AN INVESTIGATION ON THE TORSIONAL MOTION PREDICTION FROM TRANSLATIONAL ONES

The importance of torsional excitation on structural response has been indicated by several studies. However, due to the instrumental limitations to record torsional ground motions, it is important task to investigate on the possibility of torsional motion estimation from translational ones. The attempts were made to estimate the torsional motions using coherency function models based on the assumption that torsional motion induced due to spatial variation of ground motions. Spatial variation of ground motion can be defined by adequate coherency function models that some of them suggested by previous studies. While these coherency functions can be modeled the spatial variations of translational motions, their direct application in torsional ground motion estimation may not be appropriate. In this paper, the relations between power spectra density (PSD) of torsional and translational motions are examined based on Hao's coherency function using Chiba dense array data. Since the coherency function for torsional motion depends on the distance between the two stations, accelerometer pairs in Chiba array are categorized in five groups that varies form close separation distance (<30 m) to large ones (250 m - 320 m). For obtaining the model parameters, 50% of data are used in each group of separation distance then the PSDs related to each group are averaged at each frequency. Having averaged PSDs for translational and torsional motions at each separation group, the coefficients of the model are evaluated by optimization scheme. In the next step, to check the validity of the results, the identified coefficients are applied to predict the torsional PSDs for the other 50% of data that are not used in ptimization procedure for estimating model parameters. The comparison of the predicted and the calculated torsional motions reveals a weak estimation for close separation distances (<30 m) and satisfactory predictions in other distances