Residential Window Walls

[medeek]

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

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

The designers here are starting to go with 14" I-Joists on the mid floor levels so that all of the central air ducting can live inside the floor. In some cases they remove most of the web (about 12" holes in height) in strategic locations. I haven't evaluated those situations in the detail but the I-Joist manufacturers and their software has given them a green light on it. I think if large holes are in the central portion of the span where the shear values are minimal then everything should be fine, similar to a rect. chase through a typical floor truss.

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

http://www.medeek.com

 

[XR250]

On the positive side what is driving I-Joists versus other solutions?

Dunno. Good marketing and misinformed Arch/Engineers IMHO. Many builders love these things (mostly tract builders), so if they request them, I guess ya might as well specify them.
Most of the custom builders I work with prefer dimensional lumber if it makes sense.

 

[medeek]

How does the price compare between dimensional lumber and I-joists in your market?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

What do you typically see done in this situation, beveled glulam, beveled bearing plate, variable slope seat connector?

I've seen a few beveled bearing plate conditions.

Varying the stiffness of the shear panel will affect the moment distribution at the top and bottom of the shear panels.

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.

One interesting thing that popped out was the zero moment at the peak.

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.

The largest moments are at the shear panel base as expected.

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.

I understand the terms OMG and SMF but the rest is Greek to me

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.

 

[KootK]

How does the price compare between dimensional lumber and I-joists in your market?

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.

 

[XR250]

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.

Makes sense as long as the structure has sprinklers. Most single family homes do not.

 

[XR250]

How does the price compare between dimensional lumber and I-joists in your market?

Last job I checked was 2x12 versus 14" i-joist (base grade BCI).
The i-joists were 65% more. That did not include all the additional hangers etc required.

 

[medeek]

Western Washington and Oregon are also pretty much I-Joist floors with manufactured truss roofs. The occasional custom home or old timer wanting to do something different. Floor trusses are rarely used in residential applications and solid sawn is for walls and pressure treated for deck joists. In the immediate area the small builders like to use solid headers 6x6, 6x8, 6x10 and the larger builders go for the LSL headers 1-3/4, 3-1/2 etc... It interesting how certain areas build certain ways and tend to stick with it. Utah on the other hand likes to use (2) 2x10 headers, at least the plans I've seen.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

Makes sense as long as the structure has sprinklers. Most single family homes do not.

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.

 

[XR250]

Kootk,

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.

 

[KootK]

I imagine an i-joist in that situation would be toast - literally and figuratively.

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.

 

[medeek]

As and add on I've come up with a detail for the cantilevered I-Joist roof:

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[XR250]

Looks good. However, it further bolsters my case on how difficult it is to work with I-joists. That detail would be much easier with dimensional lumber if it worked out spanwise etc.

 

[medeek]

Good point, the detail in the manufacturers called out for a twist strap at the bearing point on the beam, but I didn't want little tabs sticking down that would be visible so I had to devise a different method to make the connection between the beam and the I-Joist, bit of a pain

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

One could install a non-structural ridge board and some variable slope hangers for, perhaps, more conventional detailing.

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]

Good point. I just checked the price on them and they are only about $5.00 a piece online. (Probably $10.00 from a lumber supplier) Much cheaper than I expected.