Skinny Window Mullion 1

[XR250]

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!

 

[KootK]

- 2% seems perfectly reasonable.

- 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.

 

[DETstru]

I would consider the 2x6 to brace the whole thing. The vertical LVL doesn't really have "flanges."
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.

 

[DETstru]

Also... where can I find these "really no budget" projects...?!?!

 

[OldBldgGuy]

I'm not going to comment directly on the structure because it might work, but FLW was possibly the worst building scientist ever born. He had no sense at all of what worked, what caused short term issues, and what caused long term issues. I have not been involved directly in any of his architecturally wonderful and otherwise disastrous projects, probably because I'm on the wrong side of the border, but I know and have worked with the architects responsible for the Darwin Martin House restoration in Buffalo, and I've been to his other Buffalo buildings, including the FLW designed but only recently built Buffalo boathouse for the rowing club. The man was a disaster. No drips, no dampproof courses, no respect for weatherability or frost penetration, windows that I wouldn't put in a partition never mind an exterior wall, you name it. His masonry might as well be sent straight to the compost heap, because it's getting there sooner or later. His buildings look amazing and timeless, & my best friend is an absolute devotee (which is why I've been dragged to all of these buildings) but timeless they're not & I would trust his detail about as far as I could send it as a paper airplane.

 

[dik]

Many of FLW's buildings had problems... he might find someone else to emulate...

Dik

 

[FloydLloyd]

When you say flanges, I guess you're referring to the compression side of the N.A. 2x6 is fine to brace the 9.25" compression member, you might check torsion but its not an issue.
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

 

[XR250]

Thanks for the responses. Yes, my impression of FLW's structures are they are sketchy from an engineering and building science perspective. I have shared this with the Arch. on many occasions but he still loves 'em.

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 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.

 

[FloydLloyd]

I didn't check available Parallam widths.
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!