Non-Classical Damping in the Soil-Structure System and Applicability of the Linear-Spectral Method to the Calculation of Forces

Authors:  
Rubric:     Theoretical and experimental studies   
Annotation:
Previously the author has published several papers with analyses of three-mass system for the kinematical excitation applied at the platform. This is a rough model of a soil-structure system: lower spring and dashpot model soil, the rest of springs dashpots and masses model structure. The author compared direct and modal approaches with the initial and the cut-off damping values. The results were compared in the format of transfer functions and accelerations. In this paper the author compares the results in the format of internal forces, adding linear-spectral method to those mentioned above. It turns out that for narrow-banded excitation in certain frequency range the abnormal damping effect (increasing of response along with increasing of damping) is valid for internal forces, and not only for accelerations. Spectral approach gives the results close to the time-domain approach and does not change the main conclusions. For broad-banded excitations (like seismic one) the effect of damping is “normal”: cut-off damping leads to the increase in resulting forces. Results both for cut-off modal damping and for cut-off dashpot viscosity parameter are conservative as compared to the initial values. Spectral approach gives the results close to the time-domain approach, keeping the overall effect of damping. The main conclusion is that the frequency content of the excitation plays crucial role in the applicability of modal and spectral approaches to the dynamic analysis.