Cyclic Moment-Curvature analysis issue: Lower bending moment and missing concrete pinching compared to OpenSeesPy

[magnades]

Hello STKO team,

I am performing a cyclic moment-curvature analysis on a cross-section to replicate laboratory results. I modified a standard STKO example file by implementing a Path timeSeries with the target cyclic history.

To map the laboratory curvature to the control node rotation in the model, I am using a plastic hinge length Lp of 22.5425 inches (which explains the multipliers in the monitor actors).

The cross-section was built using the STKO GUI. I verified that under a monotonic analysis, both STKO and my standalone OpenSeesPy script yield almost identical results. However, during the cyclic analysis, I encountered two major issues:

Moment_Curvature_Comparison.png (643.97 KiB) Viewed 150 times

output.png (212.52 KiB) Viewed 150 times

• Capacity Drop: The STKO cyclic curve reaches nearly half of the expected bending moment capacity compared to OpenSeesPy.

• No Pinching: The STKO model does not exhibit any concrete pinching behavior during unloading/reloading, which is clearly visible in OpenSeesPy.

I am attaching my STKO model (v4.1.0). It includes three plot monitors for comparison:

• "Moment-Curvature": The current response.

• "Moment-Rotation": With the experimental lab results as background.

• "Moment-Rotation_OPS": With the OpenSeesPy / monotonic background.

Could you please check if something in the adaptive cyclic displacement control loop or the integrator setup is scaling down the history or clipping the material hysteresis?

Thank you for your help!

Best regards,

Juan Pantoja

Pd. STKO model

05_Stuart_Stko.rar

(386.16 KiB) Downloaded 15 times

Expected behaviour

[STKO Team]

The shared folder has only the STKO model so we cannot compare it.
Moreover, the results I obtain are not the same as those posted.
Double check your model, because OpenSees (used by STKO) and OpenSeesPy share the same code, so if the model is the same they should give the same results