Load Path - Roof Diaphragm to Shear Wall

[UTvoler]

Say the attached is the upper part of a wall section (cut on the long side) of a 50' x 30' rectangular single-story building . There is a 15-ft long shear wall in the middle, so collectors are required to get the diaphragm shear into the shear wall. There is a magnificent parapet, which is partially supported by the vertical web extension of the roof truss and so the roof sheathing is not continuous to the wall sheathing. If we add blocking between the trusses, and nail the roof and wall sheathing to the blocking is that enough? How badly do we want continuous strapping over the roof and/or wall sheathing into the blocking? Are we worried about the trusses racking?
All thoughts are extremely welcome.

 

[RontheRedneck]

If I got that section view sent to me I'd ask if we could build the entire parapet into the truss.

Depending on how deep the heel of the truss is, you might be able to add some X-bracing inside the end vertical.

 

[DaveAtkins]

You could detail this with a notch in the end of the truss which allows for installation of a continuous 2 X 4 rim board. The roof sheathing and the wall sheathing both attach to the rim board.

DaveAtkins

 

[Eng16080]

The detail in the sketch seems reasonable. I can't easily think of a better/simpler alternative.

 

[XR250]

Seems OK to me.

 

[azcats]

What is acting as the collector either side of the shear wall? Is the 2-2x top plate continuous?

 

[driftLimiter]

Also where is the diaphragm chord, shear can be transferred intermittently, but the chord wants to be continuous. If your using the Dbl top plate as the chord, then there is additional flexibility in the region between the roof sheathing and the diaphragm chord. This 'could' become an issue if diaphragm deflection is high.

This could be remedied by doing a continuous strap along the wall blocking. Then that blocking can be your chord.

 

[lexpatrie]

I've entertained something like this previously and gave up.

You want a continuous ribbon at the outer edge of the truss which can't be there as shown because the end piece sticks up, furthermore shipping gets awkward, and while it's not "our problem" that piece will likely get damaged somewhere in the process.

The 2x10 blocking beneath the roof sheathing you show as [X} which is normally used to indicate a continuous piece out of plane, and it's not a continuous piece, because it's blocking. Blocking should be shown [/] if you're going old school. (I'm Midwest so yell at me if that's not how it's done elsewhere, but this is in Breyer, or was).

While you might be able to fabricate "stick on" elements that look like that desired parapet, there's still a lot of detailing to be done as the parapet forces for wind are inconveniently large. If your through connection between the diagonal strut into the truss below has nothing continuous beneath it, the decorative parapet truss doesn't have much tolerance for missing the nails and there's not much in the 2x top chord for attaching a lag screw anyway. It also looks like the parapet piece (if fabricated separately) has the center of mass outside the end of the building which will make it awkward to install anyway.

The parapet top is massive, which will make a parapet cap coping per ES-1 difficult to obtain and/or not available.

 

[driftLimiter]

The parapet isn't massive, its magnificent lol. One time an architect got sour at me because I said the parapet was really tall in a meeting with client. I wonder what they would have said it I said 'magnificent'.

 

[lexpatrie]

Try doing a ten foot parapet......

 

[UTvoler]

All, thanks for the feedback! Sorry for ghosting my own thread; I had several fires last week...

@RontheRedneck: I have seen/done the entire parapet being fabricated integral to the truss. Nice for the framers, I don't love the roof sheathing/diaphragm being held back so far from the shear wall and having to justify the vertical/horiz offsets of the diaphragm to complete the load path to the shear wall.

@azcats/driftLimiter: The collectors/chords is where the challenge lies. This is a one-story bldg; and I rarely see these kind of things thoroughly detailed in a 5-story. Was considering continuous strapping, or possibly blocking or blocking panels to transfer the shear down to the double top plate but it's kind of quirky.

@lexpatrie: I don't believe I've ever seen/noticed [X] or [/] to designate blocking versus continuous. You make valid points; I do a lot of hotel design work, and the parapets are getting way past magnificent to ridiculous. I've seen/done 10' parapets, and they add more lateral load to the building than another story does. And the framing connections are just silly to make the numbers work....

 

[Eng16080]

I gave this a bit more thought. I think I might do what Ron mentioned above and incorporate the parapet framing into the trusses. As you mentioned, that will cause the diaphragm sheathing to be held back to the parapet diagonal. To transfer the diaphragm shear into the shear wall below, I'd consider something like in the images below. Basically, 2x4s are sistered to the side of the trusses and are used to create a rectangular box between trusses which sheathing is attached to. Depending on your loads this might not need to occur between every truss, perhaps every other would work (as shown).

While this shear transfer mechanism will be some work for the framers and will require your oversight, in the balance of things, I think it's simpler than stick framing the parapet.

There should be continuous blocking supporting the edge of the roof diaphragm sheathing, which is not shown in the images above.

 

[lexpatrie]

Why not just do it as a two piece set of main trusses, with a normal diaphragm collector, then attach the "junk" as a second fabricated item? Straightforward load path, normal blocking, and then the fancy stuff gets screwed/bolted on.