Metal Deck Flexible Diaphragm with Cantilever 1

[OUe]

I have a 36 foot cantilever on a verco HSB-36, 20 gage roof deck. The depth is approximately 72' for a medical center here in Salem, OR. It is controlled by seismic design. The engineer I'm debating with on this says that the diaphragm is considered flexible under 12.3.1.1 of the ASCE 7-10. So far, I'm having a hard time fighting this one, but principles of mechanics don't seem to be properly addressed in the ASCE 7-10. The "a" dimension on the diaphragm doesn't seem to matter if under 200 ft if you look at page 6 0f 57 of the ER 2078P. I am a new plans examiner with 17 years of prior structural consulting experience. In my gut, I would design the roof deck as a rigid diaphragm and a flexible diaphragm and take the envelope solution for the horizontal distribution of forces as well as the collector design elements. Can anyone comment on the accuracy of just assuming simple tributary loading and neglecting the rotation?

In my opinion, I need to at least have him calculate the diaphragm flexibility to substantiate the design based on the esr report.

Thanks.

 

[OUe]

another issue I have a problem with is the "a" verses "L" dimension. What if the backspan of the diaphragm "beam" is smaller than the cantilever? It just seems that the ASCE 7 is doing a poor job of keeping engineers from taking negligent short-cuts...

 

[wannabeSE]

ER 2079P is valid with the 1997 UBC. The current evaluation report for Verco decks is ER-217 by IAPMO UES,

http://www.iapmoes.org/Documents/ER_0217.pdf.

It includes design requirements. As a design engineer, I would check the diaphragm deflection and building drift before passing judgement as rigid, flexible, or stiff/semirigid.

An engineer may feel comfortable taking an envelope of rigid and flexible diaphragm results. When I am using software to analyze the lateral system and the diaphragm rigidity/flexibility is uncertain, I model the diaphragm stiffness rather than enveloping rigid and flexible.

 

[OUe]

wannabeSE,

Do you actually model the flutes and ribs and orientation of the diaphragm? Seems like modeling it would give you inaccurate results. I'm a big proponent of modeling what you actually have as FEA programs are only as intelligent as the user...

Thanks.

 

[structSU10]

You can model it as a plain shell element, with modified modulus of elasticity so it acts the same. I have calculated the deflection of a specific steel deck and fastener pattern, and then modeled it and adjusted the stiffness until it matched the hand calculation. Then I would use those material properties to use with the real structure.

There is also a procedure outlined by Bentley that you could use as well:

http://communities.bentley.com/prod...ive-elastic-modulus-for-a-semirigid-diaphragm

 

[MotorCity]

Let me preface by saying that I have not read nor am I familiar with the documents posted above. However, in my experience I have never modeled an untopped roof deck as anything but a flexible diaphragm. One of the reasons for this is that the supporting structure is (hopefully) always stiffer than the roof deck. As such, the load on a flexible diaphragm is distributed based on tributary area and has little if any torsional resistance as the deck will tend to deform in plane before it can re-distribute the load to supporting members. In reality, it is semi rigid since the deck also resists out of plane loads and has an out of plane stiffness. The only way to accurately take advantage of that is to use FEA. However, in typical everyday production type of work, such a practice is not efficient and we simply select another deck that will work.