Cold Formed Backup Wall Design

[bookowski]

I'm giving the architect estimate of size/gage for CF studs in a backup wall, eventually to be designed by others. The floor to floor is almost 15ft, so clear span of the studs is a little over 14ft. The jambs are about every 8.5ft on center.

- When looking at load path should I assume windows span vertically when sizing header and span horizontally when sizing jamb? i.e. take most conservative load path for each.
- At 14+ft the jambs appear to be very heavy. At what ga does it make sense to go to structural steel (say hss).

 

[XR250]

I do the same as far as load paths go. Unless the windows are really tall, I always size the jambs using a distributed horizontal load based on the 1/2(center to center spacing + 16" to the next stud.).
You should be able to do "heavy" light gage for the jambs on something like that as long as the wind speed is not too high. I always try to do a single, heavy gage jamb rather than having them build up multiple studs and dealing with the eccentricity. 12ga or even 10ga is certainly reasonable if you want to stay out of structural steel.

 

[bookowski]

Thanks XR. I think I'm stuck with built up - at least based on my initial back of envelope calcs and based on an 8" stud. The facade is large format terra cotta so tight deflections.

 

[KootK]

I'll do a concrete slab style load distribution if the windows are inoperable. Not that it makes a huge difference or anything.



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.

 

[XR250]

What is your service wind pressure and deflection requirements?

 

[DETstru]

I typically assume the glass is vertically spanning for both the header/sill and jamb design unless the aspect ratio of the glass pane is greater than +/-1.5 vert to 1 horiz. Nearly all windows end up being vertically spanning this way.

 

[KootK]

Nearly all windows end up being vertically spanning this way.

I'm curious, what makes you say this? Certainly, I agree for most operable windows. I've often wondered how window suppliers themselves see their windows spanning.

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.

 

[jayrod12]

I've had it explained to me that most window manufacturer's assume a load distribution similar to a two way slab as Koot had suggested. Unless it's operable, then it's spanning from the hinge side to the opposite side.

 

[DETstru]

Most of the projects I've worked on usually have windows much wider than they are tall. That means the glass transfers wind load to the head and sill. You'll see long strips of windows (with vertical mullions) in office buildings, educational buildings, healthcare & MOB facilities, ect, which send the load up/down to the head/sill.

http://imgur.com/a/JjYBC

I sat through a lunch & learn a while back with a glazing guy who told us glass panes typically distribute load in a two-way fashion until the aspect ratio hits about 1.5:1. Beyond that it's still not a perfect one-way distribution because the corners are still anchored. Since it's less effort to distribute one-way, I've stuck with it for simplicity unless I believe it's clearly an incorrect assumption. I don't have a source on this but it was a local curtain wall rep and I was more interested in the free lunch...

jayrod12,
There can easily be a difference in what the window manufacturers assume and what we design for. If you have a glazing system that has square panes, each pane will distribute load in two directions, no argument there. But if this load is ultimately sent into vertical mullions, it'll end up in the head and sill, which is what matters to us in terms of design.

 

[jayrod12]

Agreed there.

I always just worst case everything in my design.

 

[KootK]

My guess is that, when windows are developed, they're developed based on four sided rigid support. Given that the wind posts are further down the load chain than the headers, I suspect that the posts tend to be a bit stiffer than the headers and thus a bit more load is probably distributed horizontally (inoperable). This is just hair splitting of course. One way, any way, is fine by me for routine design.

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.