Determining the drift of a structure with a flexible diaphragm (in RISA 3D)

[SeizeTheMoment]

Hello

I'm trying to determine the story drift of a structure with a flexible diaphragm on a standalone RISA 3D. I don't have RISAFloor so I am unable to use its diaphragm calculation features. I am designing a steel structure with a 1.5" metal deck. Would you model a flexible diaphragm to be the one on the left, or add plates to similar to the right?

And in the flexibility questions - what is the chord element if the metal deck is directly attached to the WF-beams? Do we always have to use an edge angle as the chord element?

Thanks in advance.

 

[driftLimiter]

For the purposes of this story drift check, you are implying that diaphragm deflection also be included. I believe this is a requirement in ASCE 7-22 but not in -16.

It is more complex this way because Cd R and I only apply to the MLFRS and the diaphragm deflection is based on Fpx.

Unless you can find a way to wrap all that into one model (I don't recommend this). You are left with cellulating the MLFRS drift from your model then manually determining the diaphragm deflection and adding them up.

Here is the same section from 7-22.

 

[JoshPlumSE]

Not exactly sure what you're asking. Or, what you're modeling.

So, a "flexible" diaphragm is one where the diaphragm is not assumed to have ANY stiffness and where the diaphragm loads are assigned to the frames or walls based on tributary area.

It appears to me that your model is something more like a "semi-rigid" (or semi-flexible) diaphragm. This would be where you attempt to model the stiffness of the diaphragm more accurately than just assuming rigid or flexible behavior.

I'm not sure that there is anything you can determine from this model about the ability of the diaphragm (and other members) are sufficient to support the top flanges against beam buckling.