Woodworks and Attic Trusses
Woodworks and Attic Trusses
(OP)
A couple simple questions with Woodworks software for all the Woodworks gurus.
1.) I have a small 12/12 gable roof framed with attic trusses. What is the best way to model this in the software in order to accurately model seimic dead loads from the attic floor space?
2.) Same roof line as above except now I've got a shed dormer (approx. 8ft high) on one side of the gable roof with a 3/12 pitch.
1.) I have a small 12/12 gable roof framed with attic trusses. What is the best way to model this in the software in order to accurately model seimic dead loads from the attic floor space?
2.) Same roof line as above except now I've got a shed dormer (approx. 8ft high) on one side of the gable roof with a 3/12 pitch.
A confused student is a good student.






RE: Woodworks and Attic Trusses
A confused student is a good student.
RE: Woodworks and Attic Trusses
A confused student is a good student.
RE: Woodworks and Attic Trusses
The addition of the dormers may not be signficant but I would like to take a serious look at this and see how much difference we are talking. Also related to this are the roof diaphragm loads. By creating large discontinuities in the roof diaphragm how does one rationally analyze the diaphragm.
With attic trusses the gable ends, when properly sheathed will act like drag trusses and I think one can safely assume that the gable shear walls can be regarded as normal.
A confused student is a good student.
RE: Woodworks and Attic Trusses
A confused student is a good student.
RE: Woodworks and Attic Trusses
Look at the interior shear wall line between the garage doors. A few problems:
1.) The rigidity of these smaller shear panels (9') will be significantly less than the two exterior walls (28') so they will attract less load. They won't be doing much in reality, are they worth all the trouble.
2.) The middle dormers fall right on top of them (ladder framing to girder trusses). There will not be a continuous load path to the roof diaphragm. Maybe solid blocking inline with the shearwall and between all of the ladder framing will help with this.
3.) A grade beam will need to tie the two halves together otherwise the overturning will exceed the foundation's capabilities at the int. shearwall line.
4.) The int. shear walls split up the garage space which doesn't sit well with the owner/occupant.
A confused student is a good student.
RE: Woodworks and Attic Trusses
RE: Woodworks and Attic Trusses
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: Woodworks and Attic Trusses
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: Woodworks and Attic Trusses
Personally, I try to do IBC (engineered) design for all of the houses I design. It makes more sense to me and I find it easier.
When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.
-R. Buckminster Fuller
RE: Woodworks and Attic Trusses
Unlike most roof diaphragms that I consider this one is slightly different with the addition of the attic floor sheathing. In a sense you have a triangular tube formed by the attic floor and roof sheathing combined with the attic trusses. The question really is how to determine the stiffness of this "modified diaphragm" and also the diaphragm shear loads. I would not expect this to be equivalent to a 4/12 pitch roof diaphragm with only roof sheathing.
If you disregard the IRC for a moment and look at the reasoning or need for the interior shearwall it becomes clear to me that the deflection of the diaphragm is the primary reason to include a shearwall at this central location. If the diaphram and hence wall deflection is too excessive at the mid point of the building then it definitely makes sense to provide a structural element to counteract this deflection. The loads to the two ext. shearwalls is really not a factor here since they are both long enough to provide plenty of shear stength while having manageable unit shear and holdown forces.
The interior shearwall transmits it loads to the attic floor diaphragm which is probably not as good as getting them directly to the roof diaphragm above but I think it will work in this case. To make this work I would call out solid blocking inline with the shearwall below with nailing at 4" o/c spacing. This blocking will fit between the attic floor ladder framing the full width of the building and also require a continuous strap on the underside to pickup the collector forces (tension).
The gable wall next to the stairwell should probably be balloon framed up to the roof rather than using a gable end attic truss as shown below:
The problem I see here is that there is a hinge point created with no lateral support from the floor diaphgram, but it is only about 60% of the building width so I may be splitting hairs but I thought I might point that out.
A confused student is a good student.
RE: Woodworks and Attic Trusses
I contacted Woodworks support and the provided me with their work around for dormers but it doesn't really work in my opinion. At this point I just have to sit down and run the numbers manually or program my own solution to the problem which is in the works but will take some time.
In fact, after giving it some serious consideration I would actually like to program an application similar to the Woodworks Shearwalls program but with a more flexible, updated and intuitive interface and the ability to handle dormers and attic trusses like this current problem, a bit ambitious, I know.
A confused student is a good student.
RE: Woodworks and Attic Trusses
A confused student is a good student.
RE: Woodworks and Attic Trusses
A confused student is a good student.
RE: Woodworks and Attic Trusses
Yet another problem with this design is the opening in the floor diaphragm at the stairwell on the north wall. The transfer diaphragm at the top of the stairs is going to be very heavily loaded.
Who knew residential structural engineering could be so complicated, I'm about ready to throw in the towel on this one.
A confused student is a good student.
RE: Woodworks and Attic Trusses
2) For short direction lateral, maybe it's best to use just the attic floor diaphragm. It should be pretty strong and stiff.
3) Wood lateral is the worst lateral in my opinion. We feel your pain. This, and the profit motive, are why we mostly fall into prescriptive design methods.
4) Every day I have to remind myself that my job isn't about "knowing" anything. Structural engineering is about the moderately intelligent proportioning of things, nothing loftier.
5) Take comfort in the fact that no one is doing this stuff any better than you. In fact, I guarantee that most people are being far less conscientious.
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.