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Residential Window Walls

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medeek

medeek

Structural
Mar 16, 2013
1,104
Yet another window wall, these things seem to love me, I don't love them. A couple issues with this window wall:

WINDOW_WALL_fxprrr.jpg


1. I've got 24" of wall at the corners and about 18" between the central door/window and side windows. I've also got a 7' to 8' pony wall underneath this window wall. Obviously I can't get any traditional shearwalls into the main window wall so my first thought was to use Simpson's (Wood) Strong-Wall (catalog C-L-WSW16) product which can be field trimmed to match the pitch of the roof and also is available in mult-story kits. Looking at the literature on the product it appears that it is always intended to bear on a concrete foundation and not other wood structures or members, hence the need to possibly use a multi-story kit or just balloon frame all the way up from the foundation to the inclined double top plate (ie. use a 20' strongwall).

The other option possibly would be to ignore the window wall entirely (as a shearwall) and do a three wall analysis treating the diaphragm as rigid and ignoring the window wall shear resistance. Section 4.2.5.1.1 of the SDPWS 2008 appears to allow for this since the great room is 25' wide and 16' deep with an L/W ratio of 0.64 and the diaphragm length (L) is less than 25'. Has anyone ever used this methodology in this type of circumstance?

2. The second issue appears to be a design flaw. The ridge beam needs to be supported at the gable end with a small beam that spans the polygon window above the door. How does one fit a beam in the space provided? I'm thinking we will probably just need to drop the window height until an appropriate size header can be inserted.

For reference the site criteria on this job is:

Roof Snow Load: 50 psf
Wind Speed: 85 MPH (110 MPH Ult.)
Exp. C
SDS: .739

A confused student is a good student.
Nathaniel P. Wilkerson, PE
 
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Is there a way to eliminate the ridge beam via going the scissor truss route? Saves you having to fight with that one aspect.

As for the lateral, I think 3 sided may be the only way to go.
 
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If you need a ridge beam and support at the exterior wall you can weld a kinked steel beam together.

Scissor trusses cut the interior pitch of the ceiling a bit. You may be able to use a parallel chord truss if you can tolerate 14-16" of roof framing thickness.
 
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As I was jogging on the treadmill that thought came to me, support the ridge beam with a small parallel chord truss that then is hangered from the two PSL posts each side of the door.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
 
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You could also probably design the rim over the wall on either side of the ridge as cantilevered beams to support the ridge. It looks like you have reasonable back span to cantilever distances. You'd want to review the hanger closely as you'll be near the edge of the beams.
 
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I usually just do site built moment frames on these if I cannot get a three sided building to work.
That takes care of the lateral and support of the ridge.
If you did not have the lateral problem, I have supported ridges like that on cantilevered LVL's running up the rake. They are tied to the low corner and cantilever off the main window king studs.
 
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Another option with the ridge beam is to replace it with two beams -- one at each post location -- and cantilever / two span the roof joists over those. It may be an easy sell as, in my opinion, that's the traditional framing look for this arrangement anyhow.

That pony wall's going to be pretty robust. As long as the detailing is complete and you're not violating any code limitations, I'd have no problem using the Simpson wall on top of the wood pony wall. Alternately, if the cost is tolerable, you could do the pony wall in concrete.

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.
 
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Quite a few years ago, I ran into a similar construction, with the window wall vibrating during high wind gusting. Interior was not yet finished and the solution was to place 1 LVL (horizontal) above the door/lower window set header from wall end to wall end. As the LVL protruded into the room, beyond the final finishes, It was covered and turned into a high shelf.
 
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I like the idea of the site built moment frame (portal frame). I will sketch up a rough detail and post it. My concern is how to connect the ridge beam to the two cantilevered (glulam,LVL) beams at the ridge where they meet, never tried this before. Also the connection between the PSL posts at the sides of the door and the cantilevered beam that is now on an incline. I appreciate all of the input, I thought I was stuck but I can now see there might be a few ways to skin this cat.

On the previous most recent window wall I extended my PSL posts through the rim joist and double top plate of the pony wall to the concrete stemwall. Then applied a strap across the cut double top plate. Not sure if this is the optimal way to do that...

A confused student is a good student.
Nathaniel P. Wilkerson, PE
 
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Steel frame. We do it all the time.
The 3 sided box will not work. They have a ridge vent...aka a big slice down the center of your diaphragm severing all of its ability.
Also, a 3 sided boxed analogy on a sloping roof. Good luck with that. Again, the ridge is a discontinuity in your diaphragm.
 
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Here is a first draft at the site built moment frame idea with cantilevered rake beams:

WINDOW_WALL_REV1_ymr3mz.jpg


Note that I have have not sized any of the members, straps, or holdowns just yet, the indicated sizes are my best guesses for now until I get the lateral and vertical calcs going. At this stage it is purely conceptual. The wall is symmetric so I only drew the one side.

My big question is the connection detail of the ridge beam to rake beams and the rake beam to PSL column connection at an incline (I may be worrying about nothing on that).

I mentioned steel to the designer and they gave me some very worried looks, that ended that conversation.

I also missed adding the horizontal straps connecting the headers.

The (2) 2x6 king studs next to the PSL column may need to be upgrades to something more stout as well I have to look at the out of plane bending on those and the rest of the wall studs and columns. My suspicion is that the wall may need to be 2x8.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
 
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I also don't want the two cantilevered beams to separate so either a strap over the tops of them (ie. MSTA30, LSTA30 Page 87 Simpson Catalog) at the peak or perhaps a horizontal strap on the exterior side opposite the ridge beam hanger.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
 
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It may also be helpful to create a nailed moment connection at the peak with 8d nails on a 3" x 3" grid, connecting the two rake beams. This may be something to put into RISA or build my own calculator for it. My current PFH calculator doesn't consider the inclined header and potential pinned or rigid joint at its center.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
 
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Bigmig said:
The 3 sided box will not work. They have a ridge vent...aka a big slice down the center of your diaphragm severing all of its ability.
Also, a 3 sided boxed analogy on a sloping roof. Good luck with that. Again, the ridge is a discontinuity in your diaphragm.
Click to expand...

I have never considered diaphragm continuity in a residential job of this scale. It is something that just seems to work.
I guess you could go with a ventless roof and screw bent sheet metal to the peak if you are worried about it.
 
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For the connection from beam to PSL I'm wondering if it might be possible to take a stock CCQ44 and bend the straps 30 degrees, you end up with a width of about 4.75" which can be made to fit a 5-1/4" wide PSL column with some minor notching. One could always order the CCOQ which is without the straps and field weld some on but then you would need a certified welder to make the connection. If the straps were heated (hand held propane torch) and then bent I don't think it would compromise the integrity of the column cap in any significant manner.

The main concern I have is the post slipping up along the beam without a positive connection, but looking at my detail the 2x6 blocking below the rake beam at the peak should help provide this stabilization in compression.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
 
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We have done a handful of steel moment frames for this type of thing but the contractors up here are getting really used to having lots of steel in high-end homes.
 
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The other option as suggested is to go with steel (red iron) on the wall. I've actually never done one of those before so any examples that I could potentially use a starting point would be greatly appreciated. I've seen a number of high end homes with window walls done this way but I've never had the chance to get up close and personal on one and really look it over.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
 
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I usually go with steel. FWIW, you could mortise the kings into the rake LVL a little w/o compromising them too much to prevent sliding.
Plywood on the outside and maybe some sheet metal on the inside would also likely work. Kinda depends on your roof slope and the angle of friction.
 
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The thickest LVL beam is 1-3/4", I want a minimum thickness of at least 3-1/2" and I'm not into nail lamming beams together. Wouldn't a 3-1/2" x 11-7/8" LSL beam work fine? I notice that a lot of engineers choose the LVL over the LSL, pound for pound the LVL is generally stronger in bending but LSL is usually more economical.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
 
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Why are you against nail-lamming? It's so ridiculously common that I never give it a second thought.
 
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