ETABS ASCE 41-17 Auto Plastic Hinges

[EagerBeaver77]

Hi everyone,

Background:
I’m currently working on a school project where I aim to build an Artificial Neural Network (ANN) that can predict the Park-Ang Damage Index (DI) for simple 1–3 story RC structures. This ANN will be potentially useful in preliminary seismic assessment, retrofitting prioritization, evacuation decision-making, and DI based hazard mapping. To train the ANN, I’ll need a large dataset, so I’m developing a fast, general, and automatable procedure to calculate DI. Currently i am using pushover analysis in ETABS with auto plastic hinges + N2 method to estimate the structures' nonlinear displacements, and then calculate the DI. Manually defining plastic hinges (providing P-M2-M3 and moment-curvature datapoints) would not be very practical to churn out large dataset.

Problems:
Before running the analysis, the backbone curves of the generated plastic hinges looked normal (see Image A). However, after the analysis, some column plastic hinges showed backbone curves that collapsed (or nearly collapsed) into a vertical line (see Image B). Is this behavior normal, or does it indicate a problem in modeling or analysis setup? If it’s a problem, what might be causing it and how can I fix it?

Additional information:
- I’ve attached Image C, which shows the parameters I used for the column plastic hinges.
- I'm using modal load type for the nonlinear static load case. Since the models are simple structures, where the first mode generally dominates, I've based the load case on the first mode.
- For the solution scheme, I'm using "event-to-event only" and I've also adjusted the parameters to make the result more accurate (lowering event-lumping tolerances, increasing maximum events per step, lowering minimum event step size, etc).
- I’ve also tried deleting and redrawing the elements where this strange behavior appears, but the issue still persists.

Image A:

Image B:

Image C:

Note:
- "Gravity" is a linear static load case consists of 1*live load + 1*dead load + 1*super dead load.
- "Modal pushover load" is the nonlinear static load case for the pushover analysis.

Thank you so much!

 

[raul_robalino88]

Dear friend, did you find any solution to that problem. I am facing the same in a project

 

[Luceid]

If you double-middle click while in the plot, does it fix itself? I think the X-axis is just scaled improperly

 

[EagerBeaver77]

Sorry for the late reply, I don't think scaling is the problem here, because the x axis is already zoomed in so much. As you can see it is in the scale of E-6. If it is zoomed out further, the graph would just appear as a vertical line. I have not found the solution yet, but I think I have figured out what might be the cause of this problem.

Some assumptions I used in the modelling:
1. ASCE 41-17 auto plastic hinges.
2. Modal pushover load case (not lateral load like triangular load) to push the structure.
3. Modal pushover load case to calculate axial + lateral load in the columns, and shear in the beams, which the software will use to build the hinges backbone curve.
4. Column hinges' degree of freedom is P-M2-M3

Some observations I made:
1. This problem doesn't occur when I use lateral pushover load case like triangular load.
2. The unusual hinge backbone curve (as in image B) only occurs in columns. The beams' backbone curves, after running the analysis, look normal.
3. ETABS only calculate 1 axial value to define column backbone curves.

The backbone curve in image A appears normal because it is normalized by scale factors. But the actual backbone curve, would be different, especially for columns, because of the scale factors, which depend on axial load, yield moment, and the P-M2-M3 interaction. I suspect that this problem is convergence issues, because maybe the program somehow cannot calculate appropriately the axial load, and hence retrieve the correct M2 and M3 to build the backbone curve for the analysis, or maybe the program cannot interpolate the moment in the plastic hinges when different axial load occur in the actual analysis, if you use modal pushover load case. I am not sure, because I don't really understand how the program generates the plastic hinges. I still don't know how to use the auto plastic hinges for modal pushover analysis. I would appreciate any suggestions, corrections, or explanations. Thank you so much!