Trusses with Dropped Top Chords for Outlookers 3

[medeek]

For gable roof overhangs over 12" in length I generally go with structural outlookers with a dropped top chord on the gable end truss. These outlookers can either be oriented horizontally or vertically. The problem I see with this method of construction is the connection between the roof diaphragm (sheathing) and the gable end truss. Solid 2x blocking between the outlookers is typically called out with some sort of tie for the outlookers to the truss for uplift. The blocking between the outlookers is nailed to the roof sheathing with typical panel edge nailing (8d @ 3",4",6" o/c). In my locale with high wind loads I usually call out overhang roof sheathing nailing at 4" o/c.

The load path problem is the connection between the 2x blocking btw. outlookers and the gable end truss for the resistance of shear in plane with the gable end truss/wall. I haven't really given this too much thought before and my various details don't really give much information on this blocking connections. For instance the recent detail for gable end wall bracing:

I like to use H4 clips on the outlookers, since they are small enough for application to either vert. or horiz. outlookers. However, at lateral load capacity of 165 lbs each it would be hard to rely on just these clips for any significant shear wall loads. The 2x solid blocking will either need additional toe-nailing, sheathing or LTP4 connectors to allow for more shear transfer.

Rather than use a dropped top chord truss I think it would be easier to bump up the depth of the top chord member and then notch out for the outlookers as required. This solves the connection problem described above and would seem like a much stronger connection at this location.

What do you see with residential roofs in your neck of the woods? Are dropped top chord trusses with structural outlookers the norm and if they are how do you deal with them?

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

http://www.medeek.com

 

[medeek]

The connection issue I'm attempting to describe is probably best viewed in this detail:

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

http://www.medeek.com

 

[hokie66]

In employing diaphragm sheathing at the roof level, there are so many complications like this and your other post about ridge venting, I wonder why folks in North America don't do the bracing at the ceiling level. In Australia, houses don't get built with solid sheathing on the roof. I know, NA practice is not going to change just because I wonder....

 

[BAretired]

Once again, I have to change from Firefox to Chrome in order to read the text after the sketches have been posted. In Firefox, the word wrap does not work properly.

Rather than use a dropped top chord truss I think it would be easier to bump up the depth of the top chord member and then notch out for the outlookers as required. This solves the connection problem described above and would seem like a much stronger connection at this location.

It might be cheaper to have the blocking pieces between outriggers installed and connected in the shop. I would not notch out the top chord because of the potential for cracking (stress risers).

BA

 

[medeek]

Like Kootk says we come up with all of these theoretical models and equations and then we end up throwing most of it out the window when we ignore issues such as these with our standard building practices. I am still so new in this field that when I encounter something like this I don't know if it is something that everyone else is simply ignoring or if there is a standard detail that takes care of it, hence all of the questions and posts. Some of these questions are addressed in textbooks like Breyer's but I am still finding that some of these issues are not.



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

http://www.medeek.com

 

[medeek]

Trying to keep the sheathing continuous over the top chord and blocking joint could probably be done if the sheathing is installed prior to the outlookers being installed. Basically notch the sheathing for the outlookers. However if the gable end wall sheathing is installed after the outlookers are already there this method is not feasible.

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

http://www.medeek.com

 

[Archie264]

In employing diaphragm sheathing at the roof level, there are so many complications like this and your other post about ridge venting, I wonder why folks in North America don't do the bracing at the ceiling level. In Australia, houses don't get built with solid sheathing on the roof. I know, NA practice is not going to change just because I wonder....

Might it have to do with climate? That is, in the US, particularly in northern regions, attics often wind up getting built out into habitable space. Of course, that wouldn't answer why roofs are sheathed at roof level in the southern regions of the US.

How about the prospect of snow loading? Still the same discrepancy, though.

Hmm, curious...

 

[hokie66]

Probably more to do with the shingles you use on the roof, which require a continuous surface for attachment. Those are not used in Australia to my knowledge.

 

[medeek]

I've worked with my brother at his roofing company a few years back and replaced a number of roofs on older homes in Utah. A lot of those homes did not have plywood or OSB sheathing on the roofs, just 1x4 material spaced about 12" o/c with about a 6"-8" gap between boards. The 30# felt with shakes or wood shingles would span across the gap. My thinking is that with asphalt shingles you need that continuous support of the sheathing otherwise in the heat the shingles will just bend and fall into the roof. All of those re-roofs required full tear offs and then sheathing applied to the existing 1x4 purlins.

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

http://www.medeek.com

 

[medeek]

The ceilings on most of these homes were not gypsum but a sort of lightweight fiber board material, most of which came in 12"x12" tiles or similar dimensions. The walls were lathe and plaster and the electrical wiring in some of these homes was downright scary. Makes one wonder with no roof or ceiling diaphragm how does the house hold up in the wind.

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

http://www.medeek.com

 

[emmgjld]

I am writing this based on what I have seen throughout Colorado. Most of these older house structures were probably fairly small, compact, few corners, possibly not 'platform' framed, constructed with pretty good lumber and had a good number of interior walls, which probably supported the roof system (partially or entirely). While a lot of these house structures may have had problems, most of them stood up and many will for a lot more years.

 

[hokie66]

They stand by good luck, not good management.

 

[Archie264]

Probably more to do with the shingles you use on the roof, which require a continuous surface for attachment. Those are not used in Australia to my knowledge.

Ah, ok, thanks.

 

[KootK]

Somehow truss heels are allowed to not have blocking if they're less than 6" high. Maybe the same logic, whatever that is, applies here.

It might be cheaper to have the blocking pieces between outriggers installed and connected in the shop. I would not notch out the top chord because of the potential for cracking (stress risers).

Doing it in the shop is brilliant. As a former truss slinger, that would be an easy value add for customers. If only engineers would specify the need for it... One could accomplish something similar in the field with Simpson A35's etc.

Like Kootk says we come up with all of these theoretical models and equations and then we end up throwing most of it out the window when we ignore issues such as these with our standard building practices. I am still so new in this field that when I encounter something like this I don't know if it is something that everyone else is simply ignoring or if there is a standard detail that takes care of it

I worry that I may have infected you with my pessimism. What I'm about to say will probably sound even more pessimistic but you might find it useful. I've struggled with similar philosophical issues as you for a good long time.

When I first entered the field, I had what I now believe to be a very naive perspective on things. I thought that, as structural engineers, we knew how to evaluate every mode of failure. I thought that my job would be simple in principle, if complex in execution: I would check all possible modes of failure and ensure that all were precluded. Easy peasy.

I now have a more nuanced view of structural engineering. I believe that it is entirely reactive. Something bad happens, we figure out why, and prevent it in the future. If a concrete slab pancakes onto the levels below, we devise the punching shear concept and keep it from happening again. So on and so forth, rinse and repeat. The takeaway is that we really don't understand everything about anything. Stuff is complicated. We just know how to prevent many of the issues that have manifest themselves on past projects.

While this view of structural engineering is a bit cynical, it's also gloriously pragmatic. I feel that with lateral design in general, and lateral design of wood in particular, we've somehow put the solution cart before the problem horse. Yeah, some lousy stuff has happened during typhoons and earthquakes. But I've yet to be shown evidence of anything looking like a true in plane shear diaphragm failure in light frame wood.

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]

Wow Kootk - That was very well put and echos my thoughts exactly. This is the problem we have in our area switch the new IBC lateral bracing requirements. Never have seen a lateral failure in any house anywhere in my area even after really strong hurricanes - even modernist houses with basically no shearwalls or the cardboard sheathed tract houses. Now we have to submit a lateral stability plan with all houses that is a total pain in my butt and costs the owner a pretty penny. Sure, if something looks flimsy, i am going to take care of it, but I don't feel I need to add shear collectors on every house if the wall bracing is more than 21 ft. O.C. - sheesh.

 

[medeek]

The building dept. that I work with on a regular basis pretty much requires engineering on everything now. I just finished up a small 18' x 22' garage. Granted the front wall needed a shearwall with holdowns but the rest of the loads on the structure were pretty insignificant.

The high wind loads in this area are the big concern so maybe they aren't too off base, and admittedly it is good for business. The local contractors take a different view though.

I don't think I'm pessimistic about the structural field but I do like to think that after I spend all of this time analyzing stuff and sweating about it that it was for good reason. If it doesn't really matter in the end then why do it? I also like to get things "right", sometimes to a fault, and that is what drives me to ask some of these questions on the forum.

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

http://www.medeek.com

 

[dcarr82775]

Some toenails, LTP4's, and just notching the sheathing all do the trick. Lets say the blocking is there but not really connected to anything. How will the system fail? Is the entire roof going to slide up the gable? No. Are those little 2x4 outriggers going to roll? I don't see how with the blocking there. The load path is some goofy combination of bearing and friction resistance. Nothing I would hang my hat on for something new, but there is always a viable load path to explain why something hasn't fallen down. There are so many in typical residential wood framing that things work out in the end.

 

[medeek]

I agree with you dcarr82775 that I have never seen a failure or heard of one at this location. I'm pretty sure its safe to assume that the framer/contractor will toe-nail those blocks in (or sheathing) and combined with the connection of the outlookers and top chord the whole thing should hold together. The point is I have not engineered this connection thus far, I've just assumed that the builder will put it together right, obviously you can't call out every nail or fastener in the structure but as engineers we do specify quite a bit. Sometimes I feel like it is a fine line between over specification and negligence.

I've thought about calling out LTP4 connectors for each block but this seems like overkill for most situations. The gable end truss will pick up the shear load from the shearwall below and spread it out over the entire top chord as it transfers the load into the roof diaphragm, this is probably why this connection has never shown dramatic failures before.

What prompted this thread was my reading another thread on a different forum where an building inspector noted that this method of framing a gable end overhang had this major flaw and should be avoided because of it. I don't agree with his assessment and honestly I don't see a better way to frame structural outlookers at the gable end.

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

http://www.medeek.com