1) By setting the test piece between the guards on the center of the under-water vibration base, let the displacement gauge, lowered from the top of the water reservoir, be attached. After that, let a spring be settled in a position of 9cm from the test piece bottom.
2) Let the displacement amplitude (both amplitudes) of the under-water vibration base be settled as 3mm.
3) By sine-wave vibrating the vibration base, let the frequency be increased gradually to the extent of 1Hz〜10Hz. When the motion state is stabilized, let the displacement of the under-water vibration base and test piece be measured.
4) Regarding the individuals of the water depth 5cm, 10cm, and 20cm, let measurement be made with the values of the nominal spring coefficient 0.1kgf/cm(98.1N),5kgf/cm(49.ON/cm),and l0kgf/cm (98.1N/cm),i.e. 27 cases in all.
5) By reading the displacement of the vibration base and test piece from the data obtained from the measurement, let the absolute displacement response magnification at the water depth, under-water weight, and frequency be obtained, respectively.
4. ANALYSIS AND EXPERIMENTAL RESULTS
By obtaining the time-history response displacement by means of the theoretical analysis and the one by means of the experiment, the results of the calculation of the absolute displacement response magnification(representing response displacement /input displacement, hereafter called just a response magnification) are computed. On the other hand, part of the said results sketched in a graph is shown in Fig.3. In this connection, the case of a rectangular cross section is plotted for the sake of comparison in the figure.
5. DISCUSSION
Although the analytical and experimental results shown above are restricted ones to some extent, the facts shown below can be obtained from these results.
1) The added mass is increased as the water depth is made deeper, and gliding displacement under water(the relative displacement of the structure and the ground) is increased. That is to say, the absolute displacement and response magnification are decreased. This property is the same as in case of a rectangular cross section, but freedom from seismicity in the middle frequency region is slightly lower than in case of a rectangular cross section because the gliding of the structure is slower than in case of the rectangular cross section. The same result has been obtained with respect to the theoretical analysis.
2) Difference of the response magnification depending on the magnitude of the spring coefficient is relatively small, but it can safely be said that the following description is quite adequate. When the water depth is shallow enough, the response magnification is made greater with that of greater spring coefficient as the under-water weight is increased. When the water depth is deep enough, none of remarkable difference is noted between both the cases even if the under-water weight becomes greater. In case of the moored-type structure, drift caused in case of the independent-type structure can be prevented by being moored by a spring.
3) Since the friction force is as the under-water weight is made greater, the response magnification becomes greater. Such a tendency is noted likewise both in case of the
