Flitch column

[JMASE]

I'm working up some design options for a client wanting to use heavy timber columns and beams for a series of one-bay frames parallel to each other for a greenhouse/conservatory. I have very limited options for braced frames or shear walls elsewhere, so these individual frames need to act as the lateral resisting elements. I'm considering cantilever columns for the frames, but the wind moments will be significantly larger than commercial moment resisting timber column connections can handle. So, the idea is to assemble 3-ply timber boards with, say 2 - 1/2 in. continuous plates sandwiched between the wood, and welding the plates to a moment resting base plate. I would call it a moment resisting "flitch column". I've read the DeStefano article, and the transformed section information is helpful. The difference is that in this situation the wind resisting member is a cantilever vs. a pin-pin beam, and, the combined section needs to be able to support gravity loads. Anyone have any insight on something like this?

 

[phamENG]

I did this once, but I put the moment connection at the beam to column connections. In the end, the contractor decided a steel moment frame was easier to build, and probably cheaper.

 

[JMASE]

phamENG, interesting idea: make it a portal frame. With fixed bases, should be much stiffer.

 

[phamENG]

In my case, it was a renovation to an elevated floor. So I couldn't fix the base. Also why I tried the crazy scheme in the first place - easier to carry each individual piece up to the work area.

Either way, steel is likely to be easier to construct and much stiffer and stronger than any amalgamation of wood and steel plates you come up with.

 

[KootK]

At best, with the steel plates option, I see the plates simply being a means of creating joint fixity where you desire it. I doubt that you would generate any meaningful composite behavior until you're several multiples of the member depth away from the joint. And that's unfortunate since the joints are precisely where one would most want the additional strength and stiffness to begin with. In this sense, I don't see the setup being very "flitchy".

 

[HouseBoy]

Agree with KK about the lack of composite behavior.
Here's an example of what I did once before (see pic):

 

[JMASE]

House Boy,
Nice looking project. So, the timber only served to keep the steel plate from buckling? That is, you could count on a transformed section, I, for stability?

 

[HouseBoy]

JM,
Partly that. The wood also served as the support for the log exterior (see pic) which were attached to the vertical pieces individually.
The "live roof" was also fun. That's why we used the "trussed beam" approach.