## 1 Introduction  
Non-linear structural analysis is used to predict the maximum deformation and corresponding structural damage for performance-based assessments. Different types of ground motion record, including scaled natural records or synthetic records generated to match a target response spectrum, have been widely used in research and practice to perform such analyses. However, limited research is available to investigate the influence of using different types of synthetic ground motion records on the seismic response predictions of structures, particularly for bridges. Some researchers have studied the use of synthetic records for the seismic evaluation of structures (e.g., [1, 2, 3, 4]), while other studies suggest that more research is needed on the use of artificially generated records for the purpose of seismic design and performance assessment of structures due to different results observed from such ground motion records (e.g., [5, 6]). This paper illustrates the use of artificial and natural ground motion records for the seismic evaluation of continuous multi-span reinforced concrete (RC) bridges using 2D and 3D non-linear dynamic analyses. In design it is typically assumed that the seismic response of straight bridges is independent in the two orthogonal directions and, as a result, the seismic responses in the two directions can be predicted using 2D analysis with the horizontal components of the ground motion records applied separately in the transverse and the longitudinal directions (e.g., [7]). In addition, another important issue that needs further study is the use of different methods to predict the mean values of seismic demands when structural models are subjected to a number of ground motion records. The prediction of the mean response of bridges has been carried out either by averaging the maximum absolute value displacements or by averaging the displacements in different directions. While the first method is widely used in practice, the second method has not been thoroughly investigated for different types of structure including bridges. Although the use of the first averaging method (i.e., averaging the maximum absolute values) is easy and practical, there are some problems with this method particularly when the seismic response of the structure is different in different directions. Further details about problems with this method are available in [7]. Therefore, the displacement envelopes in different radial directions are predicted and compared for the natural and artificial record sets considered in this research. Another issue that is not considered in practice is the fact that the maximum response of structures occurs in a random direction and at a random time (i.e., referred to here as radial response), while typically in practice the maximum response in the two perpendicular directions (e.g., X and Y directions) are predicted and used for seismic evaluation. As a result, the actual maximum radial displacement for each record is not predicted during the analysis. To estimate the maximum overall displacements, the 30% rule or the square root of the sum of the squares (SRSS) combination of the results from the two principal axes of the structure (e.g., [8; 9]) are typically used. The use of such combination rules is an attempt to account for the fact that the maximum displacements in the two principal directions do not occur at the same time. In this study the radial displacements of the bridge columns were determined by post processing the analysis results at each time step (very small time steps were considered for this purpose). As a result, the maximum radial displacements (i.e., actual seismic demands) are determined for each record and the results were compared to the predictions obtained using the simplified methods such as the 30% combination rule.