cracked stiffness?

[pIwhabl]

Hi guys, I have a question that I wish you can share your opinions.

I have a core wall that looks like this (see below plan view) with two side walls. I have modeled it in FEA package and the bending moment along the core wall is a lot smaller compared to a cantilever model. I believe it is due to the outrigger effect from the side walls.
My question is would you crack the slab completely to ignore the contribution from this outrigger effect? I find it not a cost-effective way to do so I am thinking about say 0.25Ig for the slab alone (if I walls are 0.25Ig that will be basically the same as the uncracked model).
What do you think?

 

[BulbTheBuilder]

Not a subject matter expert, but what property is assigned to the slab. Is it designated as rigid or semi-rigid? This normally affects the forces in the diaphragm

 

[Luceid]

What code are you designing for?

For ACI - you can either use the approximate factors (0.25Ig for the slab - as you noted) or actually analyze the degree of cracking using nonlinear modeling or by calculating the degree of cracking yourself and applying a modifier that way (likely with iteration, similar to how the nonlinear modeling would do it)

 

[pIwhabl]

Understand what you are getting at but the diaphragm effect is very minor in this case.

I am designing to Australian code, which has similar approximate factors. But what I am getting at is not really about the value but more of a design methodology. ie Would you consider 0Ig for the slab to ignore the outrigger effect so that the bending moment of the core matches the cantilever model?

 

[Just Some Nerd]

I'd also be destiffening the walls for out of plane moments (e.g 0.1)

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Why yes, I do in fact have no idea what I'm talking about

 

[hardbutmild]

That is bending moment of plate you're talking about I guess. 0,25 should IMO be a reasonable guess for the lowest expected stiffness, but you could check that by simple analysis of a completely cracked plate.
The question for me is how certain are you that during an earthquake (if it's of concern) this plate will stay "elastic".