Diaphragm analysis techniques 2

[WARose]

Kind of a odd question here but.......Is there anything in the code(s) that prohibit diaphragm analysis via FEA software?

I ask because not only do I have a diaphragm that is neither here nor there (i.e. semi-Rigid)....but also, in the seismic part of ASCE 7....it point-blank calls some things rigid and some flexible.....so I was wondering (for future reference) does a FEA analysis with something it clearly designates as Flexible/Rigid break code? Or is it a case where you will just wind up with what it says anyway?

Code verbiage loses me sometimes.

 

[KootK]

I see it like this:

1) Where the code makes recommendations, they are giving you permission to assume flexible or rigid which, most times, is a convenience.

2) You always have the option to do an FEM, semi-rigid analysis. And I'd usually consider that the Cadillac of solutions where a designer is willing to invest the time to do it.

On a related note, Agent666 previously shared a NZ design guide outlining an interesting diaphragm design method that seems to have at least semi-official acceptance over there. I think that this was the doc: Link

 

[HTURKAK]

I do not want to repeat the requirements stated at 12.3
12.3.1.1 thru 12.3.1.3

12.3.1.3 is the criteria for Calculated Flexible Diaphragm Condition.

and the following is excerpt from NEHRP Recommended DESIGN EXAMPLES FEMA_P-1051


(...Typically, the semi-rigid diaphragm will be modeled using shell elements which are semi-rigid in plane, and which have significant out-of-plane bending stiffness. However, the out-of-plane stiffness should be modified to near-zero to prevent the development of bending moments in the diaphragm, which if present, will reduce the moments in the elements of the main lateral force reciting system. (While the diaphragms have the capability to resist bending, the ductility of the diaphragm elements that resist bending is unknown, and is certainly less than the ductility of the lateral load resisting system).

 

[WARose]

Thanks Kootk & HTURKAK.

HTURKAK, I use to work with a engineer who always made at least one run of the model with the diaphragm element(s) designated as "[in] plane stress" only to get that pure diaphragm action. Your quote seems to validate that. (I've done it some myself.)