Wall of Windows (Part II)

[medeek]

The previous thread on window walls has been closed so I've had to create a new thread to continue this discussion.

Apparently, these type of tall walls full of windows is the norm around here and not the exception (Ocean Shores Washington, beach homes). My most recent project involves a tall wall with windows as shown below:

I immediately went with a 2x8 stud wall due to the height and figured PSL columns in the wall should be the solution. After running the numbers I tentatively arrived at a 5-1/4"x7" PSL column for P4. The column in question, and the most critical column, is between both columns of windows and is approximately 22' tall. As far as strength I am good (40% in bending and 41% combined bending/axial), my concern is regarding the deflection at this column (midspan). According to my calculations using Forte I get a deflection of L/192 or approximately 1.38" at midspan. Typically I don't like to see a deflection over 0.5" (my hard deflection limit) but maybe this limit is unrealistic given the parameters and height of this wall.

Per the IBC/IRC deflection limits I should be okay (L/180 for gypsum walls) but a wall flexing in and out more than an inch seems highly problematic to me. I really don't want to go to a 2x10 stud wall but that may be my only option as I've pretty much reached the upper limit of wood performance with the PSL columns.

Has anyone integrated tried integrating steel columns into these type of walls?

Does anyone have any good resources on tall wall design and other tricks of the trade related to tall columns in walls that are controlled by deflections?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[XR250]

The steel wouldn't help as much for outward pressures, but those are "usually" significantly lower than inward forces

Hmm, according to ASCE 7, the C&C external pressure coefficients are larger for outward loads than inward loads.

 

[medeek]

I am tending to agree with Brad805, the structure will in no way fail and as KootK noted the curvature is probably more the issue than the absolute deflection. A 7"x7" PSL column buys me another 3/8" of deflection:

I'm not sure if its worth it but the L/256 does look a lot better.

My real concern (valid or not) is the differential deflection between the columns. Due to the nature of the geometry of the V-shaped wall the center columns (P3) will be lateral braced against each other and their deflection will probably be quite a bit less than 0.5 inches even though my initial calcs showed them deflecting 1.16" (unbraced from each other), the P5 columns also came in at about 0.56" of deflection due to a lower column height and smaller trib. area. With the other columns on each sides of the windows only deflecting about 1/2 and inch and the column between the windows potentially deflecting a full inch how does the whole wall hang together and will the windows break or pop out?

Hence the strapping at the headers.

My thinking is that the whole wall will act like a large web to some extent and header straps will load up in tension with a large lateral wind load, which will help distribute the load and deflection into the stiffer columns from P4. I'm not sure if one can quantify this "web" action and if it really makes much difference to the overall differential deflections among the columns.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

Your concern regarding differential deflection is exactly why I suggested eliminating the intermediate columns and doubling up the end columns. You get some of the header continuity that I mentioned above. I think that it would create a more beneficial arrangement for the windows. More of that "web" behavior that we're gunning for.

This is probably KootK paranoia showing but I worry that, under some oblique wind angle gusting, you might actually get one post moving inwards and its neighbor moving outwards. If that came to pass, it would be the worst case scenario by far.


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.

 

[medeek]

After a quick check in AutoCAD I might be overly worried here. The stretch of the windows with the above mentioned differential deflection is only 0.009", approximately 1/128 of an inch, I think we can manage with that.

The wall will go from flat (180 deg.) to 177.6 deg kink at the P4 column, will the drywall crease at this line?



A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

I would have liked to post this on the previous thread but it is closed. Here is the one wall of windows finished out. I think I would have done something a little more exciting with the trim around the windows but I did not do the design on this house.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com