Convergene Issue in Nonlinear Time History Analysis

[sarif]

Hello Support Team,

I have been trying to Nonlinear Time History Analysis of a Three Story RC building. I have used InfrmFB elements for both beams and columns with 5 integration points. I have tried all the approaches to resolve convergence issues as mentioned in the manual as well as the ones mentioned in various posts in this forum. I'm attaching the model for your reference.

https://drive.google.com/drive/folders/ ... sp=sharing

[seismosoft]

Before the analysis stops, you reach interstorey drifts more than 2%, which means very significant damage. With the small and weak columns that you have, these columns fail and they cannot sustain the gravity loads, leading to the divergence problems.
Seismosoft Support

[sarif]

Thank you for the prompt response. Also, as evident in the analysis log, several columns have reached core crushing and reinforcement fracture. Could this also be due to localisation issues associated with infrmFB elements? Are the infrmFBPH elements a preferred element class to model this class of buildings?

[seismosoft]

No, probably it is because the structure cannot sustain the loading
Seismosoft Support

[sarif]

Does Seismostruct have a mechanism to monitor axial failure in columns? I am looking to capture the gravity load failure typical of non ductile rc buildings.

[seismosoft]

No, there is no automatic performance criterion for a completely axial failure.
However, you can monitor it indirectly with the strain-based performance criteria or with the definition of stress points.
Seismosoft Support

[sarif]

Thank you. To be sure if I understood you rightly, if I add a performance criterion for 'Core concrete crushing' in the columns, then any section of a column reaching that criterion can be considered as 'axial load failure' for that column. Hope this is what you meant?

[seismosoft]

No, this will not considered automatically as a failure. This is basically a monitoring mechanism, i.e. it notifies the user when a certain strain level is reached. However, you can also specify of how the member behaves with the Strength Degradation parameter (Keep Strength, or Residual Strength or no strength at all).
Seismosoft Support

[sarif]

I have tried with the Residual Strength option. But most of the time, when I run using the 'Residual strength' or 'no strength' option, it results in the analysis failing to converge. Also, I would like to know what is the percentage associated with residual strength as couldn't find that information in the manual.

[seismosoft]

Possibly, the sudden reduction in the strength upon reaching the performance criterion leads to the inability of the structure to sustain the applied loads, hence the divergence. You need to investigate this. Furthermore, the percentage of degradation is a parameter that is selected by the engineer, based on the expected decrease in the member strength upon failure.
Seismosoft Support