How Opensees deal with equation of motion

[ningxz]

# -ndm 2 -ndf 3 cantilever
O --> m node 3
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O --> m node 2 (When modeling in opensees, only the horizontal DOFs have mass, m; element: beam-column element )
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_|__ node 1
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When doing the dynamic analysis, the equation of motion can be written as below:
Ma+Cv+Kd=-MAg
then, for a 2 dimention 3 DOF cantilever described as above, the size of matrix M is 6×6 or 2×2, namely
M=[m 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 m 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0 ];
or
M=[m 0
0 m];

As a freshmen,I prefer to the first way but not quite sure. Please give me a help。

[fmk]

not sure what your question is, rephrase it please

[ningxz]

rephrased, thank you

[fmk]

m is the first way .. we do not do static condensation as there is no point in doing so as numerically (assuming exact arithmetic) the results are exactly the same which you can show with a little algebra.