torsion in a C-shaped building

[AnimusVox]

Hello & happy St. Patrick's Day everyone -

I'm dealing with a C-shaped building (at least that's the orientation of the shear walls - the fourth wall is essentially a curtain wall with pilasters that's assumed to not take any lateral load). In school I was taught that torsion is the bane of all C-shaped things, so I was leery when I realized there will be induced torsion due to lateral wind load, and in this case the basic wind speed is 133 mph.

My coworker assured me that he's dealt with a very similar circumstance and considered the side walls to act as a couple moment to deal with the torsion, however. See the attached sketch to illustrate what I'm talking about. (Sorry I didn't have a straight edge available KootK. I'll be better in the future.)

Some relevant info - 1 story structure, LFRS are int. reinf. 8" CMU, flexible roof diaphragm supported by bar joists, SDC A.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1458249956/tips/201603171603_udwxwf.pdf[/url]

edit #1: that should be vL^2/2B for the shear in the east/west walls.

(P.S., how do you include an image in the thread itself instead of just linking one? Thanks!)

edit #2: Thanks a bunch for all of the input, references, insight and opinions everyone! You've all thoroughly and collectively answered my questions.

 

[mike20793]

It depends on the length of the cantilever and really depends on the diaphragm. For this case, I believe you would need to have a rigid diaphragm or use a semi-rigid diaphragm analysis. I have handled this similar to your coworker and it hasn't been an issue, as long as you make the right assumptions regarding diaphragm and detail the load to get to the right place.

 

[haynewp]

I haven't seen warping considered for channel shaped buildings with shear walls. To me, the web and flanges that make up channels are more flexible and torsion on a channel member is different than for single story CMU wall buildings and torsion is not so much of an issue for the walls. I would go with the method you have shown unless the dimensions of the "flexible" diaphragm are too much for the cantilever action.

 

[hokie66]

I'm not a believer in channel shaped lateral load resisting elements. I always like to provide a rigid frame across that open side. You need some structure across there anyway, so why not make the frame rigid?

 

[JoshPlumSE]

Note:
This isn't your case, but this is no longer allowed in higher seismic zones. You might want to understand some of the failures that occurred in Northridge just to gain some perspective on the system. Might mean that you take a closer look at the deflections that occur for any gravity only columns along the open side of the building.

 

[wannabeSE]

A couple years ago, there was a similar discussion on Eng-Tips

http://www.eng-tips.com/viewthread.cfm?qid=360085

 

[mike20793]

The SDPWS allow for wood open front structures if certain criteria are met. Sometimes in wood buildings you have no choice but to do this, prohibited in height seismic zones of course.

 

[KootK]

Sorry I didn't have a straight edge available KootK. I'll be better in the future.

Please, that was some fine freehand sketching. We've certainly seen worse.

P.S., how do you include an image in the thread itself instead of just linking one? Thanks!

I do a lot of this and like to think that I'm quite adept now. Instructions:

1) Get the pic up on your computer screen.
2) Use the "snipping tool" app in windows and take a snip around your pic.
3) Copy the snip someplace on your hard drive.
4) Use the "upload image" feature of the forum and follow the instructions.

Step two is optional. However, through trial and error, I've discovered that this step fixes the problem that some members have where embedded graphics cause the thread to blow up with regard to screen width.

I'd recommend checking out this thread for additional background on three sided buildings. It's the most comprehensive that I can remember and contains some very interesting viewpointsLink. The more salient points there, as here, are:

1) Think hard about whether or not you want to do this in seismic areas.
2) Pay extra attention to the drift at the open end of the building and the impact that will have on cladding and gravity only framing.
3) Pay extra attention to your diaphragm and chord detailing.

For this case, I believe you would need to have a rigid diaphragm or use a semi-rigid diaphragm analysis.

I believe nearly the opposite (I think). A cantilevered diaphragm is one of the few instances where you will get a rigid diaphragm shear wall load distribution regardless of the actual degree of diaphragm rigidity. It may be that we're actually saying the same thing here, just in different ways. I certainly agree that from a software modelling perspective, the diaphragm must be assigned some rigidity in order for things to pan out.

See the attached sketch to illustrate what I'm talking about. (Sorry I didn't have a straight edge available KootK. I'll be better in the future.)

I agree with what you and your colleague have proposed. Essentially, you're treating the walls as individual linear elements rather than as a composite shape. And that actually makes pretty good theoretical sense. The flexibility of the shear wall system can be envisioned as having two discrete components.

1) A flexural component. See detail #1 below.
2) A shear component. See detail #2 below.

For a one story building of the proportions that you've shown, I would expect the shear component to do dominate by a significant margin. As such, treating the walls as individual elements is surely more accurate than treating them as a composite section.

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.

 

[hokie66]

wannabeSE and KootK have given the same link, and I agree that extensive opinions are given there. Nuff said.