Reliability-based analysis and design of bridge substructure for combination of vessel collision and scour
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Bridges crossing waterways face potential failure risk on their piers by colliding with navigating barges and ships. To decide the design vessel collision load, AASHTO LRFD provides the collapse risk assessment procedure in which vessel traffic frequency, waterway characteristics, navigation condition and bridge load-carrying capacity are incorporated. Accordingly, bridge engineers are required to calculate the annual frequency of collapse by vessel collision events directly on design process, which is a unique routine for practical structural design process. On the streambed of inland waterways, scour effect reduces the lateral load capacity on bridge substructure by lowering the bed-elevation and removing the soil surrounding a foundation. Accordingly, a scoured bridge foundation has more likelihood of failure if it collides with a vessel. The degradation of bed elevation occurs progressively due to erosion, regardless of bridge existence. Considered as a long-term scour, this type of scour has longer duration of months through years with continuous presence than a flood-related short-term scour. Being aware of the scour effect on vessel collision resistance of bridges and the concurrence of two events, AASHTO LRFD have adopted and specified a design scour depth for the vessel collision design as 1⁄2 of long-term scour depth since the recent 5th edition (2010). The 1⁄2-factor provision was established on the basis of Turkstra's rule and engineering judgments. This deterministic scour depth could result in unintended vessel collision collapse risk due to the uncertainty in predicted long-term scour depth. In this dissertation research, by utilizing the available data and models of extreme events and the associated behavior of bridge substructure, a probability-based analysis of bridge substructure subjected to the combination of vessel collision and scour is developed. Group pile foundations with waterline pile-cap are examined for the head-on vessel collision loads per different scour depth. This type of bridge substructure is a common type of bridge crossing navigable waterways and has scour-susceptible failure modes. Failure criteria of the substructure against the combination of vessel collision and long-term scour are defined as structural failure in pile section and geotechnical failure in soil. Probabilistic formulation to combine the long-term scour and the vessel collision is developed with consideration of the concurrence of two events, the total probability theorem, and the relevant set theory. The independence between vessel collision and long-term scour is assumed. The change in the vessel collision collapse risk caused by the scour uncertainty is investigated. Scour fragility concept is developed to demonstrate the influence of the scour uncertainty to the vessel collision collapse risk. The current design provision pertinent to the mean scour depth (equivalent to 1⁄2 of depth) may lead to unintended risk due to no consideration of the uncertainty of long-term scour. Especially, at deeper scour depth, the soil failure could be dominant and causes considerable increment of the collapse risk. Accordingly, a probability-based provision on the long-term scour is necessary for the vessel collision design process. To improve the current vessel collision design procedure in AASHTO LRFD Specifications, a tentative approach to obtain the design scour depth is provided. Being aware of site-specific property of vessel collision load and substructure capacity, an immediate solution is developed. An approximate approach, with a few more scour depths than current deterministic one, is to estimate the boundary of the collapse risk with considering the scour uncertainty. This proposed method shows satisfactory estimation of vessel collision collapse risk in simplified and conservative way.