Metal Deck Diaphragms (Flexible or Rigid???)

[vandede427]

The book "Designing with Steel Joists, Joist Girders, and Steel Deck" by Fisher, West, and Van De Pas is the best book out there concerning this type of construction. I've learned a lot just by reading this book. However, the question arose as to what type of diaphragm is a metal deck roof and I can't find anything in this book about it.

What is it usually: rigid or flexible?

I realize that by the IBC definition a flexible diagphragm is where the in-plane deformation of the diaphragm is 2.5 times that of the average story drift. Also, IBC 1617.5.3 states that diaphragms of untopped steel decking or structural wood panels MAY be designed as flexible. MAY is a relative word.

I understand that you'd have to run the calcs to determine the in-plane deformation, but I'm just looking for a good rule of thumb here: i.e. concrete decks are rigid, wood floors are flexible, metal decks are (fill in the blank).


p.s. JAE, you've got t-minus 30 minutes to respond with an answer, OR ELSE!!!

 

[pelotoner]

Flexible.
I never design them as anything else.
(we only have wind here in FL, verify with the p-wave folks elsewhere)

 

[DaveAtkins]

Flexible, unless that doesn't work.

Then check the 2.5 rule.

DaveAtkins

 

[youngstructural]

I hope you work with JAE, or there's some other reasonable, rational, explanation for your PS....

Maybe I've just forgotten my sense of humour at home.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[JAE]

Yikes! I lose!

 

[dougantholz]

Flexible for the most part. Smaller square'er diaphragms are often fairly rigid. Word of caution when considering un-equal stiffness vertical elements. Torsional re-distribution will occur.

 

[youngstructural]

Dead right Dougan... The assignment of loads to the supporting structure should be done with a great deal of care, particularly in seismic areas.

Length squared is a good start, proper stiffness formulations are even better. There have been several posts on the topic you should be able to find with ease.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[msquared48]

Depends on the shape of the diaphragm, the depth and thickness of the steel deck, how often the deck is stiffened from lateral buckling by the joists, plus the connections to tholse same joists.

Bottom line is I can see it going either way.

If there is a normal weight concrete infill slab too, well, that's a no brainer...

Mike McCann
MMC Engineering