tall truss heel / diaphragm continuity

[struct_eeyore]

How do you all connect your roof diaphragms when you have encounter tall truss heels?
Around my parts, on existing construction, I rarely if ever see any means to transfer roof diaphragm loads to shear walls below - residential jobs are notorious. I've come to accept that trusses sheathed on top and bottom are pretty much a rigid system, and whatever connections the truss heels have to walls below are sufficient (my area got hit directly by Ian 2 years ago, and I've come to think this assumption is sufficient for most cases). (This is not mentioning that for the shorter heels attic venting would be compromised)
However, I'm now dealing with a $$$ job with rather tall heels (+2'), and naturally am questioning my past practice. Do any of you out west with them big earthquakes have a different approach? I'm currently thinking of implementing the attached detail (cross bracing direction will alternate every bay, and base blocking will be tapconned in) - discussion with contractor about why this is necessary is now 100% expected.

Any feedback is appreciated.

 

[driftLimiter]

I almost always use truss blocking for this condition. I specify the loads the truss blocking must resist in plf. Then nail off the sheathing to the truss blocking, and use clips (like a35) to transfer from truss blocking bottom chord to wall dbl top plate. Out here (west coast) we would never get away with just sheathing on the top and the bottom as a means of shear transfer, I see it as a gap in the load path.

 

[struct_eeyore]

drift - how do you deal with situation when the heel is deeper than ~14" or height/width of the tallest available blocking? Do you see any issues with the cross bracing as I show? Also, with full height blocking - how do you guys provide vents?

 

[driftLimiter]

I've done several jobs with heels well over 14" and had no problem.

The only caveat is that when the aspect ratio H/W gets larger, we need to address overturning of the blocking. Typically I will connect the vertical legs of the truss blocking to webbing of the trusses with clips similar to those on the dbl dop plate.

The shear along the bottom is equal to the shear on the vertical legs so really we always have these connectors.

In my area, the truss manf. just fabricates the truss blocking to meet a given size. Never had any pushback about height.

 

[phamENG]

check out IRC figure R602.10.8.2(3) Braced Wall Panel Connection Option to Perpendicular Rafters or Roof Trusses. You'll obviously have to modify for your engineered loading, but it's a good starting point for an 'industry standard' detail.

 

[struct_eeyore]

Pham,

Excluding the detail with the solid soffit, the IRC figures do not actually provide a direct means of attachment between the diaphragm and the wall below - as all provide a vent space. While I'm sure they help with racking resistance, I do not think they qualify for the purposes of continuous load path.

 

[phamENG]

Look at the top part of that figure. "Where air gap at top is not used, center w/3 available for vent holes."

So you run that all the way up and connect the diaphragm to it, but you can cut a small hole out to allow for venting in the panel. It'll still have capacity to prevent racking and transfer the shear (because if they can't rack, then they'll be pushing on that blocking and the fastening will transfer shear).

 

[hokie66]

There is a good reason to avoid that detail, especially in areas which have snow and/or hail. With the outside soffit and inside ceiling at the same level, and if the gutters fill up and spill, water can go inside as well as out. Much better to bear the trusses at a regular joint, extend the tails, and just support the soffit separately back at the wall. But then, that requires common sense by the architect.