cantilever wood diaphragm

[keyPitsimplE]

Does anyone know of a GOOD reference for cantilever wood diaphragm design examples? Also, for a 3-sided building, i.e. 3 sides with shear walls and one "open" or window wall?

I am looking at a tiny 12'x18', one story cabin that has slab on grade under the back third (12'x6.5'), and the rest cantilevered over post and pier. To make it even more interesting, the outermost 12' wide wall is all window with 4 posts.

I've got a few ideas for design, but I think I'm stretching the code a bit. Steel moment frame over cant diaphragm??? Seems too much, but this building is so tiny.

Thanks,

 

[dhengr]

I’ve got some good ideas too, but I can’t see what you really have from here. How about some plans and elevations, or at least much better descriptions. Floor framing (12' jst. run north-south, long walls run east-west, etc., not left and right), ceiling and roof framing, wall heights, wind loads, etc. We know it’s 12x18' in plan, but are the side walls 8 or 16' high, plus what to the ridge? Do the outer 11.5' canti. off the slab, or where are the piers and what’s the framing onto them? You need some sort of a moment frame out at the open end, with 12 or 16" returns into the open end, thus only 10' or 9-4" of glass. Alternatively, set the glass real loose btwn. the mullions so they can rack without cracking the glass, and pray for no wind.

 

[msquared48]

I believe Amrhein has one three-sided design example.



Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[keyPitsimplE]

Can you please expand on that book title?

Attached is a 6 page .pdf of the cabin.

Thanks again gentlemen.

 

[keyPitsimplE]

Oh, regarding "praying for no wind"...out of luck there. We are 85 mph Exp. D wind and SDC D1.

We may also design for a 747 impact on the side, just for fun.

Seriously, I'm thinking of a square steel frame surrounding the windows. The bottom of which is a rim beam in the floor cavity. Then carry shear back to the foundation via cantilevered floor diaphragm. The loads will be fairly small due to the bldg size, but keeping it stiff will be the tough part. Also, piers under the frame will get larger than they show to resist HD forces.

If they'll buy it, I might go towards cantilevered columns in the front 2 corners, pole barn style. MUCH simpler, but the footings will be huge, and not I'm crazy about embedding posts in concrete for longevity/maintenance reasons.

 

[MiketheEngineer]

You might build the ceiling and floor as wood sheathed diaphragms. Then use this to support the "windowed" wall.

Just a thought

 

[keyPitsimplE]

Yes, that is a consideration Mike, but that is a "3 sided" shearwall design with torsion, some shear wall capacity reductions and other code limitations, right? I have not done one of those, but sounds like the "Amrhein" book has an example. Anyone know what book that is, or how to direct me to another example?

 

[OldPaperMaker]

keyPitsimplE,
Try Design of Wood Structures ASD/LFRD, sixth edition by Donald Breyer, et al. You could also try the Engineered Wood Association (

http://www.apawood.org).

They have a lot of information available on line and it is free.

The book has some examples of a three sided diaphragm. Generally one has to determine if the diaphragm is rigid or flexible relative to the shearwalls.If the center of the lateral force does not coincide with the center of rigidity, the diaphragm will have the ability to rotate. As a result of the rotation the lateral forces will be redistributed between the shearwalls.

I hope that this helps.

 

[keyPitsimplE]

Thank you. I will look into those.

 

[FSS]

I assume you already checked with designed and/or owner and asked about going from three windows to two so that you could lateral to work without getting crazy. Just had to ask....

 

[keyPitsimplE]

yes. matching an existing residence.

i got it to work with 4 wood "shear posts" tied into a short plywood shear wall that extends below the floor. I'll also detail the roof diaphragm to help carry load back to the side and rear shear walls as if it was 3-sided to make sure it stays stiff around the windows, but not having to prove all the 3-sided building calcs.