Residential Window Walls
Residential Window Walls
(OP)
Yet another window wall, these things seem to love me, I don't love them. A couple issues with this window wall:

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

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
www.medeek.com






RE: Residential Window Walls
As for the lateral, I think 3 sided may be the only way to go.
RE: Residential Window Walls
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.
RE: Residential Window Walls
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Window Walls
RE: Residential Window Walls
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.
RE: Residential Window Walls
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.
RE: Residential Window Walls
RE: Residential Window Walls
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
www.medeek.com
RE: Residential Window Walls
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.
RE: Residential Window Walls
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
www.medeek.com
RE: Residential Window Walls
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Window Walls
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Window Walls
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.
RE: Residential Window Walls
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
www.medeek.com
RE: Residential Window Walls
RE: Residential Window Walls
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Window Walls
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.
RE: Residential Window Walls
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Window Walls
RE: Residential Window Walls
I never spec LSL beams except as bands. I have enough problems with way over-designed LVL's sagging on me. Don't wan't another animal to deal with.
Also, they simply don't stock them at the local yards.
RE: Residential Window Walls
Here are a few numbers so far.
I have a 50 psf flat roof snow load -> sloped roof snow load of 50 psf
My glulam beam spans 16' and a 5-1/2 x 15 beam seems adequate for the job. My end reaction is 7464 lbs (2194 D + 5270 S).
Half of this reaction is picked up by each cantilevered beam, 3732 lbs.
With the cantilever beam the deflection and the bending control. I end up with a 3 ply (5-1/4" thick) LVL beam with the down force at the PSL column of 7179 lbs and an uplift at the end of the wall of 810 lbs.
I'm very nervous about the hanger at the rake beams. My SDS 1/4" screws attaching my HGU5.62 hanger are going to be quite closely spaced with 18 per rake beam and about 3" from the end of these beams, splitting may be an issue. It might be better to cut the inner ply at a scarf and then splice the two rake beams together and then hanger from this section. I probably need to draw a diagram to illustrate the point.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Window Walls
RE: Residential Window Walls
Simpson calls for 2-1/2" SDS screws for the HGU hanger series this will really only penetrate the first LVL ply.
My idea might not have any merit but below is a quick sketch of the two rake beams spliced together and then the hanger attached to the splice:
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Window Walls
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Window Walls
RE: Residential Window Walls
The one issue now is the architect wants the PSL post centered in the 18" wall section, how exactly the additional studs and trimmers frame out around it is a bit of a mystery...
I've also never encountered a cantilevered roof joist system like this before. I'm planning on installing a strap across each joist to the opposing side but beyond that is there anything else to look out for?
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Window Walls
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Window Walls
RE: Residential Window Walls
With the two beam design the 14" deep TJI Joists will be resting on the glulam at a 7:12 pitch. Based on the TJI documentation you cannot birdsmouth cut I-joists on the upper bearing points, only lower bearing. What do you typically see done in this situation, beveled glulam, beveled bearing plate, variable slope seat connector?
Based on TJ-4000 I will also need to nail (2) 8D nails from the I-joist bottom flange into the glulam and also install a backer block and twist strap (slope greater than 3:12).
The architect also wants to use flat 2x4 outlookers (gable overhang of 18") however I don't see how one can notch around the flange if they are oriented this way (detail O, page 30).
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Window Walls
Quit using those god-awful TJI's and your problem is solved!
RE: Residential Window Walls
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Window Walls
My hands are tied a lot of the time.
RE: Residential Window Walls
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Window Walls
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Window Walls
Axial Loads:
Shear:
Bending:
Numbers:
The problem with the model is that it is a little to simplistic. I don't have a member type that I can use to simulate the shear wall panels (RISA's wall panel aspect ratio limitation did not let me use a high aspect ratio wall panel), so I just used the same 3.5 x 11.875 LVL for all the members. Varying the stiffness of the shear panel will affect the moment distribution at the top and bottom of the shear panels.
One interesting thing that popped out was the zero moment at the peak. If the load from the diaphragm is balanced this is the case and there is really no need for a moment resisting connection at the peak. Granted the wind loading will not be balanced but overall the moment at the peak will probably not be significant.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Window Walls
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Window Walls
View model and download SketchUp model here:
https://3dwarehouse.sketchup.com/model.html?id=cb4...
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Window Walls
I actually meant I-joists in general. We use 2x10's and 2x12's here for most roofs . I-joists if we have to. The framers and contractors prefer the dimensional lumber.
For floors, I use mainly 2x10's or 2x12's with steel I-beams or LVL beams if needed.
Sometimes I-joists and floor trusses, if there is no other practical option.
Here is my problem with i-joists (based on my field experience)
1) difficult to frame with if sloped or skewed
2) The flimsy OSB bands suck for point loads and do not like water. Deck attachment is sketchy at best. I usually spec LVL bands behind decks.
3) Make renovations difficult as they are difficult to modify
4) They don't hold up well in fires (this is anecdotal but believable)
5) Blocking point loads thru them is difficult and is rarely done well or at all. I have seen numerous I-joists buckled from point loads. Never seen a 2x10 do that.
6) The loads from the long spans joists and their skinny profile cause them to dig into dropped beams. The blocking between them does not, so it causes a hump in the floor and will pop floor tiles
7) They do not hold up to water (as Jayrod said)
8) I have seen too many instances of them sagging alot more than they should under sustained loads - such as heavy kitchen islands.
9) They are expensive! - which is why tract builders try to span them out in length and spacing.
10) They are not designed to be used in typical roofs without structural ridge beams. This requires the expense of the beam and the expensive hangers.
11) Cannot notch them and use ledger strips - like dimensional lumber (further adds to the cost)
12) Another source of off-gassing in a house (I am guessing)
13) They seem to shrink in length just as much as dimensional lumber.
I could go on and on, but I am tired.
RE: Residential Window Walls
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Window Walls
I've read quite a bit on the fire resistance of I-Joists and I would tend to agree traditional 2x10 and 2x12 lumber is hard to beat. Similar problem with plated trusses though. I think the building industry (residential) tends to turn a blind eye to this issue, otherwise costs of building homes would become exorbitant.
I've never really encountered any water damage to I-Joist floors but I've seen some issues with roofs where there is no venting, granted the 2x10s would probably not have fairly very well either, but major mold and drywall damage was more the issue than structural damage on that particular job.
Agreed on the point loads. Squash blocking is a must, the flimsy OSB webs will simply collapse.
They are supposed to be more dimensionally stable than regular lumber, less shrinkage, warping and twisting. I do think this is where they beat out the traditional lumber. In my opinion I think they are stiffer than typical plate floor truss but some studies may prove me wrong, just my gut and anecdotal evidence on that one.
The I-beam shaped profile should theoretically give a more efficient structural shape, so less weight and cost for the same performance. On the positive side what is driving I-Joists versus other solutions?
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Window Walls
Most of the custom builders I work with prefer dimensional lumber if it makes sense.
RE: Residential Window Walls
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Window Walls
I've seen a few beveled bearing plate conditions.
If it's at all possible to do so, I would design the moment frame as though it were taking a load consistent with a simple diaphragm assumption. It will be overkill for the moment frame but, given the inaccuracies involved in estimating the share going to the moment frame, I feel that this approach is prudent.
I would model that joint as pinned in your analysis for the simple fact that it would be rather difficult/expensive to attempt to create a moment connection there anyhow.
I might model the interior PSL column into the moment frame as vertical pins / horizontal rollers at the underside of the rafters. Whatever overturning winds up being resisted by the axial loads in those posts will be overturning that doesn't have to be resisted by the base connections of the moment frames. It should improve matters some and be a better reflection of reality.
Yeah, I hear 'ya. As rn14 intimated, the trouble with using steel special moment frames in wood is getting the requisite beam bracing somehow. I believe that's fundamentally why the Simpson moment frame offering exists: it's been tested without the bracing. Whether you go with a special moment frame or an ordinary moment frame, I won't bullshit you, the design of the first couple of frames will be painful fee burners. If you go that route, I'd recommend trying to more or less mimic the Simpson frames as they were developed with wood frame constructability in mind. Include things such as:
1) Beam and column sizes that you use repeatedly so that you can use template calcs and details that you'll develop.
2) Bolted end plate moment connections.
3) Allowance for top and bottom plates in wood.
As for figuring out what's going on with steel moment frame design, I'd recommend these resources:
- AISC Steel Design Guide 28: Stability Design of Steel Buildings
- AISC Steel Design Guide 4: Extended End-Plate Moment Connections Seismic and Wind Applications
- AISC Seismic Design Manual
It's no harder than the wood moment frame systems that you've already mastered but it is a new beast with some different claws and fangs.
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: Residential Window Walls
In my market, dimensional lumber is pretty much dead for horizontal applications other than decks. All TJI, all the time except for pitched roofs. This is especially the case for taller wood buildings where shrinkage is the name of the game. When I worked for the Wood Truss Council of America, I was continually shocked at the degree of regional differences present in the use of pre-engineered wood products. Some places in Florida and Texas, the truss fabricators will ship out entire neighborhoods prefabbed. In other markets, even floor trusses are shunned and you still get guys wanting to frame pitched roof with 2x. Even where I'm at now, floor trusses are very rare despite the widespread adoption of TJI and roof trusses.
I don't sweat any of the fire issues these days other than following code. The research that I've seen in recent years all points to fire safety being almost exclusively a function of sprinkler system and alarm system performance. Statistically, it seems that that the materials of construction aren't really even very influential any more with respect to fire safety.
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: Residential Window Walls
Makes sense as long as the structure has sprinklers. Most single family homes do not.
RE: Residential Window Walls
The i-joists were 65% more. That did not include all the additional hangers etc required.
RE: Residential Window Walls
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Window Walls
Good point. To counter:
1) travel time to safety in a single family home is usually pretty minimal too.
2) It tends to be finishes and contents that contribute to fire load during the fire rating period more than skeletal structure.
3) joists are subject to the same fire testing requirements as everything else. If one wishes to go beyond what society deems "good enough", then I suppose that is another matter.
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: Residential Window Walls
I evaluate a few fire damaged structures a year. The sheetrock does a damn good job of protecting the joists. When it gets into an attic, the dimensional lumber will typically char a bit but then no more damage occurs unless the fire goes un-checked. Many times, repairs are not required. I imagine an i-joist in that situation would be toast - literally and figuratively.
RE: Residential Window Walls
I don't disagree. And if the goal is extra-code fire performance, then I'd also agree that solid sawn joists would have a leg up. I guess my point is that, as with most things engineering, I feel that it's generally the client's prerogative to decide whether or not they want extra-code performance. I know just enough about fire science engineering to know that it's actually pretty complicated and I've got little business practicing it other than to follow the basic rules. In my market, the great minds of fire engineering science have decided that TJI's are good enough to provide the socially agreed upon level of fire protection. I wouldn't insist upon more unless I'd sold it to the client ahead of time as a value add.
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: Residential Window Walls
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Window Walls
RE: Residential Window Walls
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Window Walls
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: Residential Window Walls