How would you transfer diaphragm loads from Roof Deck --> Light Gauge Roof Truss --> CMU Shear?

[BSPE90]

I am working on a project with pitched gable roof framed with a long span light gauge roof truss. The trusses are bearing on the CMU walls, where there are any and also on steel beams. The CMU walls will terminate at the bottom chords of the trusses.
I do not have any experience with this kind of construction and neither does the other structural engineer I report to. The light gauge trusses are going to be designed by the fabricator but will they be required to figure out how the diaphragm loads are to be transferred to the shear walls? If so, what do we need to provide on the drawings?

 

[jayrod12]

What is the diaphragm system at the roof level? That will affect the answers likely.

Generally speaking, where I practice the EOR (you) would be required to detail the load path from the diaphragm to the lateral load resisting system.

The delegated design engineer (light gauge truss guy) would be responsible only for the connections of his trusses to your structure.

 

[phamENG]

will they be required to figure out how the diaphragm loads are to be transferred to the shear walls?

No. That is the job of the engineering of record. They design the truss, not the diaphragm, not the collectors, and not the shear walls. Your best bet is probably going to be some sort of blocking between trusses that is fastened to the shear wall/collector and to the roof diaphragm. If your gable ends are trusses and not CMU to the ridge, you will need to provide the truss manufacturer with the in-plane drag load to get the diaphragm shear orthogonal to the ridge down into the shear walls at the ends of the building.

If neither of you have experience in this, I hope you're hiring a third party who does for a review?

 

[KootK]

I did a project with cold formed trusses a few years ago. Here's how it panned out for me:

1) The proprietary Apline system was used. You can find their details online or via a sales rep.

2) Often, cold formed trusses means:

- 4' oc truss spacing.

- 1.5" corrugated steel deck rather than wood sheathing.

- the need for explicit and robust (relative to wood) diaphragm transfer blocking at the truss heels, at hip ridges, and at gable ridges.

3) The blocking members used in the diaphragm are usually most economically handled using light gauge pieces that the truss manufacturer is both familiar with and, often, is able to supply. For this reason, I disagree with the other fellows here in that I think that it may indeed be prudent to delegate that aspect of things to the truss supplier. The truth of delegated engineering is that, while there are conventions, they differ based on the component industry and, in truth, you can delegate any damn thing that you want to as long as it's handled gracefully. Fundamentally, you want to delegate something when the supplier is likely to know much more about that thing than you do as the EOR. And, in my opinion, this is one of those cases unless you, as the EOR, are uncommonly familiar with this stuff.

4) In my case, the heel blocking took the form of a weird shaped cold formed blocking piece connecting the metal deck to the shear walls at every 2nd or 4th truss spacing depending on demand. It was super economical and would not have resembled anything that I would have come up with on my own. I'd probably have gone with rolled steel bent plates which would have been a complete waste of my time and energy as they would have gotten value engineered away before the ink on the drawings was dry.

 

[KootK]

My project was CFM shear walls at the perimeter and CMU shear walls at the interior. So I didn't have your exact condition. My guess is something like this.

 

[hardbutmild]

KootK, do you have a photo of this? From the sketch it looks like a toy piece (I'm sure you did it right, it just looks weird to me).

 

[KootK]

@ Hardbutmild: I'm afraid that I don't have a photo. Like I said above, I didn't actually have this exact condition on my project.

@ BSPE90: when the truss heel height is tall, I believe that one option is for the heel blocking to turn into, effectively, intermittent bridging. As you can imagine, once that void space gets to be 2'x4'-ish, there's space for that kind of thing.