dynamic damping; Rayleigh damping; material viscosity
Sources of dynamic damping may be various. Mostly, the damping is implemented into calculations in a form of introduction of damping forces, as a product of the velocity vector and the damping matrix in an equation of motion. In practice, the damping matrix is usually assumed to be a linear combination of the mass matrix and the stiffness matrix (so called Rayleigh’s damping). This kind of damping primarily assumes the external environment viscosity as the source of damping, even though the part of Rayleigh's damping with the stiffness matrix implies the internal damping of the material. Explicitly, the internal viscosity of the material is respected using the appropriate material models. The relation between the Rayleigh damping and the Kelvin-Voight viscosity is shown in the paper. Dynamic damping occurs even when using non-elastic materials, where the unloading takes place in a different path from the loading and thus it leads to dissipation during loading cycles. The paper deals with the comparison of different types of damping of the oscillation of a building structure. The main aim of the paper is to recommend to apply the viscous material model instead of the obsolete and physically unjustified Rayleigh damping in the nonlinear dynamic time analysis of structures.