and 3 are my preferred details but getting the contractor to line up the truss over the wall is a pain.

4 is a pipe dream, as nice as it is no one has ever done it that I have seen.

 

[CBSE]

jayrid12:

If you are using Detail 1, how are you bracing the top chief of your shear wall from buckling out of plane? Relying on Sheetrock?

Detail 3, 5, and 6 in theory could work. It doesn't tske much force to brace for out-if-plane. 1 and 3 aren't really much different than 5 and 6. They are all braced by Sheetrock.

The contractors that have built my projects have always put the blocking in...on the buildings I have inspected. If it wasn't there, it was noted to be installed.

 

[jayrod12]

Yes, I consider the gyproc enough. If the wood design manual gives me diaphragm values for gyproc then it must be worth something.

However, 3 is my most preferred detail, 1 is for lightly loaded shear walls where out of plane buckling is unlikely. The gyproc provides some nominal support even if you don't like using it for actual lateral resistance.

Have you ever read your truss shops? More often then not they have some note about the building designer to provide a rigid ceiling, how do you provide that? I use gyproc, I'm not going to specify that the ceiling is sheeted in OSB prior to drywall, I would never get another wood framed job in my life (and I actually like wood framing).

 

[RobertHale]

I have a few questions about this issue of blocking. What does the instance of blocking do in detail 3? It seems to me that you would just get buckling of all three bottom chords in the same direction. I can see this justified by either accounting on the ceiling panel or on the blocking/bracing placed for chord and erection stability but the detail doesn't really mention anything about that. Does anyone specify chord bracing in accordance with the BSCI sheets and count on that to provide stability to the top of the shear wall? I use detail 1 solely, but I have never specifically looked at the stability of the top of the wall.

 

[jayrod12]

I figure the blocking spreads the load out to multiple trusses better than relying on the gyproc solely to restraint 1 truss, If I block two bays each direction on the bottom chord I consider the lateral buckling restraint to be provided by the 3 trusses (1 over the wall and the next two) and the gyproc, it just lowers the loading on the gyproc fasteners.

 

[jayrod12]

I think of it the same as blocking floor joist spaces at the top of foundation walls.

 

[KootK]

Regarding detail three, the top chord blocking is unnecessary in my opinion. The bottom chord / wall bracing would have to rely on the ceiling drywall to behave as a diaphragm which is fine. BCSI standard bottom chord bracing could be roped into the mix but it is spaced pretty far apart and you rarely know its location with any accuracy. The more that we discuss this, the more I like detail one too.

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.

 

[Brad805]

Koot, I found it interesting that the ASCE report was sponsored by a truss plate manuf. I have tested Javelin before, and I can see it being practical for the truss plate companies to take their software to the next level in the upcoming years. I think it will take some time considering the reality would be selling smaller truss plates in many cases. It will be interesting to see what StrucSoft does with their roof framing modules in the future. This could greatly speed up the workflow in a typical engineering office. The paper by Pan and Zhang used ANSYS, and that will never be practical, but it is very interesting to see how close they can predict with some of the methods they use. I have been looking at ANSYS and ABAQUS for concrete lately.

We do use some of these details, but we have to pay attention to when we do so. I have two survey engineers, a pole building engineer and an ex-mech eng (does not care like MeDeek) that seem to virtually seal whatever their techs draw. I need to be able to defend my designs in many cases with more than, "because I said so." While the engineering is the most important aspect, the fact is we are running a business. In my area snow is our enemy, so lateral loads are not usually significant. I have seen all of these details in various forms when we do field reviews for out of town engineers (Vancouver mostly). Some of this is a pet peeve because most of them recycle their details in this region {Sa(0.2)=0.9 Vancouver; Here Sa(0.2) = 0.12 ---> no seismic req'd many cases). One job the roof required two lifts of 2x4 in the form of bracing. Now some of that was tee bracing, which I fully accept when the contractor does not compare bids carefully, but about 1/2 of it was bracing. That's fine for a high seismic zone or hurricane region, but what has changed in our area to require so much more than the old structures that were built in the 70's/80's?

I am curious to hear others intend with detail #4 and the blocking in some others? If we have specified the lateral load to the truss, and the truss manuf designs the bracing to achieve that, are we trying to shed the load to more trusses or stiffen the diaphragm locally? If it is shedding the load to another truss, I still think adding a truss is more cost effective and is more likely to get implemented correctly. If the goal is to locally stiffen the roof diaphragm, it would seem that the load has exceeded the allowable load for an unblocked diaphragm in that region and all that is needed is edge blocking that can be installed as they work up the roof.

While I am definitely not a fan of the pole building nor the poor details it is hard not to think about their performance when designing a roof diaphragm. They put virtually zero thought into the roof diaphragm design and there are countless cases performing acceptably.

Anyway, Merry Christmas.