How to carry out modal analysis for a frame?

[VISHAL BHATIA]

For a multistorey frame, how do we find out the natural frequencies and mode shapes. I tried the eigen command, but it is giving the eigen vector corresponding to each node rather than for the whole structure. Also, it is not giving the frequencies along with the eigen vectors. I used:

eigen 1;
print node.out node;

This command gives the eigen vectors for each node. Also, the mass of the strucutre has to be applied in which directions? I have 3 dof/node, so is the mass to be applied in all the three directions, because the structure can also have torsional modes?

[silvia]

yes, if you are in 3d, consider all masses of interest.
you need to use the node recorder (see the manual) and, yes, you will get the nodal displacements for each eigenvector, so include all nodes.
eigen gives you the eigenvalue(s)
eigen 1 (gives you the first eigenvalue)
eigen 2 (gives you the first 2 in a list)
you need to take the square root and process the eigenvalue to get the frequency.