Vertical effects of near-fault ground motions and the optimal IMs for seismic response of continuous girder bridges with FPB isolators

The characteristics of vertical components in the near-fault region were investigated based on 121 pairs of typical near-fault ground motions. A weak positive correlation between the vertical and horizontal peak ground acceleration (PGA) and peak ground velocity (PGV) is shown. Basically, the ratios of vertical to horizontal peak ground acceleration and velocity of the near-fault ground motions comply with the lognormal distribution, and the vertical ground motions are significantly stronger at lower periods. Fifteen scaler horizontal and nine scaler vertical IMs are compared to select the optimal scaler IMs of horizontal and vertical seismic demands of an isolated girder bridge with friction pendulum bearings (FPB isolators). It is found that acceleration-related scalar IMs perform better for vertical seismic demands, while velocity-related scaler IMs perform better for horizontal seismic demands. The optimal vector-valued IM, consisting of scalar horizontal and vertical IMs in the forms of [IMh, IMv], for horizontal seismic demands were proposed. The optimal vector-valued IMs show an apparent improvement in efficiency compared with the corresponding scaler IMs when vertical components are considered. Furthermore, the probabilistic seismic demand models (PSDMs) for horizontal seismic demands with optimal vector-valued IM were adopted to assess the seismic demands under a given intensity of vertical and horizontal ground motions. It is identified that the vertical effects significantly influence the shear force of piers while slightly influencing the curvature of piers and bearing displacements.

Automated summary

This study investigates the characteristics of vertical components in the near-fault region using 121 pairs of typical near-fault ground motions. It is found that there is a weak positive correlation between vertical and horizontal peak ground acceleration (PGA) and peak ground velocity (PGV). The ratios of vertical to horizontal PGA and PGV follow a lognormal distribution, and the vertical ground motions are stronger at lower periods. The optimal scalar IMs for horizontal and vertical seismic demands of an isolated girder bridge with friction pendulum bearings (FPB isolators) are identified. Acceleration-related scalar IMs perform better for vertical seismic demands, while velocity-related scalar IMs perform better for horizontal seismic demands. The study proposes an optimal vector-valued IM consisting of scalar horizontal and vertical IMs for horizontal seismic demands. The probabilistic seismic demand models (PSDMs) with the optimal vector-valued IMs show improved efficiency compared to the corresponding scalar IMs when considering vertical components. The study also finds that vertical effects significantly influence the shear force of piers, while slightly affecting the curvature of piers and bearing displacements.