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(ETABS v21.0.0) Why is my designed rebar percentage suddenly become overstress.

(ETABS v21.0.0) Why is my designed rebar percentage suddenly become overstress.

(ETABS v21.0.0) Why is my designed rebar percentage suddenly become overstress.

(OP)
Why is my designed rebar percentage suddenly becoming overstressed in story 9, the 2B and 3B columns have a very close amount of bending moment compared to the base level column of 2B and 3B.

I have obtained several results resembling this, when there are a huge amount of bending moments at the top of the building and designed rebar percentage at around 4% then reduced to 1% at the following stories, and suddenly it became O/S (overstress)(mostly because of P-delta exceed 1.4).

Additional Information
-ACI318-19 standard
-Spacing between columns is 7 m
-Story height is 4.2 m
-Column's concrete strength: 50MPa, 2B and 3B columns size is 400x1200 mm, 11B and 4B column size is 400x400 mm.
-Slab's concrete strength: 28 MPa, slab thickness is 350 mm.
-Non-sway
-No Lateral load, just compression
-Load combination, 1.2D+1.6L+0.5Lroof, 1.4D, and envelop.

RE: (ETABS v21.0.0) Why is my designed rebar percentage suddenly become overstress.

Quote (Ruchadaphon )

...rebar percentage suddenly becoming overstressed
Were the stresses reasonable before? If yes, what did you change in the model?

For you story height and column sizes, I don't except M2nd order> 1.4M1st order for slenderness. I don't enough information to explain why but at the top you will often have moments with low axial forces. As a result, you'll have your column behaving more like a "beam" due to flexural stresses than as a column.

Can you attach the model? and which version of ETABS do you use?

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