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SAP Modal Analysis After Nonlinear Static

SAP Modal Analysis After Nonlinear Static

SAP Modal Analysis After Nonlinear Static

(OP)
Hello, I'd like to see if you guys can help.

I am trying to run a simple test case:

1. 2-span horizontal cable
2. The two end joints pinned and the middle joint on a roller.  
3. Large mass applied to the node at the middle support.
4. Cables modeled using frame elements with small MOI.

If I run the modal analysis using the unstressed state, I get the exact natural frequency I compute manually using sqrt(k/m).

I want to run the dead load as a nonlinear static case (to get the right member geometric stiffnesses into the global stiffness matrix).

Use the structure's state at the end of the dead load analysis as the starting state for a modal analysis.  I'm trying to test the geometric matrices' effect on the computed natural frequencies.

When I run the modal analysis, the mode with the center node moving doesn't show up at all, regardless of how many modes are included.

I'm just about sure at this point that this second modal analysis doesn't have the applied mass on the center node.  Is there something that I have to do to make it retain the mass?

As another test, I applied a 100 kip load at this center joint, using the dead load nonlinear case as the initial condition.  I got a tiny increase in stiffness, so I should still get the same natural frequency as before.

Thanks in advance.

RE: SAP Modal Analysis After Nonlinear Static

(OP)
Oh yeah, using SAP v10.1.2 Advanced.

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