The vectors x¯b contains the unknown normalized displacements
and unknown boundary tractions for boundary nodes, the
corresponding matrices Ab and Ai have the dimension 2Nb 2Nb
and 2Ni 2Nb, respectively. The normalized displacement vector of
the internal nodes u˜i has the size of 2Ni 1, while, the normalized
displacement and acceleration vector u˜ and u¨˜ at all nodes has the
size of 2Nt 1, the corresponding matrices Vb and Pb for boundary
nodes have the dimension of 2 Nb 2Nt, and for the internal nodes
matrices Vi and Pi have the size of 2Ni 2Nt. It should be noted that
the sub-columns of the matrices Vb, Pb and Vi, Pi corresponding to
the known boundary displacement nodes should be taken as zero.
The vectors y¯
band y¯
i containing the known normalized boundary
and internal displacements as well as the known boundary and
internal tractions. Then Eqs. (22) and (23) can be rewritten in a
more compact form as
¨˜