Skinny Window Mullion
Skinny Window Mullion
(OP)
Working a "new" Frank Lloyd Wright house. The Arch. is a devout follower so he is trying to exactly duplicate many of his details. On this house, I have a 14 ft. tall exterior window wall. The Arch. wants to frame it with a single 1 3/4x9 1/4 LVL vertical mullion spaced 48" o.c. between the windows. The axial load is 1,400 lbs and the bending moment is 1,700 ft-lbs (service). There is a horizontal 2x6 mullion at 8'-6" AFF. If I consider the LVL braced at 8'-6", the numbers actually work out. The wall contains eight studs so the cumulative bracing force is 2% x 1,400 x 8 = 224 lbs (I am assuming using 2% is reasonable for this situation?). The studs are flanked by a masonry wall at each end so resolving the brace force should not be too much of an issue.
So here is my question:
The 2x6 mullion is going to be flush with the inside face of the wall so it technically won't be bracing one flange of the LVL. I can likely talk him into a 2x8 instead of a 2x6. Also, there will be full 1x trim on all faces of the LVL which I am not taking into account. How do y'all feel about this situation?
There is really no budget on this job so I am pretty free to come up with other more expensive solutions. I am open to suggestions.
Thanks!
So here is my question:
The 2x6 mullion is going to be flush with the inside face of the wall so it technically won't be bracing one flange of the LVL. I can likely talk him into a 2x8 instead of a 2x6. Also, there will be full 1x trim on all faces of the LVL which I am not taking into account. How do y'all feel about this situation?
There is really no budget on this job so I am pretty free to come up with other more expensive solutions. I am open to suggestions.
Thanks!






RE: Skinny Window Mullion
- if the 2x6 restrains 5.5" of the 9.25" LVL, I could consider the outside edge of the LVL to be braced by virtue of overall rotational restraint.
KootK approved.
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.
RE: Skinny Window Mullion
Good enough in my book.
What's your horizontal deflection due to wind loads? Might be wise to check that with the eccentricity it creates in the axial load.
RE: Skinny Window Mullion
RE: Skinny Window Mullion
RE: Skinny Window Mullion
Dik
RE: Skinny Window Mullion
But your L/d exceeds 50 (NDS 3.7.1.4), so you need a 'fixed' or restrained end detail to reduce the effective length of your mullions. That may simply mean framing into the sides instead of top bearing, or setting the base into concrete or between some blocking, or maybe some steel clip angles cleverly concealed within the framing so they don't offend your Arch.
I would specify a Parallam for this axial and combined loading condition, not an LVL. Also, it sounds like exterior exposure, so Parallam (treated) is the way to go. Don't forget to reduce the E value for moisture.
FLW
RE: Skinny Window Mullion
I figure the inflection point in the buckled shape is below the brace so let's say 8 ft. I should be able to use a "k" of 0.7 because of this. So L/d = 96*0.7/1.75 = 38 so I feel ok about it.
Even if I use k= 1.0, it is 54 which I am still ok with due to the sketchy composite action of the trim. Do they make skinny parallams? I though 3 1/2" was the thinnest.
RE: Skinny Window Mullion
Depending on your overall column FBD, and loading, one could argue that you'll still have too long an effective length. And I agree the slenderness doesn't seem a problem, though I'd count on the primary loading being in the other axis, and the lack of sidesway in the column, rather than any fixity provided by the trim.
Still, there is the NDS limit of 50 for L/d, so it wouldn't hurt to have some blocking or other detail, in case the issue ever came up.
Post some pictures of the completed project!