## 1 Introduction
To further investigate this issue the radial displacements (i.e., the real maximum displacement that occurs in a random direction) of the bridge columns were also predicted and compared for different record sets, in addition to the displacements in the longitudinal and transverse directions. The effect of displacement directions on the prediction of the mean response of structures was also considered in this study, as proposed by [PERSON] et al. [7]. The horizontal components of the artificial records may be generated separately and, therefore, for performing 3D analyses different pairs of records are selected from a number of horizontal components. Hence, it is important to investigate if this random selection of the ground motion pairs will affect the predictions of the mean responses of the bridges. To address this issue, the correlation coefficients between the horizontal components of the natural and artificial ground motion records were also computed and compared for the records considered. The analyses in this study were performed using one natural and two artificial record sets and the predictions from different sets of records are compared. Another issue that needs further investigation is the impact of using artificial records for incremental dynamic analysis (IDA) of RC bridges. In this paper IDA results are studied at different damage states of the RC bridges including yielding, cover spalling, bar buckling and collapse using different record sets and different number of records. An attempt is made to compare the seismic response predictions obtained using different types of artificial and natural records in different important aspects including, the use of 3D and 2D analyses for predictions of seismic demands, predicting the seismic response in different directions (i.e., around the full 360\({}^{\circ}\)) and the effects of using different typesof record (i.e., artificial and natural) on the IDA results at different damage states of the RC bridges. Also the maximum radial displacements (i.e., the real maximum displacement in seismic response) were predicted for the bridge columns and compared with the displacements along the principal axes of the bridges, as predicted in customary practice. In addition, no research is available to investigate the effects of the artificial records on the seismic response considering directionality. The mean response of the RC bridges is presented using two different methods including an advanced method that takes into account the displacement directions and also the conventional method that only considers the absolute maximum response, regardless of the direction. These comparisons are made for different record sets and important differences are presented in the paper. It is important to investigate if the use of artificial records, such as those studied in this research, provide reliable and acceptable predictions, in all aspects, for seismic response of bridges compared with the predictions using natural records. Limited research is available on these issues and this study attempts to further investigate the differences using different approaches. It is noted that although many different methods of generating artificial records may be available, this study is limited to two different sets of artificial records used in practice as examples of the use of such records for the seismic evaluation of structures.