Gable End and Interior Shearwalls
Gable End and Interior Shearwalls
(OP)
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?

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.






RE: Gable End and Interior Shearwalls
If you are requiring drag trusses to be connected to the shear walls, than where the force is applied depends on your details.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Gable End and Interior Shearwalls
A confused student is a good student.
RE: Gable End and Interior Shearwalls
RE: Gable End and Interior Shearwalls
So, the truss will experience overturning, which must be resisted by the holddown at each end of the truss.
The truss, in turn, transfers the diaphragm force into the shear wall along the top plate of the shear wall.
DaveAtkins
RE: Gable End and Interior Shearwalls
Woodworks appears to assume a balloon wall (not a drag truss). As such the shear force will be applied at the mean height of the sheathing as shown in the above sketch. It will also require straps from the gable to the wall.
When a drag truss is used, where the uplift is taken out depends on the connections details.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Gable End and Interior Shearwalls
A confused student is a good student.
RE: Gable End and Interior Shearwalls
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Gable End and Interior Shearwalls
On a separate note, for walls with full height gable studs, the shear force for OTM should be applied at the average height of the roof for "gable" sections (similar to the area between windows in your sketch) and at the highest point along the wall for "sloped" sections (similar to the end gable wall segments in your sketch) in order to maintain equilibrium. This is assuming the shear force is delivered uniformly to the shear wall along the (sloping) sheathing.
See attached link.
http://www.shearwalls.com/resources/appendix-b.pdf
RE: Gable End and Interior Shearwalls
A confused student is a good student.
RE: Gable End and Interior Shearwalls
DaveAtkins
RE: Gable End and Interior Shearwalls
http://design.medeek.com/images/misc/SLOPED_SHEARW...
A confused student is a good student.
RE: Gable End and Interior Shearwalls
A confused student is a good student.
RE: Gable End and Interior Shearwalls
T = F(cosθ)wh/b
This holdown force is less than the holdown force given by Matteson's paper (second box) and also less than holdown force given by the average wall height method (third box).
The other thing thats stands out to me from Matteson's paper is the suggestion to apply a strap at the low end of the sloping shear wall that passes over the top plate of the shear wall and connects to the chord. I've never seen this done, which doesn't mean it hasn't been done or shouldn't be, but I'm oblivious to it. Does anyone have any examples of this being done in practice, picture or diagrams would be educational. The downward force that must be provided by the strap is: Fy = F(sinθ)
A confused student is a good student.
RE: Gable End and Interior Shearwalls
Which brings me to one additional question which was already partially addressed by Dave Atkins above. Every truss in the roof that is subject to a lateral load along its top chords will experience an overturning moment with an uplift force given as:
V/2(tanθ)
The lateral force on each truss cannot be resisted by the walls beneath it since they are perpendicular to the load so the lateral load will be collected up by the diaphragm and resisted at the shearwalls. However, the vertical uplift force I would argue can be resisted by each truss connection to the wall beneath it hence the only loads that need to be dealt with at the shearwalls are the total horizontal component tributary to each shearwall. Once you have the total shear for the shearwall then if the truss above cannot act as drag truss you then have to analyze it according to the method proposed by Matteson or the avg. height method. In this reqard the Matteson method appears to be a more accurate and rational method of the two.
A confused student is a good student.
RE: Gable End and Interior Shearwalls
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Gable End and Interior Shearwalls
As long as there is no break in the sheathing at the eave height, I do not see any reason for any intermediate ties within, or at the top of the wall.
Also, you might want to consider distributing forces to walls in a line based on panel relative stiffness rather than simply wall length. There has been some discussion concerning this on here recently. (Also see 2008 AF&PA SDPWS 4.3.3.4) This will have the added benefit of drawing load away from the tall slender wall where uplift forces are highest.
RE: Gable End and Interior Shearwalls
All this being said, I agree that many engineers simply use the average wall height for the segment in question for balloon framed walls. Similarly, I think most use the eave height for interior shear walls with a drag truss and count on additional uplift being transferred through the truss and resisted by the truss ties at the bearing ends.
RE: Gable End and Interior Shearwalls
A confused student is a good student.
RE: Gable End and Interior Shearwalls
I would recomend you read up on some basic laterial design of buildings. The Reinforced Masonry Engineering Handbook: Clay and Concrete Masonry, Fifth Edition Hardcover – March 5, 1998 by James E. Amrhein (Author) should be a good one.
Please note that such designing of shear walls is not typically done for wood buildings.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Gable End and Interior Shearwalls
RE: Gable End and Interior Shearwalls
I tepidly disagree with woodman and Robert on this one. Unless a moment couple is developed between the top of a shear wall and the drag truss above it, the shear wall will feel the applied shear as though it were delivered at an elevation matching the top of the wall. That's just statics.
Moreover, if the couple mentioned above were possible, it would put the shear wall in double curvature and reduce the overturning on the shear wall. For the truss over shear wall condition, I just don't see any way for the overturning moment to be any more than it would be with the shear applied at the top of the wall.
The real question here is how to address that left over moment that reflects the elevation at which the shear is actually applied. And I believe that DaveAtkins has the right of that. I'll confess I'm a little worried because I have not been tying down my drag trusses or specifying that they have studs directly beneath them.
Two additional thoughts:
1) How darn complicated does this situation get if your shear is coming down to the wall out of a hip roof rather than a gable? Ick.
2) At some point, our profession is going to have to consider producing a lateral load design guide for the prefabricated truss folks. Those poor bastards don't stand a chance.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Gable End and Interior Shearwalls
A confused student is a good student.
RE: Gable End and Interior Shearwalls
Assume platform framing with trusses above.
Specify on the drawings the location of the drag strut trusses, and that they must be positioned above the shear walls. Specify the lateral force you want the truss supplier to design for.
I don't ask the truss supplier to do anything special at the gable ends--those trusses will be covered with sheathing.
Detail the connection of the drag strut truss bottom chord to the top of the shear wall (toenails or Simpson angles).
Detail the connection at each end of the drag strut truss for uplift due to overturning, if required (otherwise, a normal hurricane tie will be sufficient).
Detail the connection of the bottom of the gable end truss to the top of the end wall (there will be a horizontal joint in the sheathing at the top of the wall).
Detail the shear wall holddowns.
You also need to note the nailing pattern for the roof sheathing and the wall sheathing. Assume unblocked sheathing at the roof--they almost never put in blocking. Blocking is typically required in the walls, because the design tables assume walls are blocked.
DaveAtkins
RE: Gable End and Interior Shearwalls
My answers were regarding the OP drawings.
If you or the OP want to know how to design wood buildings, per the current standards, see the attached link for some good basic information.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Gable End and Interior Shearwalls
Typically a flexible diaphragm is assumed so, yes, the middle wall has twice the tributary load. However if you had a rigid diaphragm (typically defined as your shearwall deflection being twice the average diaphragm deflection) then you could/would assign load based on stiffness of the walls.
Best to use semi-rigid diaphragm and account for wall and diaphragm stiffness, like a beam on elastic foundation. I jest
As for the overturning forces - Because the resultant horizontal roof wind load is applied above the top of the top plate you will have a higher overturning moment (therefore higher chord forces). Same goes for hip roof. I think many times this gets ignored and the force gets applied to the top of the wall, but it does violate statics.
EIT
www.HowToEngineer.com
RE: Gable End and Interior Shearwalls
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Gable End and Interior Shearwalls
"I think many times this gets ignored and the force gets applied to the top of the wall, but it does violate statics."
The assumption, in typical wood construction, is the the trusses/joists that have sheathing applied to the top and bottom is that it creates a system for the transfer of forces. As a system the lateral force at each truss/joist get transferred to the shear walls but the uplift force is resisted by each truss/joist connections to the bearing walls and does not get transferred to the shear walls.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Gable End and Interior Shearwalls
Thank you, I will review it if I do another five/six story wood building.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Gable End and Interior Shearwalls
EIT
www.HowToEngineer.com
RE: Gable End and Interior Shearwalls
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Gable End and Interior Shearwalls
My question is what is the best number to give the truss manufacturer? V, v, or R1/R2
A confused student is a good student.
RE: Gable End and Interior Shearwalls
A confused student is a good student.
RE: Gable End and Interior Shearwalls
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Gable End and Interior Shearwalls
If you are going to work towards breaking even at this in the future, you should read the recommendation by woodman. There are a lot of standard practices for wood framing.
RE: Gable End and Interior Shearwalls
I'm pretty confident with the SW2 and the drag truss inline with it. The truss completes the load path and provides the collector to SWL C.
With SWB since it runs perpendicular to the trusses needs some shear panels above it to the roof diaphragm or some mini-shearwall trusses made up by the truss manufacturer. According to everything I've read a collector should connect SWB to the exterior wall SWL 3, however this is entirely open space. I think I am a little more unclear as to best handle the need for a collector at this location.
The trusses are standard trusses in this case, nothing fancy here, no vaulted ceilings etc...
Note that the exterior shearwalls are not shown. Due to the number and size of the windows there typically is not much room for exterior shearwalls hence the need to utilize interior shearwalls. FTO shearwalls, portal frames and moment frames are always options but avoided if possible due to the added complexity and cost.
Most of the residences I work on are in Exp. C or D and the basic wind speed is 155 mph (ult.) so lateral forces are usually quite significant.
A confused student is a good student.
RE: Gable End and Interior Shearwalls
I think my next big purchase will definitely be a copy of Woodworks. The ability to input complex wall and roof geometry and get instant wind and seismic loads would be a huge plus. I can calculate them manually but for large residences with crazy roof lines it can be time consuming. Beyond Woodworks is there any other software that is comparable in function?
The only concern I have with a product like Woodworks is the "blackbox" element of it all. I'm not doubting its accuracy since a lot of engineers swear by it but I like to really understand how I am arriving at my numbers.
A confused student is a good student.
RE: Gable End and Interior Shearwalls
Yup. I see it the same way except that I think the collector needs to be at the level of the plywood rather than the drywall. You can play some games with partial diaphragms and ride along roof sections but it gets pretty sketchy pretty fast. And then there's all the considerations that go into achieving rotational equilibrium for each of the three little panels.
Unfortunately, for all but four sided, flat roofed boxes, most diaphragm designs are full of load path holes and questionable assumptions. It's a tough road for the conscientious.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Gable End and Interior Shearwalls
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Gable End and Interior Shearwalls
Medeek, Risa 3D includes wood framing and you would have an option to model a roof diaphragm using FEM if you wanted to make some assumptions on the roof truss configuration. It would be time consuming, but at least you could satisfy your curiosity a few times so you could be more comfortable with simplifications. Cost wise, this would not be a good option. You can download a copy to try. I know about complex roofs. One of my guys has a package on his desk that has 158 different trusses.
You seem to have a knack for detail, and I think you would do really well at detailing if you can find forward thinking clients that understand the cost of re-works. They are hard to find, but they are out there. Something like Javelin, Boise Cascade's 3D solution or the Struc Soft add-in for Revit might be something to think about in the future. Some of these include steel stud framing as well, and that is growing in popularity in some areas. You need to find a niche. I can tell you care, but typical residential clients will wear you down quicker than others. The money is too personal and most of the time their wants out weigh their means.
I am not in a hurricane zone by any stretch of the imagination, but from what I recall from news stories the typical roof failure during a storm is the complete roof fails, segments fail or parts of the roof are dislodged. I do not recall a lot of cases where I have seen roof framing roll over due to insufficient bracing except during construction. I think that has a lot to do with all of the things we must neglect. Now that does prove anything, but if you keep that in the back of your mind you can detect when to look at certain things closely and when it only needs some basic checks. An architect that wants you to build with toothpicks is something to watch for. I realize this is about testing woodworks, and that is great, but you need to always keep practicality in mind. Truth be told, many of the residential clients do not appreciate what you do very much. A house is a house is a house in their mind.
RE: Gable End and Interior Shearwalls
I have been digging through the WFCM as of late, and yes it does have alot of helpful prescriptive rules that I've also been incorporating in my own work. However, I've noticed that most of the residential designs that I'm looking at don't fit as nicely into the rectangular box model. I'm just trying to make sure that my engineering of these more elaborate structures is not woefully inadequate.
Brad805, do you or any of your colleagues have some sample Risa files of wall assemblies, diaphragms, portal frames or shearwalls that you would be willing to share. I would like to understand how such a FE model is created for a wood structure and what assumptions and details are made to make this an accurate mode.
A confused student is a good student.
RE: Gable End and Interior Shearwalls
1) Using steel beams to make wacky cantilevers work.
2) Features stairs in steel and glass.
3) Sexy heavy timber detailing.
4) Cross laminated timber floor slabs.
These kind of things get you out of the prescriptive domain and into operating in a space where your services are more highly valued (Brad's point). Your obvious attention to detail might serve you especially well in such an environment.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Gable End and Interior Shearwalls
Is this amount of blocking common practice for a typical drag truss? I suppose it will all depend on the loads involved.
A confused student is a good student.
RE: Gable End and Interior Shearwalls
I can see an argument for bottom chord blocking but, done like this, I have two concerns:
1) 24" o/c seems excessive.
2) The blocking is only as effective as the drywall diaphragm that restrains it.
I'd rather see diagonal bracing at some interval to laterally brace both the bottom chord and the joint between the truss and the wall.
An issue with this detail, and most of itss cousins, is the truss uplift that tends to occur with seasonal moisture change. It tends to put parts of these details into cross grain tension.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Gable End and Interior Shearwalls
I don't agree. For the overturning to be taken out at each truss, the lateral force must transferred to the bottom of each truss. The ceiling diaphragm (usually GWB) is not typically considered a diaphragm.
DaveAtkins
RE: Gable End and Interior Shearwalls
I don't think I understand your point, but I"m probably missing something. Even if you didn't have a ceiling diaphragm, you could still take out the uplift at each truss / stud, right? It's like the roof is a folded plate and each stud on the uplift side is a string. They have no ability to resist horizontal forces but they can resist uplift due to rotation of the folded plate. The shearwalls then resist the tension from the lateral force applied at the top plate level.
EIT
www.HowToEngineer.com
RE: Gable End and Interior Shearwalls
Admittedly, this is an oversimplification. The entire roof (roof sheathing, trusses, and ceiling sheathing) may act as a unit. If so, many of the wall studs will assist with overturning.
DaveAtkins
RE: Gable End and Interior Shearwalls
The only way to match the design of conventional light-frame construction (per the IBC) with an engineered design is to look at the whole building. Such as, conventional light-frame construction requires an attic which has a ceiling creating a system that is not usually considered in an engineered design of wood structures. IMHO
The important thing is to design to your requirements/standards to meet the building code.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Gable End and Interior Shearwalls
As we can clearly see from the respondents posting, the simplifying assumptions can vary. Most of the literature usually only covers the simple cases. From those we all develop rules of thumb we follow for our practice. There are some interesting practitioners in the residential engineering field.
The last detail you posted makes sense, but keep in mind the labor to implement that. A typical roof truss for a common house is approx $100 (can be less). The labor to cut all the blocking and nail them into place and can be more than adding an extra truss if the shear forces suggest you need more than one truss.
The interior shear panel you posted earlier can be a time consuming detail to implement. All of us engineers like plywood in these cases because it can be cut neatly cut to fit (in our minds anyway). From the contractors point of view on site he has to cut each 2x4, nail it into place and then attach the plywood. This is more time consuming than adding additional bracing between the trusses while they are installing the construction bracing. You can add a top/bottom chord member laminated to the vertical webs to create a truss of sorts. You will have eccentricities with that detail, but practical solutions will lead to more work long run. We do not typically get direct feedback on our designs, but you can be certain the framing contractor will make comments to the owners, or the GC about time consuming details.
For a $300 fee I hope you are working towards solutions that are in the realm of 2 - 5hr. You need a salary and must use a multiplier on that rate to account for the fact you are the IT, PR, HR, and accounting guy (assuming sole practitioner).
RE: Gable End and Interior Shearwalls
The 2x4 blocking at the ceiling level seems pointless to me. The blocking at the roof diaphragm level would only seem appropriate if the shear was extreme, otherwise I would probably omit it or space it at 48" o/c.
After reading through Malone's book the thing that stood out to me was the necessity of making sure the deflection of the truss under gravity (live loads: ie. snow) loads does not compromise the lateral bracing.
A good case in point is the three configurations given below when the trusses do not line up with the shearwall. Both congfig A and config B will have problems from vertical loads and the blocking will probably be loaded beyond allowables both in bending and in shear or as in config B the blocking will be in cross grain bending or prying. Config C would be the best method to handle this situation, which allows for the vertical deflection of the trusses under gravity loads.
A confused student is a good student.
RE: Gable End and Interior Shearwalls
What I find most interesting about this detail is the continuous 2x collector that runs the full length of the diaphragm with blocking. This seems like a lot more effort for the framers but without this collector the diaphragm boundary is not compliant with the IBC.
A confused student is a good student.
RE: Gable End and Interior Shearwalls
1) I wouldn't sheath interior trusses unless you really don't trust the designer. It's costly and the truss itself should be easily capable if transferring the shear from the roof deck to the shear wall. Additionally, sheathing the trusses means interference with MEP runs.
2) I doubt the robustness of Malone configuration C to deal with upward and downward truss movement. Unless the shear wall is auspiciously centred between trusses, I suspect that there's still potential to snap that piece of plywood. I wonder if one could do something more flexible and ductile with sheet steel instead.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Gable End and Interior Shearwalls
I actually just finished helping my brother wire his house this summer. The structure is 52' x 36' with one interior shearwall parallel to the attic trusses. The sheathing extends from the interior stemwall all the way up to the ridge (12/12 attic trussed roof). I ended up boring about three 2" diameter holes through the shearwall sheathing up in the attic for the electrical work as well a number of smaller holes down lower. There was also holes for HVAC and plumbing interspersed throughout. None of this seemed too problematic, a cordless drill or sawzall makes quick work of 7/16 OSB.
With regards to truss deflection I suspect that the plywood could deflect a full inch before ultimate failure becomes a possibility. I wonder if OSB would behave better in this regard than plywood. Since you actually want the sheathing to bend easily in this situation I think it would be best to orient it along its weak axis. I might have to get into my garage and cut a 12" strip and support it at 24" o/c and load it in the center until it snaps, measuring the max. deflection just before failure. This would be an interesting imperial test of this configuration.
Assuming the sheathing bends in a arc like fashion a deflection of 1" vertically will result in a horizontal displacement of less than 1/16" for both bottom chords of the trusses, this should be acceptable.
The problem with Config A and B is the non-compliance of the framing and nails.
Sheet steel would probably work better than these two configurations however I wonder how well it would perform with the lateral loads. Would it tend to buckle or dish. Even worse would it make noise under high wind load that was cyclic in nature. If the shearwall was off center and quite close to one of the trusses I could see a much better argument for the sheet steel.
A confused student is a good student.
RE: Gable End and Interior Shearwalls
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Gable End and Interior Shearwalls
A confused student is a good student.
RE: Gable End and Interior Shearwalls
Interior Shearwalls
A confused student is a good student.
RE: Gable End and Interior Shearwalls
RE: Gable End and Interior Shearwalls
I worry about the stability of the truss to wall connection in detail three. Does the ceiling drywall provide sufficient lateral restraint? Maybe one of our resident wood-o-philes can comment on standard 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: Gable End and Interior Shearwalls
RE: Gable End and Interior Shearwalls
CBSE I like the mfg. truss block method in your attached file however if the roof is sloping that would complicate it slightly with different sizes of truss blocks, but the truss plant should have no problem in making them up correctly.
I also noticed you have toeinails as well as A35 clips into the top plate of the shearwall, normally I would do one or the other but I could see the redundancy in this connection.
I have never been a big fan of diagonal kickers but I suppose they are actually a better way to brace the bottom chord than even the horizontal blocking prescribed in this detail:
A confused student is a good student.
RE: Gable End and Interior Shearwalls
PDF and AutoCAD
A confused student is a good student.
RE: Gable End and Interior Shearwalls
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: Gable End and Interior Shearwalls
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.
RE: Gable End and Interior Shearwalls
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.
RE: Gable End and Interior Shearwalls
EIT
www.HowToEngineer.com
RE: Gable End and Interior Shearwalls
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.
RE: Gable End and Interior Shearwalls
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.
RE: Gable End and Interior Shearwalls
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/%28ASCE%291...
- Wind Engineering Conference: http://www.engr.uconn.edu/~wzhang/Pdf/C201306ACWE1...
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.
RE: Gable End and Interior Shearwalls
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.
RE: Gable End and Interior Shearwalls
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.
RE: Gable End and Interior Shearwalls
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.
RE: Gable End and Interior Shearwalls
A confused student is a good student.
RE: Gable End and Interior Shearwalls
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.
RE: Gable End and Interior Shearwalls
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.
RE: Gable End and Interior Shearwalls
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).
RE: Gable End and Interior Shearwalls
RE: Gable End and Interior Shearwalls
RE: Gable End and Interior Shearwalls
RE: Gable End and Interior Shearwalls
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: Gable End and Interior Shearwalls
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.
RE: Gable End and Interior Shearwalls
Hence, in all cases simply call out a drag truss that is inline with the shearwall. If this requires an additional truss I don't think the expense will be too great and it is the safe bet.
I'm liking the blocking at the ceiling level more and more. If the drag truss is load in compression it will tend to buckle at the bottom chord, gypsum will provide some bracing but is it enough? I agree with Jayrod12 that for a lightly loaded shearwall Detail 1 is probably adequate however the safe bet once again is Detail 3.
I think Malone's solution to the configuration in Detail 5 and 6 is good in theory but the lack of a really solid connection there just doesn't sit well with me. I don't think the plywood or OSB would snap provided the wall was centered between the trusses however if there was significant deflection, let's say an 1.0-1.5 inches, then the fasteners connecting the plywood to the truss would be pried loose both laterally and in withdrawal. Over the life of the structure they could potentially we worked loose enough that there capacity to resist the lateral loads could then become questionable.
A confused student is a good student.
RE: Gable End and Interior Shearwalls
What's the Malone book you are referring to? Is it the one on irregular buildings? Is it a good reference, have been considering purchasing it. Would appreciate some feedback.
Also you may want to consider changing (Mechanical) in your handle.
RE: Gable End and Interior Shearwalls
Yes, The Analysis of Irregular Shaped Structures by Terry Malone and Robert Rice. It is quite technical though and not as well written and as easy to follow as Breyer's book. However, it does fill in some gaps that are otherwise missed in more basic texts. I've had a bit of a hard time justifying the price tag, but so far it has helped me with interior shearwalls and portal frames.
I wish there were more texts like this that really delve into the nitty gritty of "real" wood design. Most residential designs are not your run of the mill rectangular floor plan anymore.
How do I change the (Mechanical) in my handle?
A confused student is a good student.
RE: Gable End and Interior Shearwalls
I would second the Malone book and yeah it is a bit pricey. Also the title should be Wood diaphragms and shear walls... It is good but almost too good where you are left wondering, does anyone actually do this? and How are they getting paid enough to have the time to do so? But at least it sheds light on some gray areas.
EIT
www.HowToEngineer.com
RE: Gable End and Interior Shearwalls
A confused student is a good student.
RE: Gable End and Interior Shearwalls
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: Gable End and Interior Shearwalls
RE: Gable End and Interior Shearwalls
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: Gable End and Interior Shearwalls
EIT
www.HowToEngineer.com
RE: Gable End and Interior Shearwalls
A confused student is a good student.
RE: Gable End and Interior Shearwalls
Call up the simpson guys and tell them you'll sell them the concept for a nominal fee.
RE: Gable End and Interior Shearwalls
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Gable End and Interior Shearwalls
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: Gable End and Interior Shearwalls
RE: Gable End and Interior Shearwalls
I searched around as I was certain I have seen something along these lines, but I did not find anything. I too think you should float the idea to Simpson.
RE: Gable End and Interior Shearwalls
A confused student is a good student.
RE: Gable End and Interior Shearwalls
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Gable End and Interior Shearwalls
http://design.medeek.com/resources/shearwall/SLOPE...
http://design.medeek.com/resources/shearwall/SLOPE...
I just recently ran into another shearwall on a balloon framed gable end, as they say necessity is the mother of invention.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com