Natural Periods After Nonlinear Time History Analysis/Nonlinear Static Analysis

[gordan1992]

Hi All,

I had a single column member with both fixed ends. I set up the plastic hinges properties and assign them to the column ends. Then I do the following:
1. Run the modal analysis (no load acting on the column) => get the natural period
2. Add a nonlinear static case, and apply a lateral load at the middle of the column until I see the plastic hinges (green).
3. Set a modal analysis case after the end state of step 2 (i.e. based on the reduced stiffness matrix of step 2) => the natural period (damaged state) is larger than step 1 (make sense)

4. I replace step 2 to a nonlinear time history analysis and I keep increasing the scale factor until I see the plastic hinges (green); however, when I run the modal analysis like step 3...the natural periods does not change.

Then, I tried to use a larger scaling factor in the nonlinear time history analysis (e.g. 3g) I saw some pink/red plastic hinges and do step 3 again...the natural periods changed ... (larger than the original one).

Why would it be like that???

 

[gordan1992]

I have found out the answer, thx