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# ETABS - ACI 318-19 Generation of Biaxial Interaction Surfaces of Concrete Columns Question

## ETABS - ACI 318-19 Generation of Biaxial Interaction Surfaces of Concrete Columns Question

(OP)
Good Morning.

I have a basic question, I am trying to verify manually the biaxial interaction surface curves that ETABS generates using ACI 318-19 formulas.
As expected I have no problem when doing it along the mayor axis (Angle=0). However, once i evaluate the rotate it, the curve doesnt match on the transition phase for the phi factor.

I am using the latest transition factor formula as stated on ACI 21.2.2 as
When ety<et<ety + 0.003
Then phi= 0.65 + 0.25 (et-ety)/0.003

After some trial an error, and noticing that the missmatch was at both the limits and the factor I assumed that etabs is modifying the concrete and steel strains with a factor c as shown next:

When ety + c < et < ety + 0.003 +2c
Then phi= 0.65 + 0.25 (et-ety +c)/(0.003 +c)

My question is, am I wrong with this assumption? I couldnt find anything on the codes nor Etabs manual to find a reduction factor for the strains once the section is rotated.

I appreciate any insights. Thanks

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