Methodology for uncoupling translational and torsional modes in framed structure with experimental validation

Abstract

Torsional forces may be induced in a building during dynamic excitation when the center of mass does not coincide with the center of rigidity. Estimating the modal parameters of a building structure with a lumped mass on each floor is a critical step in the design phase of a structure. The elastic dynamic response of a structure for any generalized dynamic loading is evaluated from its modal parameters. Modal parameters comprising the triad of frequencies, mode shapes, and damping of a piece-wise linear structure shall facilitate for evaluating the damage of the structure, if any, at different stages of its life cycle. Torsion might arise in symmetric buildings, also caused by torsional components of ground motion, torsional mode of vibration, or non-uniform yielding of elements in an inelastic range. Accurate yet easy methods to identify torsional modal characteristics are critical for various structural dynamic, and health monitoring approaches. This paper presents a methodology for estimating translation and torsional frequencies, mode shapes, and damping of a discrete building structure tested on a shake table with a pulse-type base excitation. The presented methodology for evaluating modal parameters depends only on the experimental response and does not need information on the input force/base excitation. The methodology is proposed such that the torsional component in the translational mode and the translational component in the torsional mode is annulled, and the pure translation and torsional effects are magnified. Along with experimental validation, SAP analysis is also carried out to verify the proposed methodology and the obtained modal frequency values are in good agreement. � 2023, Indian Academy of Sciences.