## 2 Bridge Properties and Modelling  
To investigate the influence of using different natural and artificial records on the prediction of the mean seismic demands two bridges with two different column arrangements were considered. The bridges had four spans with equal lengths of 50 m, as shown in Figure 1, and the superstructure was continuous over the columns. The bridge columns were hinged at the top and fixed at their bases. The superstructure consisted of a box girder with a uniform dead load of 200 kN/m. At the abutments the superstructure had no restraint in the longitudinal direction, but was restrained against transverse movements. All column diameters, D, were 2.0 m. For the case of the regular bridge, all column heights were 7 m, while for the irregular bridge the column heights were 14, 7 and 21 m, respectively. The bridges were designed based on the 2006 Canadian Highway Bridge Design Code (CHBDC) provisions [10]. It is noted that the seismic design provisions of the 2006 CHBDC are similar to those of the 2014 American Association of State Highway and Transportation Officials (AASHTO) design specification [9], were chosen to be compatible with both US and Canadian bridge design codes. The bridges were designed for Vancouver and site class C (i.e., \(\overline{V}_{S}=555\) m/s) with an importance factor, I of 1.0. Force modification factors of R = 3 and 5 were used for design of bridges in the transverse and longitudinal directions, respectively. The central columns of the regular and irregular bridges, C2, contained longitudinal reinforcement ratios of 1.23% and 1.88%, respectively. Columns C1 and C3 contained the minimum longitudinal reinforcement ratio. The concrete compressive strength and the yield stress of the reinforcing bars were taken as 40 and 400 MPa, respectively. The displacement limit  
Figure 1: Bridges’ properties: (**a**) regular bridge; (**b**) irregular bridge. for the critical central columns of the bridges was determined as 370 mm based on the equations developed by [PERSON] and [PERSON] [11] for the bar buckling damage state. A computer program was developed [12] to design the columns and carry out the moment-curvature analyses to compute the curvature and moment strength at different steel and concrete strains. The confinement effects in the concrete core were considered using the Mander equation [13] in the moment curvature analysis assuming confinement reinforcement provided in accordance with the provisions of the CHBDC [10]. It is noted that the non-linear behavior of RC elements strictly depends on the detailing rules of the transverse reinforcements that have a significant impact on the strength and ductility capacity of the structural elements [14; 15]. In predicting the non-linear response, the modified Takeda hysteresis model [16] was used in this study to model the behavior of the ductile RC columns with the Ruaumoko software [17] using the lumped plasticity method, as shown schematically in Figure 2a. This model has two main parameters, alpha and beta, which control the unloading and the reloading stiffness, respectively (see Figure 2b). The values of alpha = 0.5 and beta = 0 were adopted based on the recommendations of [PERSON] et al. [7]. The structural modelling considered in this study is similar to that used by [PERSON] et al. [7]. The superstructure was modelled using elastic beam elements and the superstructure mass was lumped at the nodal points as shown in Figure 2a. In addition, the column mass was considered in the corresponding nodal points. Rigid elements were defined to model the superstructure depth. The heights of these elements were considered as half of the superstructure depth (see Figure 2a). For the 3D analyses the yield interaction for moments of these circular columns had interaction factors of 2 in the Ruaumoko program [17].