Trusses with Dropped Top Chords for Outlookers
Trusses with Dropped Top Chords for Outlookers
(OP)
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?
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
www.medeek.com






RE: Trusses with Dropped Top Chords for Outlookers
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Trusses with Dropped Top Chords for Outlookers
RE: Trusses with Dropped Top Chords for Outlookers
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
RE: Trusses with Dropped Top Chords for Outlookers
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Trusses with Dropped Top Chords for Outlookers
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Trusses with Dropped Top Chords for Outlookers
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...
RE: Trusses with Dropped Top Chords for Outlookers
RE: Trusses with Dropped Top Chords for Outlookers
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Trusses with Dropped Top Chords for Outlookers
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Trusses with Dropped Top Chords for Outlookers
RE: Trusses with Dropped Top Chords for Outlookers
RE: Trusses with Dropped Top Chords for Outlookers
Ah, ok, thanks.
RE: Trusses with Dropped Top Chords for Outlookers
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.
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.
RE: Trusses with Dropped Top Chords for Outlookers
RE: Trusses with Dropped Top Chords for Outlookers
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
www.medeek.com
RE: Trusses with Dropped Top Chords for Outlookers
RE: Trusses with Dropped Top Chords for Outlookers
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
www.medeek.com