Modal Analysis: Fiber model and Elastic Model

[Syun]

Dear STKO Team,

I hope this message finds you well.

I am currently attempting to simulate the E-defense shaking table test using a fiber section model for a ten-story structure with a fixed base. However, after completing the fiber model and running a modal analysis, I noticed a significant difference in the natural period compared to the test results.

In the test, the natural period in the x-direction is 0.85s and in the y-direction is 0.58s. In my fiber model, the natural period in the x-direction is 0.44s and in the y-direction is 0.35s (Figure 1). This suggests that the fiber model is significantly stiffer than the test results.

To ensure that there are no issues with the size of my fiber model, I also created an elastic model, keeping all properties the same except for replacing the fiber sections with elastic sections. After running the modal analysis on the elastic model, I found the natural periods to be 0.78s in the x-direction and 0.52s in the y-direction (Figure 2), which are closer to the test results.

I am unsure how to adjust my fiber model to address this discrepancy and would greatly appreciate any advice or suggestions you could provide.

For your reference, I have attached both models (with units in N, m, kg).

Thank you very much for your help!