Unusual Platform Framing Scheme

[WAstruc10]

I am working on a mixed-use project that is pretty typical for the pacific northwest...3 or 4 stories of wood-framed apartments platform framed over two stories of post-tensioned concrete flat plates (for parking & retail). However, the owner and contractor think it might save them some money to get rid of the upper concrete slab and align the apartment bearing walls with column/steel beam lines below. So the "platform" level that would typically be concrete designed for the wood loads where they happen to fall, would instead be steel beams and composite deck. Or, taking it a step further, they've asked if big glulams and a wood diaphragm could get us there. My questions to you smart folks out there:
1. From a lateral perspective, if the diaphragm stresses were manageable, is a wood diaphragm permitted to be used as a "transfer diaphragm" to take the wood shearwall loads and distribute them out to the concrete walls below? No code prohibitions are jumping out at me, but still...
2. Is a wood diaphragm allowed to support the out-of-plane loads of the perimeter concrete walls/concrete moment frames?
3. Could I still rationalize a 2-stage Equiv Lateral Force analysis if a wood transfer diaphragm is used?
4. Any other issues come to mind before I go too far down this road? I think they've thought through the fire rating issue already and that is apparently not a hangup.

Thanks!

 

[MikeHalloran]

I wouldn't want the apartment above where the guy with the leaky gas tank parks, regardless of whether his wreck catches fire or not.


Mike Halloran
Pembroke Pines, FL, USA

 

[MikeHalloran]

Similarly for the guy who only reinstalls the mufflers on his motorcycle to get it inspected.


Mike Halloran
Pembroke Pines, FL, USA

 

[KootK]

Your team must have stumbled across the same APA document that I did recently:

http://www.apawood.org/docs/2013/N110.pdf

Check it out. There's a video too. It might be worth contacting APA as they're clearly trying to market this system.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[RFreund]

1. I'm pretty sure this is allowed, why wouldn't it? If it were all wood would this change your opinion?
2. Wood diaphragms support masonry out of plane, I imagine this would be similar and same rules apply (sub-diagrams etc..) but fair question.
3. I would think so. Basically it is as if you have masonry bearing/shear walls, right? Although I may be missing something here.
4. I would also be hesitant as well, but I don't have a good reason. Probably because it's pretty it is out of the norm, but the wood industry is making a hard push these days, which I think is good. I suppose one question is - what is the LFRS for the floor with the wood deck?

EIT

http://www.HowToEngineer.com

 

[MileyDiley]

The big design problem won't be diaphragm shear but dragging huge loads into the concrete walls and supporting uplifts from 4-story stacks of shear walls. Recently did prelims and was coming up with 20+ kip drags and uplifts, which would basically require pretty crazy connections (say a big angle that bolts to the side of a CMU wall and down a long length of glulam beam). Anyway, that project changed back to concrete. I've seen projects completed with it, but it seems like a nightmare. I heard of one project where a beam was not cut long enough to pick up a big uplift, and the structural engineer had to create a "FIX" for this that involved not removing the beam, which I believe was around a 10x30 glulam. I wish I could've seen what was detailed...

Just for fun, look at page 3 of that APA pdf. There are (8)-1/2" bolts to pick up the hanger (with the bottom row strangely low on the beam) and you can see the washers at what look like 1" threaded rods extended through the beam.

Beyond the structural design, the switch from concrete to wood deck leaves the plumber with quite a mess. Instead of running lines under a relatively flat PT deck, he'll basically have a 24-36" deep grid of random beams to avoid. I wouldn't be surprised if the switch to wood ends up adding about two feet of thickness to the level.

/donotwant

 

[KootK]

Yeah, the article also mentions that use of wood podium levels requires intense coordination. I think that probably amounts to an enormous project management headache for all involved.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[dcarr82775]

I have never seen a piece of wood stop a determined plumber before.