We study efficiency of the method of concentrated masses in application to approximate analysis of free oscillations of a plane elastic girder of orthogonal structure with a family of unidirectional diagonal columns. A strict structural linear static theory for the girder was developed in . The study is based on the exact statement of the problem of free oscillations of a girder with discrete mass characteristics and on the procedure of its reduction to the generalized discrete problem on eigenvalues. The theoretic result obtained can be applied, jointly with the results presented in the article , for strict statement of a more general problem of free oscillations of girders with discrete-continual mass distribution. The results are illustrated by an example.