This paper presents a new type of analytical model for an oil damper to improve the capturing accuracy of force-displacement relationship due to the small excitation such as the vertical seismic response. The structural design of buildings have been required the performance specification, and its specification has been major diversification. The building employed the three-dimensional isolation system was designed and constructed to ensure the isolation performance in the vertical direction as well as the horizontal direction. The concept of a three-dimensional isolation system for a reactor building has been proposed by the authors to ensure the structural integrity for a large reactor vessel. This three-dimensional isolation system, which consist of thick rubber bearings, disc springs and vertical oil dampers, has the horizontal natural period of 3.4 s and the vertical natural period of 0.33 s. According to the Maxwell model, the damping force is reduced as reducing the natural period (high-frequency) for the input wave. To realize the proposed three-dimensional isolation system, it is required to clarify the damping force characteristics of the vertical oil dampers because the vertical oil dampers are employed under the higher frequency (0.33 s) compared to the conventional vibration controlled buildings with oil dampers.
The purposes of this paper are divided into three main groups. The first is to clarify the damping characteristic of the full scale of vertical oil damper for both the linear type and the bilinear type, which has the maximum damping force of 2000 kN at 25 cm/s, through conducting the excitation tests which include the small amplitude with the high frequency and the seismic response excitation. The second is to investigate the variation of damping force due to the increasing temperature of the vertical oil dampers by conducting the sinusoidal excitation tests. The third is to create a new analytical model applying to the time history response analysis, which is focused on both the increasing temperature phenomena and the damping force characteristic in the small excitation such as the vertical seismic response displacement.
The primary results are summarized as follows:
1) The stable force-displacement relationships were obtained against the excitation waves with the small amplitudes and the high-frequency up to 5.0 Hz by ensuring higher stiffness of the vertical oil damper.
2) The reducing ratio for the damping coefficient up to the allowable temperature (80°C) was revealed to be approximately 7%. The increasing temperature due to the seismic response excitations were clarified to be from 2°C to 4°C. This result means that the decreasing damping force has little impact on the seismic response.
3) The new type of analytical model consisting of the double dashpot elements and the single stiffness element, which is referred to as the D.D.P model, was proposed to improve the force-displacement relationships in small excitation that results in the orifice characteristic. The analytical results obtained by the D.D.P model correspond to the test results not only for the small excitations but also the large excitations. These results demonstrate that D.D.P model can calculate the force-displacement relationships, which have the orifice characteristic, are closer to the measured value than to the ordinary Maxwell model. Additionally, the D.D.P model can accurately simulate the force-displacement relationships considering the reducing forces in response to the increasing temperature by using the increasing temperature model.
