Combined Gravity and Time-History Analysis

[Jamildouier]

Dear Stko Team,

I hope this message finds you well.

I am currently working on a structural model consisting of two foundations – an Upper Raft and a Lower Raft – connected by columns and isolators. On the Upper Raft, simplified stick models representing buildings are placed.

I initially performed the Eigenfrequency, Gravity, and Time-History analyses individually – all three produced plausible and consistent results. Notably, in the standalone Time-History Analysis, only the Lower Raft and the columns moved as expected, while the Upper Raft remained unaffected. see images Time History_1 and Time History_0.

In the next step, I attempted to run the Gravity and Time-History analyses together as a combined analysis. To do so, I adjusted the Analysis Steps and the Constraints accordingly. The analysis runs without any technical issues and produces results.

However, the deformation results from the combined analysis deviate from those of the individual analyses:

• The deformation from the Time-History Analysis within the combined analysis does not match the deformation from the standalone Time-History Analysis.
• The same applies to the Gravity deformation: it also differs from the results of the standalone Gravity Analysis.
• Regardless of whether Stage 1 or Stage 2 is selected, the model only deforms in the middle region. Furthermore, the Upper Raft now shows deformation, which was not the case in the individual Time-History Analysis.

This behavior may indicate an incorrect load transfer, an inconsistent stage definition, or a constraint issue introduced during the combination of the two analyses. see image Gravity + TH.

Additionally, the final goal of this analysis is to obtain the Response Spectrum at the level of the Upper Raft. Therefore, it is essential that the deformation behavior is correctly captured in the combined analysis.

Please find attached a small example to illustrate the concept more clearly.

In this example, only the relevant case with the corresponding properties and analysis steps is included. This example is not my actual model, but rather a simplified representation of the concept.

However, the properties, conditions, and analysis steps used in this example are the same as those used in my original model.

Do you have any idea what might be causing this behavior? I would greatly appreciate any hints or feedback.

Thank you very much in advance and kind regards

[STKO Team]

Are you sure you sent the real model?
this one doesn't run, all G are zero!