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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.

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

Hi AaronMcD,

According to "The Analysis of Irregular Shaped Diaphragms" p40:

"The drag element transferring the collected diaphragm shear forces into the top of these shear walls can consist of two different types of framing members. The first possibility would be a continuous rim joist located directly below the diaphragm sheathing. The shear from the sheathing is transferred directly into the rim joist by nailing. All joints in this member must be spliced by a steel strap or lapped and nailed to blocking. The drag forces are transferred into the shear walls by toe-nailing the rim joist to the wall top plate or by the use of light gauge metal shear clips. If a continuous rim joist is used, the shear clips typically need to be installed only over the shear wall. At an interior wall line, the continuous rim joist is normally replaced by 2x blocking or prefabricated shear panels. It becomes impractical to use the blocking as the drag element because of the closely spaced joints. Each joint would have to be spliced to transfer the tension and compression forces. In this case, the diaphragm shears are transferred into the blocking and then down into the wall double top plate, which will serve as the drag element. The shear transfer connection from the blocking to the plates must occur at each 2x block or blocking panel along the entire length of the wall line. All the joints in the wall plate must be spliced by a steel strap, or lapped and nailed plate splices as required by design. The transfer of the drag forces into the wall is accomplished by the fastening of the shear wall sheathing to the wall plates. This will complete the required continuous load path to the top of the wall."

So, yes.

I recommend you get this book. It's verbose and not very readable but it has a bit more on the subject, and the author is responsive to emails.

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