Load Dependent Ritz Vectors is an approximate solution to Eigen vectors but it produces results that are the same or nearly the same as the Eigen Vectors if enough number of Ritz Vectors is included. For the same number of modes, Ritz Vectors usually provide a better mass participation and it is computationally faster with almost the same level of accuracy.

The Ritz Vectors method is recommended where the solution with Eigen Vectors may not reliably capture lateral structural models (for instance, failing to capture 90 percent mass participation with a reasonable number of modes) or the Eigen Vectors solution captures irrelevant vertical models due to vertical vibration of slabs (although those are real modes but they are not relevant to the structural analysis). The method of Ritz Vectors also has the advantage of being generally faster. For this reason, use of Ritz Vectors is also an attractive for very big models.

The Ritz Vectors can be also used in Response Spectrum Analysis. It is observed that when mass participation is at least 90 percent or more, calculated story deflections , member forces, etc. are almost the same when a response history load case is run with Eigen vectors or Ritz vectors.

The current implementation adopts Load Dependent Ritz Vectors and the reader is referred to the following references for further technical information: Wilson, E.L., at. al. (1983) and Leger, P. at. al. (1986).