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ETABS 2016 - Diaphragm forces

ETABS 2016 - Diaphragm forces

ETABS 2016 - Diaphragm forces

Hi all, I am relatively new to ETABS although with extensive experience in FEA, and I am a bit puzzled about the results that ETABS provides compared to what is described in the CSi documentation.
I am analysing an eccentric building where the first mode is torsional (activates 37% of mass) and the second is combined translational (activates 31% of mass along Y) and torsional (activates 19% of mass about Z).

Floors are modelled as thick shells with semi-rigid diaphragm constraint, which means that the membrane stiffness adopted in the calculation is the real one. I therefore expect to obtain big shear forces at the interface of the narrow slab portion connection the two big slabs.
When I cut a section at level 7 slab between I get these results

The force 1 should be the shear along X, and force 2 should be the axial action along Y, is that correct?
If so, the shear should be 345kN?! that's it?! for a RSA?!!!!!
And when I display the deformed shape of the floor, it looks like it rotates as a rigid diaphragm!
In addition, I get slightly less shear force 320kN at the same section on the 2nd level! How can that be possible?!
Any comment is appreciated.

RE: ETABS 2016 - Diaphragm forces

I believe you're correct with Force 1 in X and Force 2 in Y. CSI's Section Cut FAQ is here if you haven't reviewed it already: Link

Really, really tough to tell whether the force is appropriate without knowing a bunch of other things and digging in. We don't know building height, level of seimicity, slab thickness, etc. Section cut is 11.2m, assuming it's cut maybe a little outside slab on each end and actual width is 10m that would give about 56 kN/m of shear (2.4 k/ft in US units). That doesn't jump out at me as unreasonable but again really tough to tell without knowing a bunch of other things.

Since you're doing RSA, the items I see issues on most often:
  • Make sure your response spectrum function is set correctly
  • Make sure your mass source is correct
  • Make sure your scale factors for modal cases are appropriate (Link)
  • Make sure your modal cases are scaled to whatever minimum values your code requires (for example: in ASCE 7-05 and -10 MRSA base shear needs to be scaled to at least 85% of ELF base shear, in ASCE 7-16 that jumps to 100%)
  • Make sure your modal combination procedure is correct (typically CQC in the US)
  • Make sure your directional combination procedure is appropriate for whatever case you're looking at
If you think your diaphragm is behaving too rigidly, one thing you could look at is to turn your in-plane stiffness modifiers (f11, f22, f12) for your floors way down and make things more flexible. Even if it's not realistic, can help you convince yourself things are working as they should. If you've got beams and columns modeled south of the section cut you may want to check to see if they're sucking up any lateral load. Would think with core being significantly stiffer that they wouldn't but with the thinner slab connection back to core I guess it's possible.

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