Diaphragm Force Transfer from lower roof to adjacent wall

[Joe_98]

Hello everyone,

I'm currently working on the structural design for a T-shaped timber building located in seismic design category E. In this project, a one-story portion of the building intersects perpendicularly with a two-story portion. The challenge I'm facing is that the wall between these two portions doesn't align with the end truss of the one-story portion and extends upward to reach the higher roof of the two-story section.

I'm trying to determine the best way to transfer diaphragm forces from the end truss of the one-story section to the shear walls of the two-story section, despite this misalignment. I'm considering options like adding two layers of wall: one to align with the end truss of the one-story portion and another that extends up to reach the roof of the two-story portion.

I'd appreciate any insights, details, or resources that could guide me in the right direction.

Thanks in advance for your help!

 

[phamENG]

I don't practice in seismic country, so don't shoot me if this isn't allowed for cat E, but I'd do away with the last truss and use a ledger fastened to the wall to catch the roof sheathing. The fasteners (preferably screws to each stud) would be sized for the gravity load and the shear transfer into that wall. You may need to specify the wall sheathing to run down past that ledger and fasten through the sheathing to make sure you have a good load path.

 

[Joe_98]

Thanks for your response, Pham.

I believe the gravity load will be carried by the ends of the truss through the side walls, so considering only the shear forces of the roof diaphragm for that ledger should be sufficient. This is also my first project in such a high seismicity area—the Sds value is 2.02g!

 

[phamENG]

I disagree. Load follows stiffness. If you forgo a truss against the wall and only do a ledger, the load must go through the ledger. If you keep the truss but fasten it to the wall at each stud, the connections at 16" o/c will be much more stiff than a truss spanning 20 or 30 feet, and the load will go from the top chord straight into the studs.

That's a pretty impressive Sds value. I'll stay out here with my hurricanes, thank you.

 

[driftLimiter]

could you make some sketches? I'm all for ledger transferring shear into a wall but it sounds your describing a more complicated scenario that may include discontinuities.

 

[Joe_98]

I hope it helps.

 

[lexpatrie]

I don't get the load follows stiffness relevance here. Whether it's a ledger or a truss nailed to the face of the wall (effectively a ledger by another name) the stiffness (of the truss/ledger) seems like it's the only way for the lateral fore in-out of the plane of the "paper" to be resolved.

I've done something similar in wind design, I think it was a truss nailed to the face of the wall, needed to resolve the end-wall diaphragm force from wind.

I seriously doubt you can design this to "slide" against the wall as a three sided diaphragm in SDC E. So there's not much choice.

Generally the shear force into the wall would involve half of the wall weight, plus the roof weight. The bottom half of the wall seismic force goes into the sill connection at the base and the soil/foundation.

 

[phamENG]

I don't get the load follows stiffness relevance here. Whether it's a ledger or a truss nailed to the face of the wall (effectively a ledger by another name) the stiffness (of the truss/ledger) seems like it's the only way for the lateral fore in-out of the plane of the "paper" to be resolved.

Sure, but the OP wasn't talking about lateral here:

I believe the gravity load will be carried by the ends of the truss through the side walls,

 

[driftLimiter]

You could use a ledger over the shear ply. Make sure you have enough fasteners to transfer your diaphragm shear.

The shear wall with the ledger attached now resists the shear force reaction from the diaphragm. This shear is in addition to any other loads coming from the 'taller' part of the structure.

 

[lexpatrie]

pham, I'm asking abut what you wrote. Not what the OP wrote.

 

[phamENG]

lex - OP talked about gravity, I talked about gravity, then you said my comment didn't makes sense because the lateral load path doesn't care. I'm very confused about your question about a question about a thing that was never said.