Composite-deck modelling in ETABS

[BAGW]

Hi,

Should the composite deck in ETABS be modelled as rigid diaphragm or a semi-rigid diaphragm? I know diaphragm needs to be modelled as semi-rigid when there is significant in-plane deformation. What does this mean? What is the limit for this?

Also, ASCE states that a diaphragm should not be treated as rigid when there is horizontal irregularity. My building has horizontal irregularity. So do you think modeling the diaphragm as semi-rigid is the way to go? When I model the diaphragm as rigid vs semi-rigid, the results in the braces are significantly different.

Thanks

 

[sticksandtriangles]

What is your diaphragm (concrete slab, slab on metal deck deck etc)?
How did you calculate your semi-rigid diaphragm properties? If concrete did you apply a cracked factor modification to the stiffness?

A post of your diaphragm layout could be helpful as well.

 

[BAGW]

Its a concrete on metal deck.

I used the thickness of concrete above flutes to model the semi-rigid diaphragm. Yup, crack modification of 0.5 has been applied

 

[sticksandtriangles]

This is a little outside my knowledge area (designer in the midwest) but I'll take a stab at it.

My guess as to why a diaphragm needs to modeled as semi-rigid when there is a horizontal irregularity is due to the expectation of high in-plane deformation of your diaphragm (think of your diaphragm as a hopefully a deep beam that needs to span out to the other portions of the disrupted LFRS).

Also if you are in ETABs, I am guessing you have the capability to determine the diaphragm shear that needs to be resisted by taking cut planes through your semi rigid diaphragm. You can compare the output of these shears to your allowable diaphragm shears and see if your diaphragm is sufficent.