Seismic Reliability Assessment of Rocking Bridge Bents With Flag-Shaped Hysteretic Behavior

Abstract eng:
Rocking as a means of seismic isolation relies on utilizing the rotational inertia of the structure through (purposely) activated dynamic motion (i.e. rigid body rotation). This is a radically different concept than conventional seismic design that regards dynamic motion as an “unpleasant” by-product of structural deformation. Rigid body rotations around predefined pivot points isolate the structure during strong ground motions. Therefore, stresses on the structure are significantly reduced, relieving it from deformation and damage. The mechanical configuration of the rocking frame has been proposed as a “damage avoidance design” for bridges. The rocking frame can be either freestanding or hybrid when supplemented with (unbonded) central tendons and energy dissipaters exhibiting flag-shaped hysteretic behavior. From the standpoint of practical bridge engineering applications, a question that persists is that of the seismic reliability of the (hybrid) rocking behavior. Motivated by the unpredictability of the seismic behavior of rocking structures, this study deploys a probabilistic approach to assess the seismic fragility of hybrid rocking bridge bents. It focuses on slender bridge frames that exhibit planar rigid rocking behavior with negative, zero, or positive post-uplift lateral stiffness. Another limitation of this work is that it ignores the fracture of the supplemental devices. This practically means that frames with zero and/or positive lateral stiffness do not overturn no matter how large the rocking rotation is. The analysis herein aims to shed light on the influence of the characteristics of the hybrid frame (stiffness of the tendons and hysteretic characteristics of the dissipaters) on the seismic fragility of the bridge bent. To this end, it compares various hybrid systems with the archetypal freestanding rocking frame. The analysis considers ground motions with near-fault characteristics, either pure coherent pulses, or synthetic ground motions that include also a stochastic high-frequency component. This study offers analytical fragility curves under (synthetic) near-fault ground motions. The proposed fragility curves are either based on a univariate or bivariate intensity measures. In both cases, the fragility analysis hinges on dimensionless and physically consistent intensity measures which minimize the scatter of the response. This work shows that there is a peak ground acceleration limit below and above which the response of the (hybrid) rocking frames scales differently. The results indicate that the examined rocking structures are more vulnerable to pure low-frequency pulses than to synthetic ground motions with additional high-frequency stochastic component of the same PGV/PGA value. Further, none of the rocking design solutions (hybrid or freestanding) outperforms all others in all cases. Thus, identifying optimal design solutions is an issue that merits further study. Finally, the analysis verifies the sensitivity of the examined rocking structures to more than one parameters of the considered near-fault ground excitation.

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