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

[Ruchadaphon]

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.

 

[BulbTheBuilder]

...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?