Torsional Moment Loads from ASCE 7 Wind

[TechPoly]

I am working on a light-framed, 2 and 3 story residence, using the analytical method of the ACSE 7-05 Standard. Does anyone know how to transfer the torsional moment in wind load cases 2 and 4 into the shearwall lines?

The ASCE is somewhat vague about this and other books I have obtained seem to skirt the issue also. Your help would be much appreciated, especially a practical example. I am heading home for the evening, but I am hoping for a response tomorrow after a lot of research. Thanks.

 

[msquared48]

In looking at figure 6-10, and the diagrams for cases 2 & 4 on page 6-9, the induced torsion would be resisted by the transverse endwalls, or the exterior walls normal to the applied force. Personally, I design the shearwalls parallel to the force for these corner forces on both the corners at the same time, as the application of the force on only one side does not seem logical to me. Conservative? Perhaps.

I cannot see that the likelihood of the extra torsional force would exceed a similar application of force in the normal direction.

Are you sure the structure could not be categorized as Method I? Method II is a pain in the *&^%$#!

Mike McCann
McCann Engineering

 

[UcfSE]

It comes down to rigid diaphragm or flexible diaphragm. Apply your loads to the diaphragm at the center of force and find the distribution into each shear wall.

 

[TechPoly]

The structure is defined by ASCE as a rigid structure based on period so we are using Figure 6-9 or 6-10, but the diaphragms are considered flexible. The new codes seem to be putting you into a classification without thought to all the implications of the classification name.

The ASCE 7-05 exepts one and two-family residential buildings of light frame construction while the 2006 IBC exempts nearly all diaphragms of light-frame construction from a rigid or simi-rigid structural analysis (IBC 1613.6.1).

So I am not sure if your post UcfSE is appropriate. I would like to be convinced if you know more than I do.

Thank you msquared48, method 2 is a bear! This structure just isn't close to the regular structure as defined by the ASCE. that the simpified method, method 1, would require.

The ASCE goes on to talk about flexible structures, but we don't meet the definition of flexible stuctures according to the building period. None of the reference books I have purchased so far give any good examples of unusually shapped buildings (other than minor ones).

I have taken your advice, msquared48, by using figure 6-10 for low-rise buildings, but the lack of examples are the same. I could use any reference help or ideas for working with method 2, 6.5.12.2.2, and figure 6-10, especially for unusually shapped buildings.

Again thanks for any help.

 

[OldPaperMaker]

TechPoly,
The Structural Engineers Association of Washington (SEAW) & the Applied Technology Council (ATC) have a three volume set that includes a commentary on the wind code provisions (volume 1) and 8, very detailed example problems (volume 2) and the third book is the SEAW's handbook of a Repid-Solutions Methodology for Wind Design (SEAW RSM-03).This is basically a graphical solution to Method 2.

SEAW RSM-03 is based on ASCE-02. I don't think that it has been revised for ASCE 7-05. You can find information about the books at:

http://www.ATCouncil.org

 

[UcfSE]

I don't have to convince you. If you knew more than me about this you wouldn't have had to post a thread about a basic subject.

You asked how to get wind load into a shear wall. I gave you the basic concept you need to figure out how to load a shear wall based on the loads to the diaphragm. It is up to you to research the concepts and learn the principles; it is not up to me or anyone else to teach you.

 

[TechPoly]

You are correct UcfSE, I am not looking to be taught, I have spent another day researching and teaching myself. I appoligize if it seemed as if I knew more than you, it took all day to try to sound like I [u}have[/u} done my homework.
You were very helpful (I clicked on the thank you button for your valuable post) and I didn't want to disrespect that.

I think OldPaperMaker is leading me toward what I am looking for. I can do a lot of calculations, but without some kind of verification I am on the right tract, such as an example, I could be flopping like a fish out of water on this one. This new IBC code after over 25 years with the UBC has me a bit flummoxed. I thank you both for your help.

 

[cap4000]

TechPoly

Better late than never.The 2006 book "Design of Buildings and Bridges for Wind" authored by Simiu and Miyata do a real nice job with a torosional moment calculation. Pages 97 and 98 are excellent.