"Normal" and "Abnormal" Effect of Damping on the Dynamic Response of Linear System
Specialists working in earthquake engineering generally are sure that increase in damping causes the decrease in response; e. g. response spectra for increasing damping decrease in the whole frequency range. Hence, underestimation of damping in analysis leads to the conservative results. The author shows that even elementary SDOF oscillators sometimes in certain frequency ranges demonstrate “abnormal” effect of damping on the dynamic response (i. e. response increases along with the increase of damping). For the modes with frequencies in such frequency ranges, the underestimation of modal damping leads to the underestimation of response. This effect is more pronounced for the narrow-banded excitations. Broad-banded excitation mitigates this effect, shifting the frequency range of abnormal damping to the low natural frequencies of oscillators. The abnormal damping effects may be of special importance in two special cases: for seismo-isolated structures with very low natural frequencies, and for high-frequency dynamic excitations (like aircraft impact).