IRC and Wood Diaphragms 1

[cleagl]

Hi Fellow EngTips people,

I am now doing residential homes, and need some opinions. No where (that I can find) does the IRC 2015 mention wood diaphragms. Nothing in Floors (Chap 5) and nothing in Roof-Ceiling (Chap 8). There is a reference in Section 804, but that is for cold-formed framing. Anyone have a reference or guidance here? Seems like they have a whole bunch of prescriptive stuff on shear walls, but what good are shear walls if the floor is absent (so to speak). Some of the floors I have designed using the Malone text(The Analysis of irregular Shaped Structures), but that is pretty long process and I am having a hard time passing the (total) cost on. I'm pretty sure RISA 3D can't do it(which I own a perpetual license) and I can't really afford RISA Floor right now. Any insights into how other residential wood frame engineers are handling this? Also, JoshPlum, if you know how I can do this with RISA 3D any help would be appreciated.
Thanks for any help.

-Cleagl

 

[msquared48]

I would default to the IBC values.

Mike McCann, PE, SE (WA, HI)

 

[phamENG]

Be careful with the IRC. As you said - it has a lot of prescriptive stuff in it. That's really all it is. If a builder or self described and unlicensed home designer can put together a set of plans using the tables in there they can get a permit and build it without a stamp from a registered Architect or a PE.

See R301.1.1, Alternative Provisions. As msquared said - use the IBC. This will probably apply to any wood house you are asked to design. Most architects violate the prescriptive limitations, especially in mid to high end custom homes.

Another situation I've run into is that nobody else involved understands the braced wall provisions. Sometimes, a house meets the prescriptive requirements but the architect/designer isn't comfortable with specifying the braced walls. We've saved ourselves a lot of time and a good deal of money understanding these and applying them. It's important to note that these are not really shear walls, though they fulfill the same purpose. Prescriptive braced walls in concert with prescriptive floor construction (under the specific limitations of the code) create a suitable LFRS as long as you're not in exposure D for wind. If you can't use braced walls, then the only real option is to do full shear wall and diaphragm analysis per the IBC and NDS/SDPWS.

It is tough, and I'll say that there are a lot of engineers out there who will ignore the diaphragm altogether. They assume the prescriptive floor construction will be sufficient and don't check it. If you're outside of hurricane and seismic country, this assumption may be valid (though I'd suggest studying the prescriptive code to truly understand its limitations before attempting it), but no matter where you are your competition will be willing to accept half of your fee to forget about the diaphragm. That's the trouble with residential engineering: everyone wants it cheaper, and few players seem convinced that an engineer's presence is anything more than a hindrance to the project (until something goes wrong, of course).

 

[JoelTXCive]

Have you checked out the AWC's you tube channel?

They have a ton of webinar/training videos available for free. I think they have a series of videos on diaphragms.

https://www.youtube.com/user/AmericanWoodCouncil/playlists

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