Curtain Wall deflection limits 1

[lanejoe10]

IBC section 2403.3 limits the deflection perpendicular to an individual glass lite to l/175 or 3/4" whichever is less.

I have a 35ft tall mullion, the first like is 15ft tall, the 2nd lite is 8ft tall and the 3rd lite makes up the remainder. I will limit my mullion deflection to l/240+1/4".

How do i check the deflection of the 1st lite to make sure it meets IBC section 2403.3? Seems that the 1st lite is almost a cantilever beam, so would I want to limit the deflection at 15ft to 2*15ft/175 or 2*3/4"?

If anyone can shed any lite (light) on this, that would be greatly appreciated.

Also, AAMA TIR-A11-15 states:
"The 19 mm (3/4 in) framing deflection limitation is then only appropriate for single lite high applications. For members supporting multiple lites, a 19 mm (3/4 in) limit places unwarranted demands on taller framing systems which may result in increased sitelines (framing widths), depths or reinforcing requirements."

This almost seems like the requirement doesn't apply to mullions that are supporting multiple lites like I have.

 

[KootK]

1) figure out the deflected shape of the mullion.

2) for teach lite, draw a straight chord on the deflection curve connecting the top and bottom of each lite.

3) Apply the limits to the relative deflection between the chords and the deflection curve.

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]

I feel that CW deflection should also be required to match the surrounding metal stud deflection (if applicable). The deflection limits for the wall studs are typically more stringent than the CW deflection (say L/600 for brick). If the curtainwall is attached to the adjacent jamb which was designed assuming a vertically spanning curtainwall, then the stud will either be forced into a greater deflection or overstressed.

 

[lanejoe10]

KootK-

Thanks for your response, I had looked at that approach and it seems when you analyze it that way the 3/4" deflection of an individual lite usually never controls the design. I assume there could be a case where it would but most of the time it seems that it wouldn't - that is why I was saying that AAMA TIR-A11 almost makes it seem like the 3/4" deflection requirement doesn't apply to mullions supporting multiple lites.

XR250-
The only connection we make between our jamb mullions and jamb studs is a silicone joint. We make sure that the silicone joint can handle the deflection assuming the stud does not deflect. So this way we are not loading the jamb studs. Usually we limit this deflection in the mullion to 1/2 the silicone joint width depending on the silicone that is being used.

 

[XR250]

The only connection we make between our jamb mullions and jamb studs is a silicone joint. We make sure that the silicone joint can handle the deflection assuming the stud does not deflect. So this way we are not loading the jamb studs. Usually we limit this deflection in the mullion to 1/2 the silicone joint width depending on the silicone that is being used.

That is good to know. Must be some flexible silicone on the taller walls

 

[KootK]

The only connection we make between our jamb mullions and jamb studs is a silicone joint. We make sure that the silicone joint can handle the deflection assuming the stud does not deflect. So this way we are not loading the jamb studs. Usually we limit this deflection in the mullion to 1/2 the silicone joint width depending on the silicone that is being used.

I question this. Firstly, wouldn't that mean that you're limiting mullion deflection to something like 3/8"? As XR alluded, that's going to be one heck of a stiff mullion for anything at all tall. Secondly, just because the mullion could move relative to the jamb studs without failing the joint, that doesn't mean that the studs won't pick up load. When the silicone stretches out, one end of it will be yanking on the studs. You may reduce the load sharing but you certainly won't eliminate it.

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.

 

[lanejoe10]

Yes, 3/8" is typically what we would limit the deflection to or we would change silicone to use one that would allow us to limit the deflection to equal to the joint width or 3/4".

Yes, there probably is some load sharing going on there with the silicone pulling on the jamb stud as the mullion deflects. But that is one reason to make the mullion stiff and limit it to 3/8" deflection. The designer of the studs should design the stud to take some load as well. When we provide design on the studs we assume that it is taking the same load as the mullion which is conservative but usually not that big of a deal.

 

[KootK]

Thanks for the clarification lanejoe.

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.