Analysis On Demand Spectra And Duration Effect Of Near-fault Ground Motions

In the field of earthquake engineering, near-fault ground motions are the research topocis of interest to the researchers for their destruction to human environment and their unique engineering characteristics which affect the seismic performance and vibration mitigation of structures. In the present paper, the ground motions records of Taiwan Chi-Chi earthquake and the Northbridge earthquake are selected as input. The demand spectra (including strength reduction factor spectra, displacement ratio spectra and energy spectra) of near-fault ground motions are analyzed systematically of Single Degree Of Freedom (SDOF) system. Moreover, the duration effect of near-fault ground motions is also studied by means of correlation analysis.Firstly, the150Chi-Chi and Northbridge earthquake records are classified according to their motion characteristics with and without velocity pulse. Using the bilinear hysteresis model of SDOF system without considering the degradation of stifiness and strength, the effects of ductility factor, damping ratio and structural elastic period on the strength reduction factor spectra, displacement ratio spectra are analyzed. Numerical results indicate that, in the short and medium period range, the ductility factor and natural period present the great influence on the mean strength reduction factor spectra and displacement ratio spectra; in the long period range, the ductility factor and natural period basically have no effect on mean strength reduction factor spectra and displacement ratio spectra. In addition, the difference of the two demand spectra for impulsive and non-pulse near-fault ground records is not large.Secondly, the effects of ductility factor, damping ratio and structural elastic period on input energy spectra, hysteresis energy dissipation spectra, energy ratio spectra and equivalent velocity spectra are examined. The calculated results demonstrate that the distinct pulse of near-fault ground motions impose remarkable effects on the peak and shape of mean input energy spectra EI and energy ratio spectra EH/EI. Moreover, the ductility level, damping ratio and natural period of systems produce important effects on the energy spectra of bilinear SDOF systems, and the effect of yield stiffness ratio can be neglected. The mean input energy spectra EI and energy ratio spectra EH/EI are fairly different for the two different earthquake events such as the Chi-Chi and Northridge earthquake.Finally, the correlation analysis between intensity measures and duration of near-fault ground motions, as well as the correlation between intensity parameters and duration of ground motions and structural response are explored. Meanwhile, combining the property of pulse-like near-fault gorund motions the velocity based duration parameters of ground motions are proposed. It is shown that, the acceleration based absolute uniform duration has high correlation with acceleration related intensity measure parameters; velocity based absolute uniform duration presents strongcorrelation with the velocity and displacement related intensityindices. Meanwhile, the natural periods of SDOF systems significantly affect the correlation between intensity parameters and structural responses. Furthermore, the structural responses have high correlation with acceleration related intensity parameters in the short period region (acceleration sensitive region); and they are strongly correlated with velocity related intensity parameters in the medium and long period region (velocity sensitive region) and with displacement related strength parameters in the long period region,(displacement sensitiveregion). Additionally, in the short period region, the acceleration based absolute uniform duration has good correlation with structural responses, the duration parameters of non-pulse ground motions have strong negative correlation with structural responses. In the medium and long period region, the velocity based absolute uniform duration is well correlated with structural responses.