Since it is well known that the vibration characteristics such as the natural period and the damping ratio of buildings are dependent on the shaking amplitude of the building. In the previous paper, these characteristics of the horizontal 1st mode for the low and middle raised RC and SRC buildings were studied. In this paper, those of the horizontal 1st and 2nd mode for 8 high rised steel buildings are studied.
The observed seismic data including the 2011 off the Pacific Coast of Tohoku Earthquake (hereafter, 3.11 earthquake) are analyzed using the ARX (Auto-Regressive eXogenous) model. The results are estimated for 2 averaged drift angle levels (the micro amplitude (10-6) and the middle amplitude (10-3)) to consider the amplitude dependency. The effects of aging of the buildings are also investigated. From above study, following results are obtained.
1) In both 1st and 2nd natural periods, comparatively small amplitude dependency is seen and their angles of inclination are almost the same. The variation of 1st and 2nd natural periods between before and after 3.11 earthquake is small, since all buildings studied in this paper were in almost the elastic condition though all observed data.
2) The 1st damping ratio before and after 3.11 earthquake have a tendency that the ratio increases corresponding to the increase of the amplitude from the micro amplitude level.
3) The 1st damping ratio during 3.11 earthquake, when the amplitude was large, tends to decrease corresponding to the increase of the amplitude. The value becomes less than 2% when the drift angle is more than 10-3. It is considered this tendency is not due to the nonlinear condition because all the buildings seemed to be in the elastic condition.
4) The 2nd damping ratio has almost the same tendency as the 1st, qualitatively. Some differences are seen between them, quantitatively. The accuracy of the 2nd seems to be not high.
5) The relationship between the experienced maximum drift angle during 3.11 earthquakeand the variation of natural periods is studied for all buildings. The ratio is between from 1.0 and 1.1. However, the amplitude dependency is not seen clearly.
6) Concerning to the secular change of the 1st and 2nd natural periods, clear tendency cannot be seen while some previous research showed obvious change.