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]

Did you (or forte) take a 30% reduction in deflection for wind?
I have used tube steel or flitch plates in these before and it worked out nicely. I went to 2x10's on a recent one as it peaked at 29'

 

[Canuck67]

Medeek,

I am not sure your spacing for your 2x8 studs, however, if at 16" o/c, this could be reduced to 12" o/c, or I have also used 2 ply studs @ 16" o/c or 2 ply studs @ 12" o/c.

While I have used steel columns in the past, I am hesitant to do this unless you can get sufficient insulation all around the column. A cold column within a wall space can end up with large amounts of condensation within the wall cavity.

I would try to limit my deflection to a maximum of 1", I have done this in the past. Double studs may very well help in this matter. Consider deflection criteria for interior finishes (drywall, etc..), but also keep in mind that exterior finishes also can have some more stringent deflection criteria (i.e. brick, or in some cases, one of the more popular exterior stucco/insulation systems requires l/600).

 

[manstrom]

I would use l/240, but you can get away with l/180 if it is just gyp. There a bend in plan at that wall so that will stiffen it up. A few more tricks: you can reduce C&C loading by 0.7 when calculating deflection. Also you do get partial composite action from osb. If you do the math, it makes a big difference. 1/2" deflection over 22' is no biggie. I wouldn't worry about it.



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

 

[KootK]

I'd be willing to relax the absolute deflection limit. For lateral deflection of walls, I really do feel that curvature is the parameter of interest. Although I suppose that occupant comfort / terror factors in too.

It's a shame that there isn't a simple way to get flexural continuity across your horizontal members. The L/180 limit doesn't bother me much so long as all of the posts are travelling in the same direction at the same time with little differential movement.

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.

 

[BadgerPE]

One additional consideration is window deflection. In this case, it is probably not an issue due to the smaller window dimensions so less differential deflection across each window. I did a wall a long time ago the windows were 12' tall and 4' wide. I believe the deflection criteria on those windows were L/360 or < 1/2".

I have run into tall column issues before and being in the upper Midwest, thermal transfer is always a concern. Usually we can take it by increasing column size, but I just had this thought pop into my head. Maybe it would be worth exploring. The steel wouldn't help as much for outward pressures, but those are "usually" significantly lower than inward forces. The wood "blocking" would be designed to take outward pressures and would be trimmed down an amount to allow the steel to be recessed into the wall system a small amount.

Thoughts on this "idea"?

 

[XR250]

@Canuck;

Does IRC specify L/600 for brick w/ wood stud backup? This is common for metal stud backup, but wood is typically L/240 min as I recall (brittle finish). I don't think any of the stud-height tables in the IRC would actually work if it was L/600.

 

[KootK]

@BadgerPE: I think that your idea slick! Why wouldn't it help for outward pressures? The moment of inertial of the assembly would be the same in both directions would it not?

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.

 

[Canuck67]

XRC,

Code in Canada (Ontario) calls for l/180 for drywall and exterior finishes not susceptible to cracking, and l/360 for brick/stone veneer on wood studs.

However, beyond Code, there are exterior stucco systems that specifically call for l/600 deflection limits in their product literature.

 

[jayrod12]

I also agree that BadgerPE's detail is a great idea.

That is getting downloaded into the vault. There are always times to use something like that.

 

[BadgerPE]

@KootK: I was thinking more for strength provisions, but for deflection I agree with you regarding the moment of inertia.

 

[jayrod12]

Strength rarely governs wood walls and columns for this type of application. It's normally a serviceability failure.

 

[KootK]

@BadgerPE: I would think that the WT would be stronger for outward pressure as that would put the stem in tension. I imagine that we'd be uncomfortable using the wood blocking to brace the stem against buckling when in compression.

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.

 

[BadgerPE]

@KootK: Good point.

 

[manstrom]

Good article on wall deflection.

http://www.structuremag.org/wp-content/uploads/2014/08/C-CodesandStand-Sprague-May081.pdf

My understanding is that it's L/600 for CFS only - the wood guys made it that way to keep their competitor down

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

 

[jayrod12]

KootK and Badger,

Why would you be uncomfortable using the wood blocking to brace the stem?

If you were to enact a bit of the sheathing through nails into adjacent studs and into the blocking I don't see why it can't provide the necessary bracing force. At least to account for some lower unbraced length.

 

[KootK]

@Jayrod: I rescind my concern. I was thinking of local compression plate buckling of the stem tip. b/t ratio stuff will take care of that. I agree with considering the tee braced for LTB/column buckling.

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]

Just to clarify the critical column is the one indicated below with its trib. area (wind load).

I could always go to a 7"x7" PSL.

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

http://www.medeek.com

 

[KootK]

Perhaps you could eliminate the column and double up the one to the right which is going to be buried in the wall.

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.

 

[Brad805]

Go with a 7x7 and move on with life. You are not getting paid enough on these jobs. Wood framers usually hate steel, and they would rather alter the width of your shearwalls to fit the windows the owner wants if that is an option.