Confined Concrete Backbone Difference: Fiber Hinge Result weaker than Nonlinear Material Data

[darrennthnl]

Why is the concrete backbone used when calculating the fiber hinge so much weaker than the backbone in nonlinear material (ETABS)?

For the same section, it's as if the transversal reinforcement effect is set to default. Look at the hinge result (left) of a section with 4 legs. It is far weaker than what is shown from the nonlinear material data (top-right). Coincidentally, it is very similar to the nonlinear curve, if the section only had 2 legs instead of 4 (bottom-right).

My manual calculation matches very well with the nonlinear material one, so I feel like that's the correct one. It doesn't seem to be a safety factor issue either, since the shape is entirely different.
Any help is appreciated.

ETABS.V18

 

[Luceid]

Can you provide any other screenshots of the setup of the nonlinear material and the fiber hinges? Always easy to double check that there isn't some user input error....I'll check my fiber hinges in a model I'm working on to compare later today.

 

[darrennthnl]

Concrete:
unconfined strain = 0.002
ultimate strain = 0.0035
final compression slope = -0.1

Rebar:
Strain Hardening = 0.01
Ultimate Strain = 0.086
Final slope = -0.1

 

[darrennthnl]

Can you provide any other screenshots of the setup of the nonlinear material and the fiber hinges? Always easy to double check that there isn't some user input error....I'll check my fiber hinges in a model I'm working on to compare later today.

By the way, I have an additional question, do you define the column with regular concrete frame section, or by using section designer?

 

[Luceid]

Hm - I've only used fiber sections with wall shells, not columns, so I'm not too sure about that

 

[darrennthnl]

how do you go about modeling nonlinear column hinges then?