curvature problem.

[jboshra]

Dear Sir,
I performed a time history non linear inelastic analysis (using a given ground record) on a 2D frame representing a three-span bridge with two piers fixed at bottom and hinged to the deck at their top; I recorded the moment and the curvature at each pier base. Also, I scaled the ground record to give Sa(T1)=2.4g and applied it to the bridge for two different cases, these two cases are :
1-The steel ratio in the first pier is ≈ 1.8% while in the second pier the steel ratio is ≈ 1.5%; the absolute maximum curvature at this scale was 0.024 and 0.092, respectively.
2-I increased the steel ratio in the first pier to be ≈ 3.2% while in the second pier the steel ratio is still ≈ 1.5%; the absolute maximum curvature at this scale was 0.024 and 0.092, respectively.
The problem is, when I increased the steel ratio in the first pier I got an increase in the moment demand only, not in the curvature. This means that the curvature demand is not affected by any increase in the reinforcement ratio. I am questioning what is the reason for the unchanged curvature?
Kindly note that, however, when performing a section analysis for the two cross sections (the one with a steel ratio of 1.8% and the other with steel ratio of 3.2%, I get different maximum curvature (0.057 and 0.046, respectively). I mean by this maximum curvature capacity the curvature corresponding to a pre-specified maximum strain attained in confined concrete. That means the steel ratio affects the curvature value in section analysis while it has no effect while performing the time history analysis.
Note:
1-I used nonlinearBeamColumn element for columns (Force-based elements).
2-I used minmax material for concrete as follows:
uniaxialMaterial MinMax 4 1 -min -0.014 -max 10
3-I used steel01 material for reinforcement as follows:
uniaxialMaterial Steel01 3 42000 20000000 0.01

[silvia]

if you are enforcing a strain limit on a section you will, likely, get the same curvature limit.

[jboshra]

I used limits on the material strain in both time history and cross-section analysis, and I got difference in the curvature limits between two piers (the one with As ≈ 1.8% and the other with As≈ 3.2%) in the cross-section analysis, but I didn’t get any difference in the curvature limits between the two piers while doing the time history analysis.
Thanks for your prompt response; I really need your help to solve this problem.

[silvia]

look at what is going on everywhere.
the demands may not be high enough, it is all model dependent, so i can't help you too much on this one.