Evaluation de la vulnerabilite sismique des ponts routiers au Quebec rehabilites avec l'utilisation d'isolateurs en caoutchouc naturel

From a total of 2672 multiple-span bridges in the Province of Quebec, more than 60% are concrete and steel girder bridges. Due to the advanced age and the lack of seismic detailing, these bridges may be vulnerable to future earthquake events. In an effort to mitigate the seismic risk and to prevent the interruption of the transportation network that could be catastrophic to Quebec's Province, this study proposes the replacement of typical elastomeric bearings used in Quebec by natural rubber seismic isolator devices. The seismic vulnerability of typical bridge classes retrofitted with seismic isolation devices is assessed through the development of fragility curves. Nonlinear time history analysis with 3-D detailed models for typical configurations of multi-span continuous (MSC) and multi-span simply supported (MSSS) are conducted and a comparison between the seismic fragility of as-built and retrofitted configurations are provided. Retrofitted bridge fragility curves provides a powerful tool to evaluate the impact of a retrofit measure on the performance of different bridge classes and supports the decision to prioritize vulnerable structures. Using the design-of-experiments principles, an analytical screening study reveals that the most important modeling parameters that affect critical components responses of MSC and MSSS bridges are: the isolator effective stiffness, abutment stiffness and gap between abutment and deck, in addition to variation in gross bridge geometry and ground motion contents. This screening study gives also an indication that the variation on other modeling parameters such as structural damping, bridge mass and skew angle affects the seismic response of MSC or MSSS bridges and should be considered carefully in a seismic vulnerability assessment for these portfolios of bridges. A comparative study using a suite of artificial ground motions representative of a hazard level of 2% in 50 years is presented for MSC and MSSS concrete bridges to evaluate the impact of seismic isolation on the response of critical components and the effectiveness of the retrofit measure to keep these bridges functional even after a strong ground motion. The use of seismic isolation devices effectively reduces the curvature demand placed on columns but has a negative impact on the deformation demand placed on abutment walls, considering that no special details are adopted to respect minimal clearances for isolated bridges. Experimental results of square bearings with different sizes and shape factors were used to account with the incertitude in mechanical properties of seismic isolators. Critical load tests were conducted in slender seismic isolation bearings and a finite-element model calibrated to define the limit states for seismic isolation bearings. A comparison between fragility curves for different key components of the bridge system were done and the results revealed that seismic isolation is effective in reducing significantly the probability of damage for columns and foundations. However, due to the lack of clearance between superstructure and abutment wing walls, the probability of damage in wing walls in increased and the fragility of this component controls the bridge-system fragility for all classes of bridge evaluated. Concrete girder-bridges are found to be more vulnerable than steel girder bridges due to the higher superstructure mass involved in the seismic response. The results from this work can be used to enable regional risk assessment in Quebec, prioritization of bridge retrofit and can form the basis for retrofit cost-benefit studies.;Keywords: Seismic Vulnerability; Highway Bridges; Seismic Isolation; natural rubber.