ABSTRACT
Highway bridges are critical infrastructure facility in the transportation system. Reliability-based design concept provides a powerful tool in quantifying the safety level of bridges and producing a strong support for decision making in bridge construction and maintenance. Since the full implementation in 2007, the American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) Bridge Design Specifications (AASHTO 2017) have become the main platform for the practice of reliability-based bridge design in the United States. Much effort continues to be spent on further improvement of the LRFD to produce more consistent design and to make it easier to use.
Based on a statistical analysis of the bridge failures in the United States in the past three decades (Lee 2013), scour and hydraulic issues caused over 47% of failure in the existing bridge population. Current design guidelines/specifications address the risk of scour-related foundation and structural failure by including a predicted scour depth in the foundation design. While the scour predicting formulas are generally conservative to offer some margin of safety, they are not formulated and calibrated in a consistent framework with the AASHTO LRFD Bridge Design Specifications. With limited knowledge in the uncertainties in scour prediction formulas and uncertainties in their influence on foundation performance, it is difficult to further refine these formulas to better support risk-based asset management.
This paper presents a reliability-based framework to include scour depth and the associated uncertainty in the evaluation of bridge reliability. The probability distribution of maximum scour depth in bridge design life is developed on the basis of flood discharge distribution, taking natural uncertainty, hydrology modeling uncertainty, hydraulic model uncertainty and equation uncertainty into account. The resultant scour depth distribution is then converted into a reduction to the geotechnical or structural resistance, followed by the computation of the bridge reliability with existing statistical models of structural loads. A case study on a typical two-span bridge seating on deep foundations is conducted to demonstrate the procedures and preliminary results.
