cable and modal analysis

[idrogio]

Hi everybody, I would like to make a modal analysis of a structure to find its frequencies, the problem is that the cable element, as stated in the help, is used only in nonlinear combinations, so if you put a cable, the modal analysis will thing, and this is wrong, that it is a frame with compressive resistance.

thanks

 

[stressed]

Modal analyis is always linear. You can define the nonlinear tension only (for example) properties of the Frame/cable, but modal analysis is linear. Modal analysis can capture behavior which can then be used for a nonlinear analysis.

 

[idrogio]

Hi, could you please explain better what do you mean with:

"Modal analysis can capture behavior which can then be used for a nonlinear analysis."

Do I have to apply to the structure a force (in the non linear analysis) with the same frequency that I found on the modal analysis?

thanks

 

[ElmerTheFudd]

You'll have to pick a point at which to perform the modal analysis. For example, on a suspension bridge, you will want to perform the modal analysis after the dead load has been applied, which stiffens the structure. In the SAP2000 Analysis Case dialog box for the Modal analysis, select the load case after which you want the eigenvalues calculated.