general link

Question:

Hi, Sir/Madam

The ballast of track was modeled by general links. the ballast properties are give in vertical, transverse and longitudinal. The rail and beam are linked diagonally with general links:

is it possible to define general link properties in global coordinates to follow given values? for example, I know vertical spring between rail and structure will be 2100k/ft and transverse will be  420k/ft, longitudinal spring in y direction will be nonlinear.

thanks,

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Answer:

Hello,

The best way to go about this would be to create vertical elastic links and provide required stiffness to the relevant components.  Kindly refer the image below.

Hi, 

The elastic multi-linear link doesn't work for time history analysis. Do you have any non-linear link works for Both of

work for both nonlinear construction analysis and time history analysis?

Hello,

That's a good question!  So, to go about with the expected behavior, both the general links and multi-linear links would have to be defined between the 2 nodes.
Kindly refer the attached documentation on how both could be defined and used simultaneously.

Regards,
Nandeep
Technical Manager, MIDAS

Hi, Nandeep

In your analysis guide, it didn't show. is the elastic multiline link good for pushover analysis?

thanks,

Hello,

We will update the manual.  In the mean time, kindly refer the global control for pushover analysis.

Hi, Nandeep

I don't understand how it works. if I define both of General springs and multi-linear link with very small initial value, no matter construction stage nonlinear analysis or non-linear time history analysis, both needs Eigen value analysis, how do you calculate Eigen value with those initial soil spring stiffness?

Hello,

Eigenvalue analysis would be required if the analysis type is modal.  In that case, since the links have negligible property, for eigenvalue analysis, those would be used, rendering springs to have almost no stiffness.
Generally with non-linear time history, this issue could be avoided by using the Direct Integration analysis rather than modal.

Dear Nandeep

My problem is about the damping: I need structural frequencies to determine the Rayleigh damping coefficients. so, how can I get the structural nature frequencies to calculate the Rayleigh coefficients.

see below screenshot.

thanks,

Hello,

So, for non-linear time history analysis, General Links would be considered.  However, for Eigenvalue analysis, the stiffness of the link would be linear.  So, for mode shape, the link stiffness would have to be approximated to be linear, especially for eigenvalue analysis.  Theoretically, it isn't possible to give an absolute answer as to what this linear stiffness should be.  Thats completely engineer's preference.
Based on my recommendation earlier, for eigenvalue analysis, these springs would have negligible stiffness.  If that isn't acceptable to find the mode shapes, I'd recommend different models for TH and CS analysis, especially if Rayleigh damping has to be done.  If however, it is acceptable, then the mode shapes could be found in the same model and the damping ratios could be entered.