It is recalled the interest of modal analysis of heavy structures on stick model with reduced number of degrees-of-freedom (DOF), at every age of the structure: at the presizing stage, at the design stage, for assessing the results given by large tridimensional Finite Element (FE) models, and at every further seismic review (Seismic Margin Assessment studies, Seismic Probabilistic Safety Assessment studies).
This article indicates a practical means for a simple programing of a modal analysis based FE code on spreadsheet able to handle up to 50 DOF. The beams, of Timoshenko’s type, are supposed moving along a vertical plane, with 2 DOF per node: the horizontal translational DOF and the rotational DOF around the horizontal axis perpendicular to the vertical plane. The analysis follows the classical steps of any FE code.
The programing of such dynamic modal analysis on a spreadsheet is besides quite easy and provides a very convenient tool i) for the intimate understanding of the dynamic behavior of structures ii) for any accurate modal analysis and iii) for any sensitivity studies.