High Heel Trusses

[jtwright4216]

Working on a project with attic trusses over the entire upper floor. However, these have a high heel height of 8'-0". How would you guys analysis the lateral (transverse to the trusses) for this. Typically, I would call out for shear panels at high heel locations but 8'-0" tall x 2'-0" wide shear panels between trusses doesn't seem like a good option. I thought of considering it as another story but then you get into issues with load path since we don't have a top chord. I guess you could block and strap across?

Has someone ran across this before? OR have any ideas.

 

[dik]

Just like any other truss... you can put bracing (temp or permanent) and sheathing to the heels. A caution, make sure you don't have a joint at the top plate because of the 8' dimension. I like to extend the wall sheathing across the plate to tie the roof to the wall.

Dik

 

[jtwright4216]

Thanks for the reply Dik.

In that case if it's just a typical truss with a high heel you would have to have shear panels, right? Otherwise how would you transfer the force to the roof diaphragm? And like I mentioned an 8x2 shear panel seems uneconomical. Good point about the top plate and extending the sheathing.

 

[dik]

It's just a matter of transferring all loads...

Dik

 

[KootK]

Depending on how seriously you take your lateral load paths, these situations can become quite complex. My recommendation is shown below. Additionally:

1) If you want the roof/wall sheathing meeting cleanly, you're stuck notching the sheathing. Alternately, you can block lower on the truss and tolerate some rollover discomfort on the heel of the truss. This is a common issue on non-attic trusses and this doc seems to imply that a 6" offset would be tolerable in many instances: Link

2) With option #2, you could stagger the blocking vertically to eliminate the need for the framing clips.

3) With both options, you could eliminate the framing clips if you could get away with blocking at only every other truss.

4) Your best friend in these situations in unaccounted for interior drywall. Hopefully there's wall sheathing inside the attic space and continuous drywall on the underside of the truss. Those things will improve strength and stiffness of the lateral system considerably if inadvertently.

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.

 

[focuseng]

Following KootK's lead: if you are only in the design phase, the trusses could be constructed to have a "let-in" on the upper part of the heel for the band joist similar to a floor truss. This would also be similar to the configuration of using a flush ledger or midchord bearing only upside down. This would only work with the "less good" configuration though. But to be fair, the rafter tail would make the notched sheathing loose some of its shear capacity/continuity and make it harder to construct as well.

Even more thoughts: Usually the top chord is only a 2x4 so you might make the upper blocking taller, say a 2x8 or 2x10 and then combine the two options into one option and then the sheathing doesn't need notching. My let-in won't work with this idea though due to the tail.

KootK: you haven't copyrighted "Less Good Option" by any chance? Sounds like the title a nice fiction novel about a structural engineer fighting for Good options in a world gone mad.

______________
MAP

 

[dik]

KootK... I would avoid the joint at the top plate... that was what I was trying to caution the OP about.

Dik

 

[KootK]

KootK: you haven't copyrighted "Less Good Option" by any chance?

Public domain all the way...

Usually the top chord is only a 2x4

I designed these for a couple of years right out of highschool. I've never seen an attic top chord less than 2x6. Often 2x8.

KootK... I would avoid the joint at the top plate... that was what I was trying to caution the OP about.

I see what you're getting at but stand by my solution as detailed. You can get tension tie down from truss to stud with the usual Simpson hardware, just like with a regular truss. In my area, most walls are shop fabricated and the builders are grateful for a friendly sheathing joint placement.


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.

 

[focuseng]

KootK, what is your typical design snow load? 2x4 top chords are ALL the rage around me at 30-45 psf P_g. Sometimes a 2x6 and only bigger in girders or less conventional profiles.

______________
MAP

 

[KootK]

KootK, what is your typical design snow load? 2x4 top chords are ALL the rage around me at 30-45 psf Pg. Sometimes a 2x6 and only bigger in girders or less conventional profiles.

Anywhere from 20 psf to 60 psf depending on whether you're on the plains or in the mountains. I agree that a non-attic truss will generally have 2x4 top chords. The attic TC situation, however, has little to do with snow load. It's about that stretch of untrussed top chord that you usually get between the top of the vertical webs that form the attic walls and the ends of the collar tie looking thing that forms the attic ceiling. Great big moments and shears in there.

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.