Rim Joist at Multi Floor Wood Framed Structures

[ntattose]

I am designing a three story multi-family structure. The floor framing consists of wood trusses. I am using a rim board to transfer
the shear to the floor below. Is this too conservative? Can I use the sheating to transfer the shear and omit the rim board?

 

[XR250]

I imagine just the truss ribbon and exterior sheathing could be made to work. Make sure to check your top and bottom plates for offset loads. i.e. 24" O.C. trusses and 16" O.C. studs. There has been a discussion on here about that in last few years.

 

[ryaneng]

I don't think it's excessive to require the rim board; I would even say it's quite typical.

Rim board pros:
1. Provide stability to joist ends during construction to prevent racking.
2. Clear load path from each floor diaphragm to rim board to wall top plates.
3. Some framers prefer to not sheet the rim board area, which allows them to use exact 8' or 9' sheets of material in most cases.

No rim board pros:
1. A bit cheaper?

 

[driftLimiter]

The rim Joist can also serve the role of transferring wall and sometimes post loads down, as well as can be used as a chord for diaphragms. The rim can also support the floor diaphragm before the wall is constructed if detailed as such. I don't think I have ever designed a floor like this without a rim joist. However I have used other load paths to get the shear down to the next level.

 

[jdgengineer]

Would modified balloon framing work instead of the rim joist? Essentially, the top plates would be installed high and the trusses hung off of them with top flange hangers. I think this approach is fairly common for multi-family to reduce the potential for shrinkage, but is potentially less critical with engineered framing.

 

[driftLimiter]

Balloon framing is an option definitely. There are some downsides associated with longer cut lengths, anchorage of the wall to the diaphragm for out of plane and in plane forces requires some different detailing. If I understand correctly there are also some fire concerns when balloon framing but that isn't really in my wheelhouse.

 

[ntattose]

To balloon frame a structure lie this would require studs on the order of 31'
long. This would require engineered lumber for studs. It would also be difficult
to get a ledger connection to work. Also, often these multi family structures have
a balcony at each unit. The simplest way to frame the balcony is to cantilever the
floor truss over the exterior wall. I think balloon framing would create a bunch of
new problems.

 

[jdgengineer]

I think the balconies could sway it one way for sure, but modified balloon framing would not require studs 31' tall. It's called "modified" because it is somewhere between platform framing and balloon framing where the top plates are located at the top of the floor joists to minimize shrinkage caused by the rim joist. It may not be applicable to your situation here, but it's used often with multi-family.

We would typically frame balconies with steel rather than cantilever out the trusses due to balcony collapses caused by rot at the interface between the balcony and the exterior wall.

 

[driftLimiter]

The modified method isnt really about avoiding shrinkage due to the rim itself, not to mention you'll probably use an engineered rim. Remember most of the shrinkage is happening cross grain of sawn lumber(flat plates). This method reduces the shrinkage depth per floor significantly. These guys discuss it towards the end and show the detail that you speak of as a way to reduce shrinkage.
Link

 

[RontheRedneck]

I don't see a point in it. Using rim board reduces the amount of area the floor trusses can bear on.

In 3 story structures I typically put 3 verticals on the end of a truss - Two full height one and one that's shortened for a ribbon board.

 

[JStructsteel]

Just a quick look at it, I would want that there for the shear load, or at least some blocking between the trusses.

 

[phamENG]

There was another thread on this recently - or maybe I found it in a search recently - about this issue. I recall one of KootK's nice hand sketches exploring the implications of extending the shear wall below up to the ribbon board rather than stopping it at the top plate so it could effectively brace the trusses at bearing. If I can find it again I'll post the link...

 

[HouseBoy]

I'm curious why the 2x4 vertical (next to the truss) is "needed". Seems like the end of the truss would heave 2 or 3 verticals (per Ron) plus the rim (seems like plenty of gravity load support).

Also, I did not know LSL rims would be made 1 1/2". I usually see 1" or 1 1/8". Would you ever see 2) 3/4" plywood rims? (just curious)

Also regarding Ron's comment about a ribbon board, THAT would preclude spreading the load over the top plate.

I don't do any residential floors with greater than 19.2" max floor framing spacing. Might be more common in multi family though. 16" spacing is by far the more common spacing that I see (usually somewhat "high end" single family)

ONE QUESTION - Do YOU specify where the plywood sheathing joints are located? I have had some plan examiners "require" that an elevation drawing be submitted to show ALL plywood panels (usually I argue against that).

 

[WiSEiwish]

Jumping in a couple of months late. I see this sort of detail all the time, and would say that it is pretty typical. The rim being used to transfer in-plane shear is really nice because it does allow for the wall to be panelized which many walls are these days. The thing to keep in mind is making sure that the connections are capable of resisting the shear force applied. A lot of times the toe-nail from the rim to the top plate is overridden by proprietary clips such as the Simpson A34/A35.
Also make sure you really need 1 1/2". Depending on the height of your building a 1 1/4" may be appropriate.

 

[Aesur]

I don't see why you can't use the sheathing for shear transfer (not looking at gravity loads/blocking requirements for said vertical loading), however the rim would still be needed to transfer the diaphragm loading down to the wall below as the sheathing only transfers the wall loading above unless you overdesign the shear wall sheathing attachment above to drag the floor diaphragm loading up to the rim, then into the shear wall sheathing and down to the sheathing below.