Seismic Vulnerability Assessment of As-Built and Retrofitted Multi-Frame Box-Girder Bridges
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Most long bridges have at least one in-span hinge to accommodate thermal expansion and contraction without inducing large forces. These bridge types are categorized as multi-frame bridges because in-span hinges divide the structure into separate frames. There is a significant amount of evidence to suggest that adjacent frames often vibrate out of phase during strong earthquakes. Therefore, two different types of displacement problems can occur. The first type occurs when the two next frames move away from each other. In this case, deck unseating is expected if the seismically induced displacements are excessively large. The second case occurs when the two adjacent frames move towards each other. In this case the span pounding can cause localized damage and collisions between two adjacent frames and also between the deck and the abutment back-wall can cause the deck to rotate about the vertical axis. If the rotations are large and the deck seat lengths are small, the bridge can become unseated at the acute corners of the deck for skewed and curved multi-frame bridges. For this reason, multi-frame bridges have suffered severe levels of damage during past strong earthquakes such as San Fernando and Loma Prieta. However, the seismic fragility analysis of multi-frame bridges has not been sufficiently studied as other bridge types, and accordingly it is the focus of this study. Fragility curves are conditional probability statements that give the likelihood that a structure will meet or exceed a specified level of damage for a given ground motion intensity measure. In this study, PGA and spectral acceleration at 1 second period (Sa (1.0)) were considered as intensity measures (IM). For a thorough and advanced assessment, uncertainties associated with the earthquake, structural geometries, and materials were considered and nonlinear time history analysis (NTHA) was performed on 3D prototype bridges modeled in OpenSees. In this study, probabilistic methods were used to develop the time-dependent fragility curves for old and new multi-frame reinforced concrete box-girder bridges. Since long-term deterioration of existing bridge infrastructure is clearly a growing cause of concern, both the non-deteriorating and deteriorating columns with different levels of reinforcement corrosion was considered to develop the fragility curves for multi-frame box-girder RC bridges in this study.In this study, an improved understanding of the role of shear keys in affecting the seismic response of various regular and irregular bridge configurations was provided. This study also provided the engineers and researchers with the tools to make best judgment for selecting Rayleigh damping characteristics based on a clear picture of each parameter consequences through a probabilistic and fragility analysis methodology. In addition, this study performed a probabilistic seismic assessment of both older and newly-designed skewed multi-frame reinforced concrete box-girder bridges in California. Moreover, the effectiveness and implications of using three retrofit measures to limit excessive movement of decks and prevent deck unseating at the seat-type abutment and in-span hinge locations in multi-frame bridges was explored in this study.The developed system fragility curves can be readily used for seismic risk assessment along with planning, design improvement, and financial loss estimation for old and new multi-frame bridge construction. Moreover, the developed component fragility curves can be beneficial in informing seismic retrofit prioritization and decisions for existing multi-frame box-girder bridges. The developed fragility curves can also be used to optimize initial bridge inspection priorities and rapid initial estimate of loss through ShakeCast near-real-time alerting system.