Study On Randomness Of Near Field Vertical Ground Motion

Generally we consider vertical ground motion is about 1/2~1/3 of the corresponding horizontal one. We often ignore the effect of vertical ground motion in general structure seismic design. In current seismic design code there is only one design response spectrum for horizontal ground acceleration, whereas the corresponding vertical design response spectrum equals that the horizontal design response spectra times 65%, the characteristic period of vertical design response spectrum is assumed to be same as that of horizontal component.Recently more and more near field strong seismographic records are obtained, and in these records there are many great vertical ground accelerate, some even greater than the corresponding horizontal one. At the same time, the quantity and the height of building are continuously increasing. Further researching vertical ground motion is not only feasibility but also has academic and practical engineer value.Based on hundreds of strong ground motion records of Europe, a study on difference between vertical and horizontal ground motion is proposed. Several parameters of them are compared including peak acceleration, acceleration response spectra and predominant period. In addition, effects of several factors such as local site conditions, focal mechanism, epicentral distance and seismic magnitude on these parameters are discussed, and several significative results and conclusions are obtained. In all records there are about 1/3 vertical ground motion's peak acceleration bigger than 2/3 of the corresponding horizontal one, so the action of vertical ground motion should not to be underestimated. Vertical design response spectrum equals that the horizontal design response spectra times 65% is probably unsafe when period is short, and suggest adopting different coefficients when at different periods. The predominant periods in our current seismic design code is smaller than the results of calculation, and our design code is probably unsafe. The ratio of vertical and horizontal predominant period is 0.7-0.9, so independent characteristic period of vertical design response spectrum should be defined.According to this study, we gain some profound understand of random characters of vertical ground motion. Some conclusions are recommended to be used in the correction of current design response spectrum. This provides a means by which the results of structure seismic design would be more reasonable.