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.

 

[RobertHale]

I agree with woodman88. Even if you assume a drag truss there will be additional overturning force simply because the lateral load is not imparted at the eves to the system as a whole. Personally, I used the midheight of the roof structure, I do not use dead load to offset the tension (I specify the nailing of stud groups so typically I have much greater capacity than I need for compression; my stud group thickness are governed by the minimum for the holdown) and I clearly spell out what the in-plane shear needs to be for drag truss. I can almost guarantee the truss manufacturer will not size the truss members for right load if you do not explicitly define the demand.

 

[KootK]

This has been an important and enlightening discussion for me. Thanks for getting it started Medeek.

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.

 

[medeek]

I'm glad I worked through this a bit last night. I feel a little more educated on the topic now. However, with so many differing opinions I'm not exactly sure how to design these shearwalls anymore.

A confused student is a good student.

 

[DaveAtkins]

Here is the way I have always done it:

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

 

[woodman88]

KootK

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.

 

[RFreund]

I think there are a few things going on here and maybe some talking past eachother, but lots of good information/ideas.

Take for instance the interior shearwall in the image at the top of the discussion. Based on standard approach this shearwall will see double the load that either exterior gable shearwall will see. Being that this wall is only a small segment of the entire wall length this is not realistic, the ext. shearwalls will take a larger fraction of the load. The question is how to determine what is appropriate.

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

http://www.HowToEngineer.com

 

[KootK]

@Woodman: thanks for the PDF. Here's one for you in case you want to know how to design wood buildings Canadian style: Link. You guys doing six story yet in AZ?



The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[woodman88]

RFreund
"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.

 

[woodman88]

KootK

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.

 

[RFreund]

Thanks woodman, I knew there had to be an answer of why we were ignoring it.

EIT

http://www.HowToEngineer.com

 

[KootK]

I thought it time for a summary sketch to ensure that I've really got the gist of this now. I went with a flat roof as the sloping diaphragm business detracts from the principle of most interest to me. Any objections?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[medeek]

Your statics looks solid to me:

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.

 

[medeek]

or the collector force?

A confused student is a good student.

 

[KootK]

Nice. Exactly the calcs that I would have provided were I more industrious. I say give the truss guys everything. It's just ink and, the more information we provide, the better the odds.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[Brad805]

Most truss companies delegate the connections for lateral forces to the EOR. The only number of interest to them is the total shear applied at the top chord. In my neck of the woods it is my job to decide how to connect their truss to the shear resisting element. They will eventually provide you a simple 2D shop drawing denoting the applied forces and their assumptions. This will be a standard output from a company like MiTec or another truss plate manuf. Truss engineers do not get paid a lot. Years ago it was something like $50 per truss. They are not interested in excess information. It is a cut throat industry and low price gets the job always.

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.

 

[medeek]

Since we are on the topic of interior shearwalls I've sketched a rough schematic of a typical residential interior shearwall situation that I commonly encounter in some form or another. I apologize that the drawing and dimensions are completely not to scale:

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.

 

[medeek]

As I mentioned, my putting Woodworks through its paces, is what got me started on this conversation. How exactly it was arriving at its numbers for shearwalls had me digging pretty deep through the documentation. Not to prove the software wrong but to understand what I had been doing different or possibly missing in my own manual analysis.

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.

 

[KootK]

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.

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.

 

[KootK]

@Brad805: you and woodman certainly are right in that documents like the WFCM are productivity gold mines. What would you consider to be the analog document in Canada?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[Brad805]

Koot, the CMHC had a framing book years ago, but I don't know if it has been updated.

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.