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2 Story Residential Addition Lateral Design

PVPE24

Structural
Joined
Aug 21, 2024
Messages
6
Location
US
Hey everyone,

I'm designing a lateral system for a 2 story addition connecting to a 1 story existing home. The addition has a loft area on the second floor for half of the structure, and an open area from floor to roof in the other half. I've attached a sketch of the roof and 2nd floor plans. There are interior partition walls, but I've left them out for clarity. The shear walls can be placed on all 4 sides without much trouble, so I've shown a rough location for the shear walls at each level.

My question here is how to analyze the lateral system of the addition for seismic. My thinking is as follows:
Assume a 2-story ELF distribution.
For the roof level, send half of the full height section to the roof weight, and the upper portion of the loft area to the roof weight.
For the second floor, assume the lower half of the loft walls, upper half of the ground level walls, and second floor weight all go into the second floor seismic weight. Nothing from the open area is assumed to go into the second floor seismic weight. - Second image attached shows an elevation view of the addition with representation of how I'm distributing the weights to the levels.
Distribute the loads to the shear walls by tributary area (flexible diaphragm).
For walls that intersect with the existing home, do a separate 1-story analysis of the existing home and add the shears from the existing home and the addition.

Does this seem like a reasonable way to approach the lateral analysis? Have other people approached this type of structure differently? Looking forward to hearing some insight on how people have approached structures like this. Thanks!
 

Attachments

What you showed makes sense for a wind load distribution. For seismic, I lump the weights into the center of the diaphragm, where the weight is the floor plus the walls above. I don't use walls below for that floor. It makes the analysis a lot easier. I skimmed ASCE 7, I don't think it specifies if you should take the tributary height of the walls into the seismic weight of that floor. However, I do multistory buildings so maybe it's different for wood.

I know you didn't ask about the perpendicular direction force. But anyway, are you connecting to the existing home, or is the addition independent? Drag struts such as top plates and rim boards will be transferring horizontal load across that open area.
 
Thanks for the response! Make sense what you do, just looking at the floor itself plus anything above. For this specific structure, I think that would result in similar forces as far as the shear in each shear wall and does make the analysis easier to conceptualize. So by that logic, the open section would then really only be contributing the roof weight to the roof diaphragm and nothing to the loft level (if I'm understanding that correctly).

As for the perp direction, yes we will be tying back into the main home. We will be balloon framing the full height walls for that open area, so my plan was to detail some blocking between the studs at the height of the loft and strap across all the blocking to create the collector. We will then tie that segment back into the main home with another strap connecting the addition into the ceiling joists. Thoughts on that approach for the perp direction?

And the final note for the perp direction, we have a hip roof for the main structure. One thing that's always tripped me up, does it necessitate a shear panel from the ceiling joists to the existing sloped diaphragm where we tie the addition into the main structure? Or are the ceiling joists assumed to be the "collectors" for the roof diaphragm and we just need to connect to the joists to adequately transfer the load back into the main home roof diaphragm? More just food for thought, I'll likely be doing the shear panel as we have typically done that for similar connections in the past.

Thanks again for the response!
 
I thought about this more deeply. This is both a correction and a further explanation so you can understand how I got to my conclusion. When I learned dynamic response many moons ago, I would lump the seismic load into a single point, as shown below. (Source: The Seismic Design Handbook)

1754171780795.png

However, there's no reason these nodes should be one thing at each floor. Can't we say that every stud (or even molecule) in the building participates in the seismic response? We have a ground motion where the earth below moves (which we invert to instead apply lateral forces rather than a displacement at the bottom), but the building hasn't moved yet. So the building wants to straighten itself out. The cladding and studs want to go back into equilibrium as well. However, the cladding and studs cannot go back into equilibrium on their own. They need to rely on the floor they're sitting on, and the diaphragm strength of such, to move back into place. So in that case, the seismic weight would be calculated as the elements sitting on that floor.

There is some part of this that I haven't fully resolved yet, which is the mass participation of the seismic weight affecting the floor above. I think I got as far as I could with this, with my limited knowledge. Either you accept the explanation, or someone else will come along and say I'm totally wrong (happens all the time lol).

So by that logic, the open section would then really only be contributing the roof weight to the roof diaphragm and nothing to the loft level (if I'm understanding that correctly).
Yes. I would just double check a worked-out seismic analysis example from Woodworks or APA or a textbook to make sure. As I said, I don't really work with wood; this is based on my experience with multi-story.

Regarding the last point, I've never done a shear panel as you said. Collectors are the roof joists themselves and top plates and things like that, not ceiling joists under a hip roof. But maybe a sketch would clarify what you're saying.
 

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