科研之友
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摘要
Seismic design by the theory of response spectra equivalents to simplifying dynamic design to static design. The maximum value of complex construction under input ground motion can be determined by the superposition principle. Except for amplitude and frequency characteristics, the duration characteristic of ground motion is not taken into account in the response spectrum theory. However, damage to structure caused by duration should not be ignored. Elastoplastic properties and stiffness change of structural elements are taken into account in elastic-plastic time-history analysis. Under input ground motion, the displacement, velocity and acceleration time-history response of each mass point can be obtained through computation step by step. Although the duration characteristic of ground motion is taken into account in this method, it takes a lot of time and the calculations are very complex. In addition, the accuracy of results depends on finite element division, material constitutive relationship and seismic waves. Elastic-plastic time-history analysis has high requirement to hardware for processing a large amount of data. Even though seismic design to a structure by elastic-plastic time-history analysis under certain input ground motion is safe, it may not quite safe under other input ground motion. In order to sole this problem, Normalized Time-Frequency Response Spectrum (NTFRS) was proposed and studied.
Fist, the time-frequency response spectrum was introduced. Then, such spectra of the Tianjin record in NS direction (Tianjin people's hospital recording station), ChiChi record in NS direction (Nantou Hsinjie middle school recording station) and Wenchuan record in EW direction (Wolong recording station) were analyzed. After that, the conception of Normalized NTFRS was proposed. The NTFRS of the above three records were calculated. The potential destructiveness and the spectral patterns of them were analyzed and the predict results were verified by elastic-plastic time-history analysis. Taking the ChiChi record in NS direction for example ( peak acceleration were adjusted to 500 gal which equivalents to the level of 8 degree severe earthquake), the earthquake response of a 12-storey reinforced concrete frame structure was analyzed by using the elastic-plastic time-history analysis method. The earthquake response results of the structure and the performances of structural damage curves on normalized time-frequency response spectrums of the ChiChi record in NS direction were compared.
The spectral patterns of NTFRS of the Tianjin record in NS direction, ChiChi record in NS direction and Wenchuan record in EW direction are clearly different. Their spectrum amplitudes distributed along the period axis and duration axis are different from each other. The elastic-plastic response results of the 12-storey reinforced concrete frame accord with analysis results of NTFRS of the ChiChi record in NS direction.
Earthquake waves with different NTFRS may cause different effects on structures. Therefore, seismic design should take suitable ground motions for certain structures, or the seismic design results may be less reliable. Under strong motions, the natural vibration period of structures would increase step by step. The maximum seismic response of structure might not be the cause of its collapse. It implies that the traditional response spectrum theory has limitation because in the theory the maximum elastic response of structures is used to make seismic design. It can be concluded that the Normalized Time-Frequency Response Spectrum is better for analyzing the characteristics of ground motions and the structural failure mechanism in earthquakes.
