Vulnerability assessment of flag-shaped hysteretic rocking bridge bents

Abstract

Rocking motion implies that a structure under seismic loading exhibits rigid body rotation around pre-defined pivot points. Its stability primarily relies on utilizing the rotational inertia through (purposely) activated dynamic motion. This paper focuses on slender bridge frames that exhibit rigid planar rocking behavior. It examines rocking frames enhanced with elastic central tendons to provide supplemental re-centering capacity and buckling-restrained braces to dissipate seismic energy; transforming a freestanding frame into a hybrid rocking frame with flag-shaped hysteretic behavior. The objective of this study is twofold. Firstly, it investigates the existence of 'optimal' intensity measures for hybrid rocking structures by examining their 'practicality', 'efficiency' and 'sufficiency' with an appropriate engineering demand parameter. The analysis shows that the slenderness-based intensity measures reduce the scatter on the response and they exhibit independence from seismic hazard parameters. Further, this study also focuses on the seismic vulnerability assessment of the examined freestanding and hybrid rocking frames. To assess their seismic performance, the analysis employs a suite of recorded earthquakes and performs a parametric investigation in terms of the vulnerability of different structural configurations. This paper develops analytical fragility curves based on the most 'optimal' scalar intensity measures. The results reveal the mitigated fragility of the positive stiffness rocking frame compared to the other structural systems. For some intensity measures though, this seismic enhancement becomes marginal. Hence, a more sophisticated approach would require the introduction of a second engineering demand parameter tailored to the needs of hybrid rocking structures or an appropriate vector intensity measure.