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Do you have a 3D model? If you have your model in SAP2000, I have developed a software which can easily export your model from SAP2000 to OpenSEES. for both 3D and 2D Steel Moment Resisting Frames. So I think I can convert your SAP2000 model to OpenSEES.

Why don't you write your script in MatLab and run OpenSEES from your MatLab script?

First, you must know that, eigen command calculates N-1 number of eigen valuse which N is Number of DOFs of the structure which have masses.
Second, Are you sure your structure has not any stability problems?

Eigen command in OpenSEES, calculates eigenvalues using tangent stiffness of the structure in its existing state in the Domain. You can check it on a SDOF Perfectly Plastic model by exciting it with a sinusoidal excitation and calculating its eigen values in intervals.

It depends on the global coordinate system that you have supposed. If you have placed your structural nodes in such a way that your two horizontal axis are in 1 2 directions, then you must use directions 1 and 2 to enforce your EQ to your structure.

Hi,
The other option is rotating the translational masses which is assigned in nodes to any angle you want.

Hi,

In addition to correcting the Wsection command line try the settings below for your analyze:

constraints Lagrange
numberer Plain
system BandGeneral
test EnergyIncr 1.e-7 6
algorithm ModifiedNewton
integrator LoadControl 0.1
analysis Static
analyze 10

hi there. i have examined a 3-D linear 8storey model with SSI effect and a TMD attached. in both SAP2000 V12 and opensees and i got the same results in both of the softwares. the errors in eigen value problem were almost zer0 (about 0.0001%!). i did time history anlaysis under bi-directional ground ...

[quote="silvia"]you can only solve for N-1 eigenvalues, where N is the number of dynamic dofs.[/quote]
thanks for your reply, but i solved it in a different way by assigning some negligible masses to some nodes.

Hi, I'm getting an error message when im trying to retrieve eigenvalues of higher modes from my model. the error body is: BandArpackSolver::Error with _saupd info = -9999 Could not build an Arnoldi factorization.IPARAM(5) returns the size of the current Arnoldi factorization. The user is advised to ...