大型震動台実験に基づくRC造建物の振動特性の分析

Natural period and damping factor are important values in seismic design because of their impact on the response of superstructures when their behavior during an earthquake is estimated. However, commonplace earthquake observation records contain various effects, as respresented by the soil–structure interaction. This study focuses on the records of large shaking table tests and investigates the vibration characteristics and time shift for reinforced concrete (hereinafter RC) specimens subjected to more than one large amplitude seismic motion. The experimental data are analyzed using the autoregressive exogenous (ARX) model. Moreover, the effects of the maximum deformation caused by the large seismic motion on the vibration characteristics are investigated.The main results obtained are as follows:1) The first damping factors for the three different types of RC specimens with first natural periods of approximately 0.2, 0.4, and 0.5 s were approximately 8~10, 4~8, and 1~2%, respectively. The result showed a slight variability in these values. Meanwhile, the second natural damping factor only had a little variability in the specimens with first damping factors of approximately 3, 2, and 1%, respectively.2) The first and second natural periods and damping factors tended to progressively increase even when the RC specimens were seismically loaded at levels from a nearly linear response to a slightly nonlinear response, except for the first ones of specimens 1 and 2. This finding was thought to be the reason why the minimal injury of specimens cumulatively increased.3) The natural period had an amplitude dependency that underwent a gradual transition from left to upper right on the logarithmic axis of the drift angle according to the resulting maximum deformation, which was the same as that in previous studies20). Moreover, the natural period increasingly became longer and subsequently kept the condition every time the specimens were seismically loaded in amplitudes larger than the previous value. A trend toward increase in amplitude dependency according to the maximum deformation was found.4) The more the natural period tended to increase, the more the maximum deformation that the specimens increased. The rigidity lowering rate or ratio of the natural period showed gradual changes according to the resulting maximum deformation, regardless of the specimen sizes and structural form. The trends roughly agreed with the characteristics of the skeleton curves typical