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Do you use the double top plate or the rim joist as your chords/collectors?

Do you use the double top plate or the rim joist as your chords/collectors?

Do you use the double top plate or the rim joist as your chords/collectors?

Just wondering what you all do in practice, and what your seismic loads are like. Most of my searches online come up with the assumption that the top plates are the collectors. This doesn't make much sense to me, and except for extremely low demand collectors I can't think of even one reason to use the top plates.
(1) the rim joist is closer to the plane of the diaphragm
(2) the rim joist is nailed directly to the diaphragm
(3) the rim joist is inline with any roof beams over full height openings where the double top plate is non-existent
(4) the rim joist is has fewer splices and is easier to splice (just strap over the diaphragm or on the side where shear wall nails are not required).

This last point may not be true in very low load collectors under a couple kips, where a double top plate with 20 splice nails might be sufficient. Also, if the rim joist is the collector, we need clips both sides @8" to handle the shear in moderate to highly loaded walls with the 1.25 multiplier per ASCE 7-10, (SDC D-F, certain common irregularities (including diaphragm discontinuity and reentrant corners)).

Most of my jobs are SDC C, D, or E.

RE: Do you use the double top plate or the rim joist as your chords/collectors?

I usually go with the top plate, since most of the guides and examples I have seen follow this assumption. I assume that the rim joist acts as blocking for the floor joists if the load is small (sometimes I will block the floor joists at the top plate and consider the blocking as a strut that drags the load into my wall, at which point I will nail the blocking to the top plate every 4' or so with the nails needed for the collected force).

RE: Do you use the double top plate or the rim joist as your chords/collectors?

follow the load path.

the rigid or flexible diaghram will impart a load that needs to be collected, taken through to, and resisted by the shear wall.

drag struts, blocks, top plates, can all be mechanisms used to transfer those loads.

RE: Do you use the double top plate or the rim joist as your chords/collectors?

I have a small point to make. I have only designed in SDC A or B, so I'm not very familiar with seismic detailing. But like you said in (4), rim joists need an additional strap, whereas top plates are lapped over each other anyway. The top plate splice is a more familiar detail to typical contractors; some of them haven't even caught up to FTAO detailing yet and mess it up, so I tend to stick with traditional methods.

I recommend you email Woodworks with this kind of thing; it's their bread and butter.

I'll crack open my copy of "The Analysis of Irregular Shaped Diaphragms" and see if they mention rim joists in there. I had a similar thought process as you, but I didn't find it to be a standard practice so I gave up on it. But I guess I'm too afraid to innovate.

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