Structural aspects of a double-stud wall
I like Lstiburek’s ideal double-stud wall with the structural wall on the interior. Durability and resilience are important to me and I think this wall is good from that perspective. I’ve been thinking about the more structural aspects of this wall and would appreciate any input or links to relevant info.
The BSC graphic (https://www.greenbuildingadvisor.com/article/lstibureks-ideal-double-stud-wall-design) that is often referenced shows “1/2” plywood connecting interior and exterior walls” at the top. At the level of the floor joist, it shows some sort of plywood piece sandwiched between plates in the outer wall and connected to what looks like a 2×2 attached to the outside of the sheathing. (I’ve never seen a 2×2 that I would want to use for anything structural beyond maybe infill in a deck railing.) I assume the bottom plate would be bolted to the foundation.
If the interior wall is the structural wall, how are windloads being effectively transferred to the structural wall? Do both walls have to be designed to meet IRC bracing requirements? I don’t see double-stud walls mentioned in the IRC, and the IBC only seems to address them in terms of firestops. It seems possible from a building science perspective to leave off the outer sheathing in this wall in favor of a membrane and rainscreen siding, but from a structural perspective, is this possible?
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The interior wall is sheathed to resist lateral forces. Is that your question? And no the outer wall would not need sheathing, just as a Larson truss does not need sheathing
I understand that the interior wall is sheathed to resist lateral forces. And I understand that a Larson truss does not need sheathing since the trusses are directly attached to the structural wall. But how are wind forces safely transmitted to the interior wall in a double stud wall?
Lateral forces are transferred through your roof or floor diaphragm to resisting members. It is not the wall that is being pushed by the wind that is resisting its forces, it is the wall perpendicular.
I understand that. But the exterior wall surface that is experiencing wind loads has to transfer those loads to the perpendicular wall/roof/floor diaphragms. Does that mean that the exterior wall corners need sheathing?
Elizabeth,
There are so many variations on the double-stud wall it's hard to make blanket statements about how their structure works. Some are tied together with common top and bottom plates and gaskets so that they act as one structural component, others are more like lightly framed scaffoldings for exterior insulation.
The need for both lateral resistance and wind-loading both also vary widely by location. There are areas of the US where houses are routinely built that rely entirely on their interior drywall and cladding to provide both.
All this is a long-winded way of saying: It depends. Your design may be fine for where you are, or it may require some engineering.
I think I must not be understanding your question. That image you showed had the outside wall connected at the top plate/floor diaphragm which is where the load then transfers into that diaphragm. It is not wall to wall connections that transfer the load (generally). For instance if you build a 20x20 building without a roof and go into the middle of one of those walls and push, it will move considerably in and out. By attaching it to the roof diaphragm you transfer that load into the roof sheathing and then to the perpendicular wall via the roof to wall connection.
I think i am probably still not answering your question so I will let others, maybe an engineer will answer more thoroughly.
Thank you both! I think I'm probably over-thinking this after watching too many APA videos on wall bracing and structural connections.
There is a wind load on exterior walls, it is not just the roof. I've wondered this myself, somehow the wind load needs to transfer into the interior load bearing wall. The wind load can be thought of as a pressure. I was recently using the Boise Cascade BCcalc software to do a tall wall design. Wind loads come in around 24-25lbs/sqft of wall area, using an exposure B. Open fields, exposure C, and you come up to the low 30s lbs/sqft. I don't know how this would transfer with an exterior non-structural wall to an interior load bearing wall.
Based on what you wrote, yes, it is possible to eliminate the outer sheathing.